The RX-888 (Mk2) and external clocking

Figure 1: The RX-888 with external clock input (right) The enable/disable switch is barely visible behind the USB connector. Click on the image for a larger version.

• Improving the thermal management of the RX-888 (Mk2) - link - This page talks about highly recommended modifications to the RX-888 to reduce internal heat to improve reliability.
  

• Measuring signal dynamics of the RX-888 (Mk2) - link - This is a discussion of how much (and little) signal is needed to stay within the dynamic range of the RX-888 and the effects of gain and attenuator settings.

Adding an external clock connection

While the internal 27 MHz TCXO in the RX-888 (Mk2) is pretty good, there may be instances where one wishes better accuracy and stability.  Fortunately, the RX-888 (Mk2) has provisions for doing so in the form of a jumper to disable the internal clock (when the jumper is removed) and a small connector (a tiny U.Fl) on board to accept that clock.

Unfortunately, it is up to the user to add the cable to feed an external clock - but short 4-6" (10-15cm) cables already fitted with a U.Fl male and SMA chassis-mount female connector are easily obtained from the likes of Amazon, EvilBay and others - just be sure that you do NOT get a "Reverse" (RP) SMA by mistake!

This leaves the jumper.  While many people simply remove the jumper and mount the external clock connector between the HF and VHF inputs - or sometimes to the right of the USB connector knowing - from then on - their RX-888 will be unusable unless there is an external clock input - I prefer to make use of the ability of the internal clock to be switched - using (ahem) a switch allowing for testing/use of the RX-888 in a "stand alone" configuration - but this is up to you.

If one is careful, it's possible to mount the external clock SMA connector and switch on the same panel as the USB connector, orienting so that its handle is toward the "Clock In" connector to indicate that an external clock is to be used - but labels or markings are always nice, too!

If one takes the route of mounting the external clock input between the HF and UHF inputs, the switch could be placed to the right of the USB connector - or, if as in the case of one of my RX-888s where I put a heat sink on the FX3 chip and there wasn't room there - I found a very small toggle switch that just fit between the case screw and left side of the USB connector and tip of this switch may be spotted just behind the USB connector in Figure 1, above.

IMPORTANT:  As the external clock input is simply wired in parallel with the internal 27 MHz clock.  What this means is that with the internal clock enabled, it will be present on the external clock input.  Similarly, if you supply a 27 MHz external clock without disabling the internal one, the two will "fight" each other and you'll get "garbage" results.

What type of signal to use as an external clock

• The best external clock source is a 27 MHz sine wave of between 1.25 and 3.3 volts peak-to-peak.
• A series coupling capacitance of between 100pF and 1000pF (470pF typ.) should be present on the "center pin" between the RX-888 to eliminate a DC path to ground on the signal line.
  

While a capacitively-coupled 27 MHz sine wave is recommended for reasons that will be mentioned later, a lot of devices offer square wave outputs - and getting these to work reliably requires at least a little bit of attention.

Using the Leo Bodnar Precision GPS Clock to drive an RX-888:

Because the RX-888 natively requires a 27 MHz clock this means that if you already have a 10 MHz standard (GPS, Rubidium, etc.) kicking around, you will not be able to use it directly.  While it's not too difficult to synthesize 27 MHz from 10 MHz (a number of Si5351-based devices can do this) it's most common for users of the RX-888 to use a device such as that sold by Leo Bodnar, which can be programmed for almost any frequency (from audio through UHF) with good precision and accuracy.

You can look at these products here:  https://www.leobodnar.com  (I have no stake in Bodnar, but I have used them and I and others have had good success.)

The most commonly-used device is the Bodnar "Mini" - which has one output - and this single output is often "daisy-chained" between RX-888s.  There is also the functionally similar LB-1420 with a single output and the "Precision GPS Reference Clock" which has two signal outputs - but there is very limited ability to set the "second" output to a specific frequency and it's mostly useful for outputting the same frequency on the two ports - or outputting a 1PPS signals on the "unused" port.

Testing with a square wave - such as that produced by the Leo Bodnar GPS reference revealed that the drive level was far more finicky - and this has to do with the fact that a "square" wave with a reasonably fast rise time does NOT remain a square wave for very long as it quickly turns into something rather spiky and distorted as depicted in the image below:

Figure 2: A typical square wave output from a Bodnar GPS reference at the end of about 3 feet (1 meter) of unterminated cable.  Ringing is evident!

With multiple "spikes" that can occur on such waveforms due to distortion, it's possible - even likely - that certain combinations can result in multiple triggering peaks of the waveform.  In an extreme case, such distortion can cause the Si5351 to be triggered at twice the actual clock rate - but rather the result may be instability resulting in the RX-888 clocking which can be manifest as anything from no signals being "present" to those that are being off-frequency, varying, or just "noisy" - and this errant behavior may vary with temperature and slight changes in operating voltage.

It's important to realize that like the RX-888, the Bodnar is ALSO DC-coupled which explains why the above waveform in Figure 2 largely rests above the center line (zero volts) with the exception of some "ringing" which extends negative and is likely being clamped somewhat by the '888's internal diodes.

With a 3.3 volt waveform emanating from the Bodnar, we can reasonably expect that - if the signal isn't too "ringy" that a signal exceeding about 1 volt positive just once per cycle is likely to trigger the 888's Si-5351 correctly.

IMPORTANT:  If you try to directly drive an RX-888 with the output of a Bodnar, it will probably NOT work reliably!  I have observed this with my own Bodnar/RX-888s and many others have reported the same issue.

Remembering that the external clock input of the '888 goes directly to very sensitive logic devices, a simple resistive attenuator pad will do double duty:

• Rather than a very high impedance circuit that has a low resistance path from the outside world to a sensitive logic gate, resistance to ground offers a degree of protection by offering a relatively low resistance to ground and the series resistance provides at least some limit to input currents.
  

• While theoretically OK, the output of the Bodnar will not reliably drive the input of the Si5351 in the RX-888 directly, but being reduced to half or third of its original output seems to be pretty reliable and is less likely to cause clipping of diodes on the input circuit which can exacerbate ringing and other types of waveform distortion.
  

A 6 to 12 dB resistive pad - either 50 or 75 ohms - is a reasonable choice offering a bit of voltage reduction - but staying well above the 1 volt usability threshold - and such a pad, even if it is not connected to a 50 ohm load, will provide a bit of resistive termination, likely reducing the tenacity of reflections.  While a resistive pad does not offer DC decoupling between the center pin of the '888's external clock input, it works with the Bodnar as that device sources a square wave referenced to zero volts so the pad simply acts as a voltage divider for that square wave.

Testing has shown that the '888 seems a bit more forgiving of signal drive levels if there is a DC blocking capacitor on its signal input - something that could be provided by placing a "DC block" device (available in SMA, BNC or F-type connectors) between the '888 and the external clock source.

Caveats and warnings - and why the '888 is so finicky about its external clock

The external clock input of the RX-888 - as described in better detail in the next section of this blog post - is connected DIRECTLY to inputs within the '888 and as such, it has a few undesirable properties:

• There is a DC connection between the external clock, the oscillator output and the input to the 888's internal Si5351 synthesizer.  This exposes the clock input directly to extremely static and voltage-sensitive inputs.
• Because of this, it's very easy to damage the RX-888 when using and external clock, particularly if there are voltage potentials between different pieces of equipment.

• There is diode clamping between ground and the 3.3 volt input.  In the '888, this is primarily a BAT99 dual diode, but it also includes the protection diodes of the other devices in the circuit - namely the output of the onboard 27 MHz oscillator and the input of the Si5351 itself.  At first this might seem like a good thing - and it sort of is - but this means that any signal input to the RX-888 should be capacitively coupled - or directly to a 0-3.3 volt signal.  This is one aspect of the '888 that was definitely not well considered or implemented.
  
• What this means is that if you try to drive the RX-888's clock input with a source that is DC "grounded" - which includes devices that are transformer-coupled (e.g. a splitter to send the clock to multiple units) that the voltage output will be bipolar.
• For example: 
• If you were try to use a T1-1 isolation transformer to break a ground loop between the external clock input and the Bodnar - as well as other devices - the signal input may be 3.3 volts - but bipolar - that is, it will go above and below "ground" by about 1.65 volts - but since there is diode clamping, the negative-going signal will distort the waveform.
• The result of this can either be finessing required to find the precise drive level to make it work at all or - sometimes - you will find the signals at the wrong frequencies (sometimes at about half the expected frequencies) if the badly-distorted waveform triggers the input of the Si5351 synthesizer in the '888 twice on every clock cycle.

All of these factors often confound users of the RX-888 (Mk2) trying to feed an external clock - and things get more complicated if multiple devices are use.  For example:

• As with any sensitive piece of RF equipment, having multiple, disparate connections between pieces of equipment will usually end up with circulating currents - and since every conductor has resistance, this can cause noises to appear in the RF input.  A few examples:
• The RX-888 - or any SDR - will have multiple connections to it - typically the antenna and power input.  In the case of the RX-888 and many other SDRs, this means an antenna and USB connection.
• Isolating the RF signal lines from longitudinal currents (e.g. common mode) is a useful tool.
• Often, this can take the form of small coaxial cable (RG-142 or RG-174) wound with 8-12 turns on an FT-140 or FT-240 core of 31 or 43 material (the former being better for lower frequencies).  This is useful for HF (160-10 meters) but it loses efficacy below this and is not helpful if your interest extends into the AM broadcast bands and lower frequencies (e.g. longwave - including LF and VLF which includes the 2200 and 630 meter amateur bands.)
• Another tool can be an "voltage balun" - essentially an isolation transformer with no DC connection at all.  Often, these are built around the Mini-Circuits T1-1.  These lose their efficacy below a MHz or so so they may have excessive attenuation on LF and VLF frequencies.  At higher frequencies (above 10 MHz) their common-mode rejection also starts to drop meaning that in a very noisy environment, signals can "leak in" at high HF from the surrounding equipment - something that needs to be checked if you try it.
  
• Power supplies and computers (via a USB cable) are notoriously noisy, so you WILL get circulating currents flowing between the devices.  Having a choking USB cable (e.g. 6-12 turns on an FT-140 or FT-240 core of 31 or 43 material) can help significantly, as can doing similar on a DC supply line and also choosing a "known RF-quiet" power supply.
• Adding a "third" connection to the receiver - such as the external clock, in case of the RX-888 (Mk2) - can further complicate issues as it adds yet another  avenue of common-mode currents and noise.
• This connection, too, should be appropriately isolated - but doing so is complicated by the way the external clock input is implemented.
• The fact that the external clock device is connected to a potentially-noisy power supply and  a GPS antenna - which may or may not have its own grounding (which can further introduce circulating currents) is yet another thing about which you should be wary!
  

One issue that also arises is that output of devices like the Bodnar are square wave.  This, by itself, isn't a problem - and a direct connection between the Bodnar and '888  - since they both have 3.3 volt signal levels - works OK, at least with very short cables when using a 6-12 dB pad.

 

Conveying this square wave signal - particularly over greater distances and considering that the clock input to the RX-888 is high-impedance with a bit of capacitance means that long runs (anywhere near 1/4 wave at the clock frequency or longer) can result in reflections due to unterminated cables.  What one can do is put a 50-75 ohm termination at the far end of the cable. This, however, does not help with the issue of DC/galvanic isolation between individual receivers.

 

Testing the stability of your external clock mechanism:

 

As properties of solid-state devices change over temperature - and signal levels may vary depending on what other devices are connected to your clock source - it would be a very good idea to varying the clock signal to determine if you have enough margin to allow it to work if levels change, or if you are on the "ragged edge".

Reducing the signal level is the most obvious test:  The use of a step attenuator - or use a variety of fixed attenuator pads (be sure that they pass DC) and reducing the level by between 1 and 15 dB - and then observing when clocking becomes unreliable:  This will give you a good idea as to the margin between what you are feeding to the '888 and when it will quite - and it may prompt you to reduce your signal level slightly.

Using HDSDR under Windows

Determining when the clocking signal into the '888 becomes unreliable is a bit trickier in some cases.  By far the easiest is to use a program like HDSDR with the "SDDC" ExtIO driver on a fairly fast Windows computer with USB3 ports:  A higher-end Intel i5 or medium-high end Intel i7 will suffice.  Connecting the '888 to an external antenna and tuning in a reliable signal (like a shortwave broadcaster or a time station like WWV/H or CHU - or tuning it your own signal generator) while watching the waterfall will tell you immediately when the external clocking fails.

If you are using Linux with ka9q-radio, you can use the "Monitor" program to tune a signal with the audio being sent to the default audio device - but doing this is beyond the scope of the document.  If you are using a Mac, I don't have a suggestion unless someone speaks up.

Transformer-based signal isolation NOT recommended for the '888's clock input - sort of...

It is important for any receiver to minimize the amount of current circulating through the "ground" connections.  Such currents in an analog receiver can induce hum in unbalanced audio lines and if the receiver is actually a transceiver, those same signal paths can induce RF into seemingly unrelated equipment in the ham shack.

Sometimes overlooked is the fact that these same currents can induce RF currents on the cables interconnecting equipment and it is likely that these will find their way into the receiver's front end and degrade performance by raising the noise floor.  This is especially true when a computer-connect software-defined radio - like the RX-888 - is involved as we now have a connection (via the USB cable) to a device that is likely to be "noisy" at RF - namely the computer - but this also means that noise can come from other devices to which this computer is connected directly or indirectly, namely its power supply, other peripherals, its power supply - and noisy devices on the AC mains into which this power supply is plugged.

Current "balun"

For receiver RF connections one way to deal with this is to use a common-mode RF choke which is typically a dozen or so turns of coaxial cable wound on a T-140 or T-240 toroid - usually with 31 or 43 type material.  This will break up common-mode currents on the cable - at least at HF - and can reduce such issues and this works for both the signal (antenna) and external clocking lines.

At DC and mains frequencies such chokes offer little/no efficacy and at low frequencies (below a MHz or so) these chokes lose their effective series resistance owing to limited inductance.  What this means is that if you have strong circulating currents (e.g. current flowing between your antenna "ground" and house mains "ground") they will have little effect.

Voltage "balun"

A possible alternative is to use a transformer to couple between RF sources:  A reliable, low-cost, commonly-available device for this is the Mini-Circuits Labs T1-1 which provides complete galvanic isolation between the source and load with a reasonable degree of longitudinal isolation.

While the T1-1 works well for the RF input, it will not work so well for the RX-888's external clock input by itself and the reason for this is that the output from a transformer winding is, by definition, bipolar about the zero volt point.  In the case of an external clock signal of, say, 1 volt peak-peak, each half would be above and below zero volts and with a direct DC connection to the Si5351's input it is unlikely to properly drive/trigger it.

If the signal is of higher amplitude - such as our 3.3 volt square wave - half of this "ugly" waveform will lie below ground potential and that below the 0.6 volt diode conduction voltage will be clamped, potentially distorting the waveform even more.

If a transformer-based method of isolation is used it is strongly suggested that a capacitor be placed in series with the '888's signal input to allow the waveform and voltage to float above ground and avoid negative clamping.  As mentioned earlier, a "DC Block" device could be used if you choose not to build your own device.

Example homebrew devices:

Here are a few (relatively) simple devices that one could build on a piece of scrap PC board - or you could go through the effort of designing and building a board with these features.

Figure 3, below, shows a simple resistive coupler incorporating the features suggested above:

Figure 3:  A simple 10-ish dB resistive pad with DC blocking to keep the external clock input of the RX-888 "happy" and to prevent clipping of negative-going voltage by built-in protection diodes.  The "small" capacitor value also minimized the amount of stored charge dumped into the '888 due handling/shorting of the input cable.

This diagram shows a resistive pad that offers about 10 dB of attenuation - the values being determined assuming a 50 ohm system - but since the '888's input impedance is almost exclusively capacitive (a few 10s of pF) it is operating more as a voltage divider presenting a resistive load that just happens to be around 50 ohms.  The coupling capacitor between the pad and the '888 offers DC blocking to make it more forgiving to varying signal levels.  While the capacitor blocks DC, the signal being input to the Si5351 will find its own level due to the clamping effects of the protection diodes in the '888.

Also shown is the optional inclusion of a 1000pF capacitor that can be inserted at point "X":  This will decouple DC and mains AC currents that might flow between the clock source and the RX-888 itself - but it is low enough impedance that it does not necessarily offer RF decoupling between devices.  With the circuit shown above, however, you can precede it with decoupling device - such as a common-mode choke (e.g. current balun - the type with a dozen or so turns on a toroid) or even a T1-1 transformer.

Figure 4, below, shows another possible approach:

Figure 4:  This circuit provides both common-mode isolation and a degree of band-pass filtering of the 27 MHz clock signal:  Filtering to a sine-like waveform reduces glitching due to cabling issues (reflections, misterminations) as well as offers a degree of protection to the RX-888's input as the filter will limit the amount of energy that could be imparted.  It also provides a (small) degree of termination (<150 ohms).   The "optional" 1000pF capacitor shunts low level leakage of the 27 MHz signal due to transformer imbalance - but it is suggested that one use a common-mode choke to restore isolation at HF frequencies.

This device is slightly more complicated, but it offers several advantages:

• "L1" is a trifilar-wound toroidal transformer (that is, its turns consist of three wires gently twisted together before winding on the toroid).  Its intrinsic inductance is around 0.22uH and with the 150pF capacitor seen on the lower half of the diagram, it resonates broadly at 27 MHz - the external clock frequency for the '888.
• The resistors shown offer a bit of resistive termination to the signal source (a bit below 150 ohms) which can help to reduce reflections on the cable.
  
• These series 150 and 100 ohm resistors "decouple" the resonant circuit from the signal path somewhat and the values were chosen to allow sufficient "Q" to offer reasonable filtering of the input signal into a fairly good sine wave.

• Figure 5:  The (nearly) sine wave output from the circuit depicted in Figure 4. Click on the image for a larger version.

  As this is a transformer-coupled circuit, there is no DC connection at all between the input and output.  Because it is resonant at 27 MHz, it will also offer a degree of rejection of other signals that might be present.  As the resonant circuit is wired to the "RX-888 side" of the circuit, it offers excellent protection to it.
  
• As with the previous circuit, an optional 1000pF capacitor is shown as well:  Including this will reduce the common-mode isolation between the input and output but it will suppress a bit of leakage of the 27 MHz clock signal that can occur owing to the fact that the transformer that is L1 is not perfectly balanced.

The disadvantage of this circuit is that it requires the winding of a toroidal transformer and tuning it to 27 MHz - something easily done with a NanoVNA or an oscilloscope and an oscillator.  

Figure 5 shows the resulting waveform that has passed through the circuit depicted in Figure 4:  It is nearly a sine wave and as such, it is much more resistant to causing false triggering on "ringing" edges as compared to a square wave.

Figure 6:  The prototype transformer/filter circuit depicted in Figure 4 connected at the Bodnar, connected to the '888 with a short BNC<>SMA jumper. Click on the image for a larger version.

Figure 6 shows the circuit of Figure 4 in action, connected directly to the Bodnar's output and - via a very short BNC to SMA cable - to the RX-888 sitting atop it.

This prototype unit was built in a piece of copper-clad PC board material.  On the top side, the components were wired with flying leads to the connectors and "dead bug" on the copper itself:  Between the "Bodnar" and the "RX-888" side the copper was cut to provide the two separate signal "grounds" with only the transformer coupling between the two.

At some point, it may be worth designing a small PC board for this, but for the meantime a small number of these prototypes have been built and put into service very successfully.  As suggested earlier, the a step attenuator was inserted between the Bodnar and this circuit and the signal reduced until the '888 no longer reliable locked to the external clock and it was found that there was plenty of margin to assure stable operation under varying conditions.

Lots of other possibilities

Now that you know what the RX-888 "wants", you have a better idea of what you are likely to be able to "safely" use to drive the external clock input of the RX-888.

* * * * *

This page stolen from ka7oei.blogspot.com

[End]

It should not have escaped your attention - at least if you live in North America - there there have been/will be two significant solar eclipses occurring in recent/near times:  One that occurred on October 14, 2023 and another eclipse that will happen during April, 2024.  The path of "totality" of the October eclipse happened to pass through Utah (where I live) so it is no surprise that I went out of my way to see it - just as I did back in 2012:  You can read my blog entry about that here.

Figure 1: The eclipse in progress - a few minutes before "annularity". (Photo by C. L. Turner)

The October eclipse was of the "annular" type meaning that the moon is near-ish apogee meaning that the subtended angle of its disk is insufficient to completely block the sun owing to the moon's greater-than-average distance from Earth:  Unlike a solar eclipse, there is no time during the eclipse where it is safe to look at the sun/moon directly, without eye protection.

The sun will be mostly blocked, however, meaning that those in the path of "totality" experienced a rather eerie local twilight with shadows casting images of the solar disk:  Around the periphery of the moon it was be possible to make out the outline of lunar mountains - and those unfortunate to stare at the sun during this time will receive a ring-shaped burn to their retina.

From the aspect of a radio amateur, however, the effects of a total and annular solar eclipse are largely identical:  The diminution of the "D" layer and partial recombination of the "F" layers of the ionosphere causing what are essentially nighttime propagation conditions during the daytime - geographically limited to those areas under the lunar shadow.

In an effort to help study these sort of effects - and to (hopefully) better-understand the propagation effects, a number of amateurs went (and are) going out into the field - in or near the path of "totality" - and setting up simultaneous, multi-band transmitters.

Producing usable data

Having "Eclipse QSO Parties" where amateur radio operators make contacts during the eclipse likely goes back nearly a century - the rarity of a solar eclipse making the event even more enigmatic.  In more recent years amateurs have been involved in "citizen science" where they make observations by monitoring signals - or facilitate the making of observations by transmitting them - and this happened during the October eclipse and should also happen during the April event as well.

While doing this sort of thing is just plain "fun", a subset of this group is of the metrological sort (that's "metrology", no "meteorology"!) and endeavor to impart on their transmissions - and observations of received signals - additional constraints that are intended to make this data useful in a scientific sense - specifically:

• Stable transmit frequencies.  During the event, the perturbations of the ionosphere will impart on propagated signals Doppler shift and spread:  Being able to measure this with accuracy and precision (which are NOT the same thing!) adds another layer of extractable information to the observations.
  

• Stable receivers.  As with the transmitters, having a stable receiver is imperative to allow accurate measurement of the Doppler shift and spread.  Additionally, being able to monitor the amplitude of a received signal can provide clues as to the nature of the changing conditions.
  

• Monitoring/transmitting at multiple frequencies.  As the ionospheric conditions change, its effects at different frequencies also changes.  In general, the loss of ionization (caused by darkness) reduces propagation at higher frequencies (e.g. >10 MHz) and with lessened "D" layer absorption lower frequencies (<10 MHz) the propagation at those frequencies is enhanced.  With the different effects at different frequencies, being able to simultaneously monitor multiple signals across the HF spectrum can provide additional insight as to the effects.
  

To this end, the transmission and monitoring of signals by this informal group have established the following:

• GPS-referenced transmitters.  The transmitters will be "locked" to GPS-referenced oscillators or atomic standards to keep the transmitted frequencies both stable, accurate - and known to within milliHertz.
  

• GPS referenced receivers.  As with the transmitters, the receivers will also be GPS-referenced or atomic-referenced to provide milliHertz accuracy and stability.

With this level of accuracy and precision the frequency uncertainties related to the receiver and transmitter can be removed from the Doppler data.  For generation of stable frequencies, a "GPS Disciplined Oscillator" is often used - but very good Rubidium-based references are also available, although unlike a GPS-based reference, the time-of-day cannot be obtained from them.

Why this is important:

Not to demean previous efforts in monitoring propagation - including that which occurs during an eclipse - but unless appropriate measures are taken, their contribution to "real" scientific analysis can be unwittingly diminished.  Here are a few points to consider:

• Receiver frequency stability.  One aspect of propagation on HF is that the signal paths between the receiver and transmitter change as the ionosphere itself changes.  These changes can be on the order of Hertz in some cases, but these changes are often measured in 10s of milliHertz.  Very few receivers have that sort of stability and the drift of such a receiver can make detection of these Doppler shifts impossible.
  

• Signal amplitude measurement.  HF signals change in amplitude constantly - and this can tell us something about the path.  Pretty much all modern receivers have some form of AGC (Automatic Gain Control) whose job it is to make sure that the speaker output is constant.  If you are trying to infer signal strength, however, making a recording with AGC active renders meaningful measurements of signal strength pretty much impossible.  Not often considered is the fact that such changes in propagation also affect the background noise - which is also important to be able to measure - and this, too, is impossible with AGC active.
  

• Time-stamping recordings.  Knowing when a recording starts and stops with precision allows correlation with other's efforts.  Fortunately this is likely the easiest aspect to manage as a computer with an accurate clock can automatically do so (provided that one takes care to preserve the time stamps of the file, or has file names that contain such information) - and it is particularly easy if one happens to be recording a time station like WWV, WWVH, WWVB or CHU.

In other words, the act of "holding a microphone up to a speaker" or simply recording the output of a receiver to a .wav file with little/no additional context makes for a curious keepsake, but it makes the challenge of gleaning useful data from it more difficult.

One of our challenges as "citizen scientists" is to make the data as useful as possible to us and others - and this task has been made far easier with inexpensive and very good hardware than it ever has been - provided we take care to do so.  What follows in this article - and subsequent parts - are my reflections on some possible ways to do this:  These are certainly not the only ways - or even the best ways - and even those considerations will change over time as more/different resources and gear become available to the average citizen scientist. 

* * *

How this is done - Receiver:

The frequency stability and accuracy of MOST amateur transceivers is nowhere near good enough to provide usable observations of Doppler shift on such signals - even if the transceiver is equipped with a TCXO or other high-stability oscillator:  Of the few radios that can do this "out of the box" are some of the Flex transceivers equipped with a GPS disciplined oscillator.

To a certain degree, an out-of-the-box KiwiSDR can do this if properly set-up:  With a good, reliable GPS signals and when placed within a temperature-stable environment (e.g. temperature change of 1 degree C or so during the time of the observation) they can be stable enough to provide useful data - but there is no guarantee of such.

To remove such uncertainty a GPS-based frequency reference is often applied to the KiwiSDR - often in the form of the Leo Bodnar GPS reference, producing a frequency of precisely 66.660 MHz.  This combination produces both stable and accurate results.  Unfortunately, if you don't already have a KiwiSDR, you probably aren't going to get one as the original version was discontinued in 2022:  A "KiwiSDR 2" is in the works, but there' no guarantee that it will make it into production, let alone be available in time for the April, 2024 eclipse. 

Figure 2: The RX-888 (Mk2) - a simple and relatively inexpensive box that is capable of "inhaling" all of HF at once. Click on the image for a larger version.

The RX-888 (Mk2)

A suitable work-around has been found to be the RX-888 (Mk2) - a simple direct-sampling SDR - available for about $160 shipped (if you look around).  This device has the capability of accepting an external 27 MHz clock (if you add an external cable/connector to the internal U.FL connector provided for this purpose) in which it can become as stable and accurate as the external reference.

This SDR - unlike the KiwiSDR, the Red Pitaya and others - has no onboard processing capability as it is simply an analog-to-digital coupled with a USB3 interface so it takes a fairly powerful computer and special processing software to be able to handle a full-spectrum acquisition of HF frequencies.

Software that is particularly well-suited to this task is KA9Q-Radio (link).  Using the "overlap and save" technique, it is extraordinarily efficient in processing the 65 Megasamples-per-second of data needed to "inhale" the entire HF spectrum.  This software is efficient enough that a modest quad-core Intel i5 or i7 is more than up to the task - and such PCs can be had for well under $200 on the used market.

KA9Q-Radio can produce hundreds of simultaneous virtual receivers of arbitrary modes and bandwidths which means that one such virtual receiver can be produced for each WSPR frequency band:  Similar virtual receivers could be established for FT-8, FT-4, WWV/H and CHU frequencies.  The outputs of these receivers - which could be a simple, single-channel stream or a pair of audio in I/Q configuration - can be recorded for later analysis and/or sent to another program (such as the WSJT-X suite) for analysis.

Additionally, using the WSPRDaemon software, the multi-frequency capability of KA9Q-Radio can be further-leveraged to produce not only decodes of WSPR and FST4W data, but also make rotating, archival I/Q recordings around the WSPR frequency segments - or any other frequency segments (such as WWV, CHU, Mediumwave or Shortwave broadcast, etc.) that you wish.

Comment:  I have written about the RX-888 in previous blog posts:

• Improving the thermal management of the RX-888 (Mk 2) - link 
• Measuring signal dynamics of the RX-888 (Mk 2) - link
  

Full-Spectrum recording

Yet another capability possible with the RX-888 (Mk2) is the ability to make a "full spectrum" recording - that is, write the full sample rate (typically 64.8 Msps) to a storage device.  The result are files of about 7.7 gigabytes per minute of recording that contain everything that was received by the RX-888, with the same frequency accuracy and precision as the GPS reference used to clock the sample rate of the '888.  

What this means is that there is the potential that these recordings can be analyzed later to further divine aspects of the propagation changes that occurred during, before and after the eclipse - especially by observing signals or aspects of the RF environment itself that one may not have initially thought to consider:  This also can allow the monitoring of the overall background noise across the HF spectrum to see what changes during the eclipse, potentially filling in details that might have been missed on the narrowband recordings.

Because such a recording contains the recordings of time stations (WWV, WWVH, CHU and even WWVB) it may be possible to divine changes in propagation delay between those transmit sites and the receive sites.  If a similar GPS-based signal is injected locally, this, too, can form another data point - not only for the purposes of comparison of off-air signals, but also to help synchronize and validate the recording itself.

By observing such a local signal it would be possible to time the recording to within a few 10s of nanoseconds of GPS time - and it would also be practical to determine if the recording itself was "damaged" in some way (e.g. missed samples from the receiver):  Even if a recording is "flawed" in some way, knowing the precise location an duration of the missing data allows this to be taken into account and to a large extent, permit the data "around" it to still be useful.

Actually doing it:

Up to this point there has been a lot of "it's possible to" and "we have the capability of" mentioned - but pretty much everything mentioned so far was used during the October, 2023 eclipse.  To a degree, this eclipse is considered to be a rehearsal for the April 2024 event in that we would be using the same techniques - refined, of course, based on our experiences.

While this blog will mostly refer to my efforts (because I was there!) there were a number of similarly-equipped parties out in the fields and at home/fixed stations transmitting and receiving and it is the cumulative effort - and especially the discussions of what worked and what did not - that will be valuable in preparation for the April event.  Not to be overlooked, this also gives us valuable experience with propagation monitoring overall - an ongoing effort using WSPRDaemon - where we have been looking for/using other hardware/software to augment/improve our capabilities.

In Part 2 I'll talk about the receive hardware and techniques in more detail.

Stolen from ka7oei.blogspot.com

[END]

As a sort of follow-up to the previous posting about the RX-888 (Mk2) I decided to make some measurements to help characterize the gain and attenuation settings.

The RX-888 (Mk2) has two mechanisms for adjusting gain and attenuation:

• The PE4312 attenuator.  This is (more or less) right at the HF antenna input and it can be adjusted to provide up to 31.5dB of attenuation in 0.5dB steps.
  

• The AD8370 PGA.  This PGA (Programmable Gain Amplifier) can be adjusted to provide a "gain" from -11dB to about 34dB.

Note:

While this blog posting has specific numbers related to the RX-888 (Mk2), its general principles apply to ALL receivers - particularly those operating as "Direct Sampling" HF receivers.  A few examples of other receivers in this category include the KiwiSDR and Red Pitaya - to name but two.

Other article RX-888 articles:

RX-888 Thermal issues:  I recently posted another article about the RX-888 (Mk2) discussing the thermal properties of its mechanical construction - and ways to improve it to maximize reliability and durability.  You can find that article here:  Improving the thermal management of the RX-888 (Mk2) - link. 

Using an external clock with the RX-888:  The 27 MHz external clock input to the RX-888 is both fragile and fickle.  To learn a bit more about how to reliably clock an RX-888 from an external source, read THIS article.

* * * * *

Taking measurements

To ascertain the signal path properties of an RX-888 (Mk2) I set its sample rate to 64 Msps and using both the "HDSDR" and "SDR Radio" programs (under Windows - because it was convenient) and a a known-accurate signal generator (Schlumberger Si4031) I made measurements at 17 MHz which follow:

Figure 1:  Measured performance of an RX-888 Mk2.  Gain mode is "high" with 0dB attenuation selected.

For convenience, the noise floor is shown both in "dBm/Hz" and in dBm in a 500 Hz bandwidth - which matches the scaling used in the chart below.  As the programs that I used have no direct indication of A/D converter clipping, I determined the "apparent" clipping level by noting the amplitude at which one additional dB of input power caused the sudden appearance of spurious signals.  Spot-checking indicated that the measured values at 17 and 30 MHz were within 1 dB of each other on the unit being tested.

Determining the right amount of "gain"

It should be stated at the outset that most of the available range of gain and attenuation provided by the RX-888's PE4312 step attenuator and AD8370 variable gain amplifier are completely useless to us.  To illustrate this point, let's consider a few examples.

Consider the chart below:

Figure 2:  ITU chart showing various noise environments versus frequency.

This chart - from the ITU - shows predicted noise floor levels - in a 500 Hz bandwidth - that may be expected at different frequencies in different locations.  Anecdotally, it is likely that in these days of proliferating switch-mode power supplies that we really need another line drawn above the top "Residential" curve, but let's be a bit optimistic and presume that it still holds true these days.

Let us consider the first entry in Figure 1 showing the gain setting of 0dB.  If we look at the "Residential" chart, above, we see that the curve at 30 MHz indicates a value very close to the -113dBm value in the "dBm in 500 Hz" column.  This tells us several things:

• Marginal sensitivity.  Because the noise floor of the RX-888 (Mk2) and that of our hypothetical RF environment are very close to each other, we may not be able to "hear" our noise floor at 30 MHz (e.g. the 10 meter amateur band).  One would need to do an "antenna versus no antenna" check of the S-meter/receiver to determine if the former causes an increase in signal level:  If not, additional gain may be needed to be able to hear signals that are at the noise floor.
  

• More gain may not help.  If we do perform the "antenna versus no antenna" test and see that with the antenna connected we get, say, an extra S-unit (6dB) of noise, we can conclude that under those conditions that more gain will not help in absolute system sensitivity.
  

Thinking about the above two statements a bit more, we can infer several important points about operating this or any receiver in a given receive environment:

• If we can already "hear" the noise floor, more gain won't help.  In this situation, adding more gain would be akin to listening to a weak and noisy signal and expecting that increasing the volume would cause the signal to get louder - but not the noise.  
  

• More gain than necessary will reduce the ability of the receiver to handle strong signals.  The HF environment is prone to wild fluctuations and signals can go between well below the local noise floor and very strong, so having any more gain that you need to hear your local noise floor is simply wasteful of the receiver's signal handling capability.  This fact is arguably more important with wide-band, direct-sampling receivers where the entire HF spectrum impinges on the analog-to-digital converter rather than a narrow section of a specific amateur band as is the case in "conventional" analog receivers.

Let us now consider what might happen if we were to place the same receiver in an ideal, quiet location - in this case, let's look at the "quiet rural" (bottom line) on the chart in Figure 2.

Again looking at the value at 30 MHz, we see that our line is now at about -133dBm (in 500 Hz) - but if we have our RX-888 gain set at 0 dB, we are now ((-133) - (-113) = ) 20 dB below the noise floor.  What this means is that a weak signal - just at the noise floor - is more than 3 S-units below the receiver sensitivity.  This also means that a receiver that may have been considered to be "Okay" in a noisy, urban environment will be quite "deaf" if it is relocated to a quiet one.

In this case we might think that we would simply increase our gain from 0 dB to +33dB - but you'll notice that even at that setting, the sensitivity will be only -131dBm in 500 Hz - still a few dB short of being able to hear the noise in our "antenna versus no antenna" test.

Too much gain is worse than too little!

At this point I refer to the far-right column in Figure 1 that shows the clipping level:  With a gain setting of +33dBm, we see that the RX-888 (Mk2) will overload at a signal level of around -31dBm - which translates to a  signal with a strength a bit higher than "S9 + 40dB".  While this sound like a strong signal, remember that this signal level is the cumulative TOTAL of ALL signals that enter the antenna port.  Thinking of it another way, this is the same as ten "S9+30dB" signals or one hundred "S9+20dB" signals - and when the bands are "open," there will be many times when this "-31dBm" signal level is exceeded from strong shortwave broadcast signals and lightning static.

In the case of too-little gain, only the weakest signals, below the receiver's noise floor will be affected - but if the A/D converter in the receiver is overloaded, ALL signals - weak or strong - are potentially disrupted as the converter no longer provides a faithful representation of the applied signal.  When the overload source is one or more strong transmissions, a melange of all signals present is smeared throughout the receive spectrum consisting of many mixing products, but if the overload is a static crash, the entire receive spectrum can be blanked out in a burst of noise - even at frequencies well removed from the original source of static.

Most of the adjustment range is useless!

Looking carefully at Figure 1 at the "noise floor" columns, you may notice something else:  Going from a gain of 0 dB to 10 dB, the noise floor "improves" (is lower) by about the same amount - but if you go from 25 dB gain to 33 dB gain we see that our noise floor improves by only 1 dB - but our overload threshold changes by the same eight dB as our gain increase.

What we can determine from this is that for practical purposes, any gain setting above 20 dB will result in a very little receiver sensitivity improvement while causing a dramatic reducing in the ability of the receiver to handle strong signals.

Based on our earlier analysis in a noise "Urban" environment, we can also determine that a gain setting lower than 0 dB will also make our receiver too-insensitive to hear the weakest signals:  The gain setting of -25dB shown in Figure 1 with a receive noise floor of -79dBm (500 Hz) - which is about S8 - is an extreme example of this.

Up to this point we have not paid any attention to the PE4312 attenuator as all measurements were taken with this set to minimum.  The reason for this is quite simple:  The noise figure (which translates to the absolute sensitivity of a receiver system) is determined by the noise generation of all of the components.  As reason dictates, if you have some gain in the signal path, the noise contribution of the devices after the gain have lesser effects - but any loss or noise contribution prior to the gain will directly increase the noise figure.

Note:

For examples of typical HF noise figure values, see the following articles:

• Measurements on a Multiband R2P Pro Low-Noise Amplifier System - link to the Web Archive.  Refer to page 9 section 7.
• Antenna and Receiver Noise Figure - link.
  

Based on the articles referenced above, having a receiver system with a noise figure of around 15dB is the maximum that will likely permit reception at the noise floor of a quiet 10 meter location.  If you aren't familiar with the effects of noise figure - and loss - in a receive signal path, it's worth playing with a tool like the Pasternack Enterprises Cascaded Noise Figure Calculator (link) to get a "feel" of the effects.

I do not have the ability to measure the precise noise figure of the RX-888 (Mk2) - and if I did do so, I would have to make such a measurement using the same variety of configurations depicted in Figure 1 - but we can know some parameters about the worst-case:

• Bias-Tee:  Estimated insertion loss of 1dB
  
• PE4312:  Insertion loss of 1.5dB at minimum attenuation
• RF Switch (HF/VHF):  1dB loss
  
• 50-200 Ohm transformer:  1dB loss
  
• AD8370 Noise figure:  8dB (at gain of 20dB)

The above sets the minimum HF floor noise figure of the RX-888 (Mk2) at about 12.5dB with an AD8370 gain setting of 20dB - but this does not include the noise figure of the A/D converter itself - which would be difficult to measure using conventional means.

On important aspect about system noise figure is that once you have loss in a system, you cannot recover sensitivity - no matter how much gain or how quiet your amplifier may be!  For example, if you have a "perfect" 20 dB gain amplifier with zero noise, if you place a 10 dB attenuator in front of it, you have just turned it into an amplifier with 10 dB noise figure with 10dB gain and there is nothing that can be done to improve it - other than get rid of the loss in front of the amplifier.

Similarly, if we take the same "perfect" amplifier - with 20dB of gain - and then cascade it with a receiver with a 20dB noise figure, the calculator linked above tells us that we now have a system noise figure of 3 dB since even with 20dB preceeding it, our receiver still contributes noise!

If we presume that the LTC2208 A/D converter in the RX-888 has a noise figure of 40dB and no gain (a "ballpark" value assuming an LSB of 10 microvolts - a value that probably doesn't reflect reality) our receive system will therefore have a noise figure of about 22dB.

What this means is that in most of the ways that matter, the PE4312 attenuator is not really very useful when the RX-888 (Mk2) is being used for reception of signal across the HF spectrum, in a relatively quiet location on an antenna system with no additional gain.

Where is the attenuator useful?

From the above, you might be asking under what conditions would the built-in PE4312 attenuator actually be useful?  There are two instances where this may be the case - and this would be applied ONLY if you have been unable to resolve overload situations by setting the gain of the AD8370 lower.

• In a receive signal path with a LOT of amplification.  If your receive signal path has - say - 30dB of amplification (and if it does, you might ask yourself "why?") a moderate amount of attenuation might be helpful.
  

• In a situation where there are some extremely strong signals present.  If you are near a shortwave or mediumwave (AM broadcast) transmitter that induces extremely strong signals in the receiver that cause intractable overload, the temporary use of attenuation may prevent the receiver from becoming overloaded to the point of being useless - but such attenuation will likely cause the complete loss of weaker signals.  In such a situation, the use of directional antennas and/or frequency-specific filtering should be strongly considered!
  

Improving sensitivity

Returning to an earlier example - our "Quiet Rural" receive site - we observed that even with the gain setting of the RX-888 (Mk2) at maximum, we would still not be able to hear our local noise floor at 30 MHz - so what can be done about this?

Let us build on what we have already determined:

• While sensitivities is slightly improved with higher gain values, setting the gain above 20dB offers little benefit while increasing the likelihood of overload.
  

• In a "Quiet Rural" situation, our 30 MHz noise floor is about -133dBm (500 Hz BW) which means that our receiver needs to attain a lower noise floor than this:  Let's presume that -136dBm (a value that is likely marginal) is a reasonable compromise.

With a "gain" setting of 20dB we know that our noise floor will be around -128dBm (500 Hz) and we need to improve this by about 8 dB.  For straw-man purposes, let's presume that the RX-888 (Mk2) at a gain setting of 20dB has a noise figure of 25dB, so let's see what it takes for an amplifier that precedes the RX-888 (Mk2) to lower than to 17dB or so using the Pasternak calculator above:

• 10dB LNA with 7 dB noise figure:  This would result in a system noise figure of about 16 dB - which should do the trick.

Again, the above presumes that there is NO  loss (cable, splitters, filtering) preceding the preamplifier.  Again, the presumed noise figure of 25dB for the RX-888 (Mk2) at a gain setting of 20 is a bit of a "SWAG"  - but it illustrates the issue.

Adding a low-noise external amplifier also has another side-effect:  By itself, with a gain setting of +33, the RX-888 (Mk2)'s overload point is -31dBm, but if we reduce the gain of the RX-888 to 20dB the overload drops to -18dBm - but adding the external 10dB gain amplifier will effectively reduce the overload to -28dBm, but this is still 5 dB better than if we had turned the RX-888's gain all of the way up!

Taking this a bit further, let's presume that we use, instead, an amplifier with 3dB noise figure and 8 dB gain:  Our system noise figure is now about 17dB, but our overload point is now -26dBm - even better!

The RX-888 is connected to a (noisy) computer!

Adding appropriate amounts of external gain has an additional effect:  The RX-888 (and all other SDRs) are computer/network connected devices with the potential of ingress of stray signals from connected devices (computers, network switches, power supplies, etc.).  The use of external amplifiers can help override (and submerge) such signals and if proper care is taken to choose the amount of gain of the external amplification and properly choose gain/attenuation settings within the receiver, superior performance in terms of sensitivity and signal-handling capability can be the result.

Additional filtering

Only mentioned in passing, running a wideband, direct-sampling receiver of ANY type (be it RX-888, KiwiSDR, Red Pitaya, etc.) connected to an antenna is asking a lot of even 16 bits of conversion!  If you happen to be in a rather noisy, urban location, the situation is a bit better in the sense that you can reduce receiver gain and still hear "everything there is to hear" - but if you have a very quiet location that requires extra gain, the same, strong signals that you were hearing in the noisy environment are just as strong in the quiet environment.

Here are a few suggestions for maximizing performance under the widest variety of situations:

• Add filtering for ranges that you do not plan to cover.  In most cases, AM band (mediumwave) coverage is not needed and may be filtered out.  Similarly, it is prudent to remove signals above that in which you are interested.  For the RX-888 (Mk2), if you run its sampling rate at just 65 MHz or so, you should install a 30 MHz low-pass filter to keep VHF and FM broadcast signals out.
  

• Add "window" filtering for bands of interest.  If you are interested only in amateur radio bands, there are a lot of very strong signals outside the bands of interest that will contribute to overload of the A/D converter.  It is possible to construct a set of filters that will pass only the bands of interest - but this does not (yet?) seem to be a commercial product.  (Such a product may be available in the near future - keep a lookout here for updates.)
  

Figure 1: The RX888 showing the "top" and RF connectors.  While the heat sinks attached to the sides are visible, the large one on the "bottom" plate are not. Click on the image for a larger version.

Note:

If you are considering buying an RX-888 - or have already bought one, be sure to read the following BEFORE you power it up!

* * *

The RX-888 Mk2 SDR is a USB3-based software-defined receiver that, unlike many others, is JUST and analog-to-digital converter (with a bit a low-pass filtering and adjustable attenuation and amplification) coupled to a USB 3 PHY chip.  With a programmable sample rate and a 65-ish MHz low-pass filter, it is capable of simultaneously inhaling the entire spectrum from a few 10s of kHz to about 60 MHz when run at 130 Msps - a rate which pretty much "maxes out" the USB 3 interface.

(Note:  There is also a frequency converter on board which will take up to a 10 MHz swath of spectrum between about 30 and 1800 MHz and shift it to a lower frequency within range of the A/D converter - but that's not part of this discussions.)

The purpose of this post is to discuss the thermal management of the RX-888 Mk2 which, in two words, can be described as  "marginal" and "inconsistent".

Other RX-888 articles:

After posting this entry I produced another article about understanding the gain and properties of the HF signal path on the RX-888 (Mk2) - including information that can also be applied to other direct-sampling "all band HF" Software Defined Radios like the KiwiSDR, Red Pitaya and others.  You may read that article here:  Measuring signal dynamics of the RX-888 (Mk2) - Link.

Using an external clock with the RX-888:  The 27 MHz external clock input to the RX-888 is both fragile and fickle.  To learn a bit more about how to reliably clock an RX-888 from an external source, read THIS article.

Please note:

Despite the impression that the reader might get about the RX-888 (Mk2)'s thermal design and potential reliability, I would still consider it to be an excellent device at a good price - warts and all.

Its performance is quite good and especially since it lacks the FPGA that many other direct-sampling SDRs use, it is quite "future proof" in the sense that support of this receiver - and others like it that will no doubt appear soon - will be based on code running on the host computer (typically a PC or SBC) rather than on an FPGA contained within that requires specialized tools and knowledge for development and is limited by its own capacity.

If you think that an FPGA is needed, consider this:  For a few "virtual" receivers using "conventional" DSP techniques (e.g. HDSDR, SDR-Radio, etc.) a moderate Intel i7 is sufficient:  If using an optimized signal processing program like ka9q-radio along with a modest Intel i5, hundreds of virtual receivers covering the entire HF spectrum can be managed - but these are topics for another discussion.

In other words:  If you need a fairly simple, modestly-priced device to receive multiple RF channels it is well worth getting an RX-888 (Mk2) and performing some simple modification to it to improve its durability.  We can hope that future versions of this - and similar devices - will take these observations into account and produce even better hardware.

What's the problem?

There are scattered anecdotal reports of RX-888 (both the original and Mk2) simply "dying" after some period of time.  For most of these reports there are few details other than comments to this effect in various forums (e.g. little detailed analysis) but this was apparently enough of a problem with the original version of the RX-888 that with the Mk2, "improved" thermal management is one of the new features noted by its sellers.  (I do not have an original RX-888, but I would expect that the same general techniques could be applied to it as well.)

In short, here are a few comments regarding the thermal management of the RX-888 Mk2:

• DO NOT run it outside its case.  There is a compressible thermal pad that goes between the exposed metal pad below the A/D converter that is intended to transfer heat to the case and without this in place the A/D converter and surrounding components can exceed 100C at moderate ambient temperatures.  If you plan to shuck the case, you should be aware of this and make appropriate arrangements to draw away heat via the same method. 

Figure 2: Showing the paper double-sided "sticky tape" used to mount the heat sinks.  Despite improper materials, these work "less badly" than expected, but it's best to re-attach them properly. Click on the image for a larger version.

• The heat sinks are held on by double-sided tape.  The heat sink on the A/D converter appears to be some sort of thermal table like that seen on Raspberry Pi heat sink kits, but  those on the exterior of the case (one on each side, another the top) are held on with standard, paper-based double-sided tape:  People have reported these falling off with handling.  Additionally, because both the case and heat sinks are extruded their surfaces are not flat and all of the RX-888 (Mk2) units that I had a gap between the heat sink and the case through which a sheet of paper can be slid meaning that the heat sinks should be flattened a bit and/or attached using a material that will work as a thermally-conductive void filler.
  

• The thermal pad may not be adequate.  Unless the small-ish thermal pad is placed precisely in its correct location, it will not be effective in its thermal transfer.  Additionally, these pads require a bit of compression between the board and the heat sink to be effective and it seems that the spacing between the board and the case is somewhat "loose" in the slot into which the PCB slides and that thermal contact may be inconsistent - more on this shortly.
  

• Other components get very hot.  Next to the A/D converter are the 3.3 and 1.8 volt linear regulators which run very hot.  While this may be OK, they are next to (what appear to be) electrolytic capacitors which - if run very warm - can have rather short lifetimes.  While it is unknown if this is the case here, many regulators will become unstable (oscillate) if their associated capacitors degrade with lower capacitance and/or increased ESR (Equivalent Series Resistance) and if oscillation occurs due to capacitor degradation, this is likely to make the device unusable until the components are replaced.
  

Figure 3: The top of the RX888 board.  The ADC's heat sink was removed for the photo, but glued in place later to improve its thermal transfer. Click on the image for a larger version.

• The FX3 USB interface chip can get very warm.  This chip is right next to the A/D converter.  There are anecdotal reports (again, nothing confirmed) that this particular chip can suffer reliability problems when running near its maximum rated temperature:  Whether this is due to a failure of silicon or (more likely) a mechanical failure of a solder connection on its BGA (ball grid array) as a result for thermal cycling remains to be seen, but either one could explain one of the RX-888's reported failure modes of no longer appearing to be the expected type of USB device, making the unit non-functional even though it seems to enumerate - albeit improperly.

Several different people have made spot measurements of the temperatures within an RX-888 and come up with different results, further indicating inconsistency in the efficacy of the passive cooling and showing the inherent difficulty in making such measurements - but here are a few comments that are likely relevant:

• Unless you need coverage >30 MHz, do NOT run a sample rate higher than 65-70 Msps.  As with most devices, more current (and higher heat dissipation) will occur at a higher sample rate so keeping it well below its maximum (around 130 Msps) will reduce heating and potentially improve the lifetime.  

If you do run at a sample rate 64-70 Msps, it is recommended that a 30 MHz low-pass filter be installed as this will prevent aliasing due to this lower rate and the fact that the RX-888 (Mk2) has only a 60 MHz low-pass filter internally.

• At normal "room" temperatures (68F/20C) the thermal properties of the RX-888 Mk2 are likely "Okay" if run at just 65-70 Msps - but increasingly marginal above this.  On several RX-888s, the temperature of the A/D converter and other components was fairly high, but not alarmingly so, although this seemed to vary among samples (e.g. some seemed worse than others.)  Since thermal resistance can be characterized by a temperature rise, it makes sense that as the ambient temperature increases, so will the components by the same amount meaning that if the unit is in a hot location - or placed such that it will become warm (convective air movement across the heat sinks is restrictive or in/near the hot air flow of other equipment) then thermal stresses of the components also increase.
  

Again, the reader should be cautioned that the reported inconsistency between units (e.g. the efficacy of the thermal pad) may mean that the above advice may not apply to all units as some may have, say, a misplaced thermal pad or extra "slop" in the spacing between the board and the case which reduces the compression of the pad causing extra thermal resistance.

"Board slop"doesn't help: 

Figure 4: Measuring the "board slop" in the mounting rails.  As noted in the text, the board's looseness was nearly 1 mm - the far extent of which exceeding the 5mm thickness of the pad. Click on the image for a larger version.

On this latter point (e.g. "slop" in the board position) with the covers removed I measured a variance of 0.170-0.205" (4.32-5.207mm) from the board to the case due to looseness in the board fitting in the rail on one of my RX-888.  Of the three units that I have to measure, this was the worst - but not by much as the the photo (figure 4) from another unit shows.

Considering that the thermal pad is nominally 5.0mm thick, this means that the board MAY not be effectively conducting heat to the case if the gap is closer to 5.2mm.  Also considering the fact that the thermal pad will work better when it is compressed it would be a very good idea - if possible - to reduce this gap - more on this later.

I also observed that with the USB end plate fitted, it happened to push the board "down" (e.g. reduced the gap between the board and the case) by about 0.02" (0.5mm) and since this is the end of the board closest to the A/D converter chip, it likely reduces the gap by about 0.015" (0.38mm) owing to geometry (e.g. the fact that the A/D converter is located away from the edge.)  If desired, this fact could be exploited by adding a shim to the top of the USB connector and filing the bottom a bit to allow the end plate to push "down" on the board a bit, better-compressing the thermal pad and potentially reducing its thermal resistance. 

Figure 5: The screwdriver tip points to where the end plate is pushing down on the connector and board to reduce board-to-case distance to better-compress the pad. Click on the image for a larger version.

On the opposite end of the board, the RF connectors fit rather loosely in their mounting holes meaning that one could, in theory, move the connectors to the "bottom" of their holes and tighten the nuts on the SMA connectors.  This would not be advisable without adding a washer of appropriate thickness between the plate and the SMA connector as the connectors themselves are not right at the edge of the circuit board and firmly tightening the nuts would likely bend/break them loose.

Before getting out the file, however, I suggest considering the methods/modifications mentioned below to improve the thermal performance of the RX-888 (Mk2) in several other ways.

Ways to improve the thermal performance:

There are two ways to improve the thermal performance and reduce the temperature of the onboard components.

Add another heat sink and a fan

A "brute force" approach to this would be to move more air through and around the unit. using a small fan.  If you do this I would recommend two minor modifications:

• Glue the heat sink to the A/D converter.  As noted earlier, the heat sink the A/D converter is held on by tape, but I would recommend that this be removed from the heat sink and the chip itself (using a bit of paint thinner or alcohol to remove residue) and it be reattached using thermally conductive epoxy rather than conventional "clear" epoxy.  This epoxy is readily available at the usual places (Amazon, etc.) but it should be noted that the gray (not clear!) "JB Weld" epoxy (available at auto-parts and "big box" stores) also has reasonable thermal conductivity and works quite well in this application.   Do NOT use an adhesive like "super glue" as it is not void-filling by its nature and it is unlikely to endure the heat.
  

• Add a heat sink to the FX3 chip.  This chip - next to the A/D converter - should also be cooled and a small heat sink - such as that which comes with a Raspberry Pi heat sink kit - may be attached.  Again, I would recommend thermally-conductive epoxy rather than supplied double-sided sticky tape.

As for the fan mounting, several people have simply removed both side plates and fabricated the attachment for a small fan (say, 20x20mm to 30x30mm) on the side with the USB connector to blow air through the case on both sides of the board.  Others have temporarily removed the board from the case and put holes in "top" of the case (on the side with the labels) into which a fan is mounted.

Either of these will be quite effective - but since these are not passive cooling, the failure of a fan could result in excess heat if other methods are not also employed.

Improve passive cooling by using a much larger thermal pad

This is likely the favored approach as it does not depend on a fan which will have a defined useful lifetime, and the failure of which could result in immediate overheating in certain circumstances.  There are two parts to this approach:

Replace the thermal pad. 

At reasonable ambient temperatures I believe that the area of the external heat sinks on the RX-888 are of adequate size, provided that they are open for air flow and not placed in the heat exhaust of equipment and properly attached to the case - more on that shortly.

As noted, the thermal pad is seemingly marginal and it is only as large enough to draw heat away from the area immediately proximate to the A/D converter - an issue that may be exacerbated by the inconsistent board-to-case spacing mentioned above.  Improper placement of this pad will prevent it from conducting heat from the A/D converter - the major heat producer - to the case - and subsequent heating of adjacent components.

Figure 6: A piece of 45mm x 65mm thermal pad on the bottom of the board.  This piece is large enough to cover all heat- generating components. Click on the image for a larger version.

It is also likely that the thermal pad material supplied with the unit is of lower thermal conductivity than other materials that are available (to save cost!) so the use of better thermal material and a larger pad will draw more heat away from all of the heat-producing components on the board and conduct it to the heat sink.

A suitable pad material is the Laird A15340-01 which may be found at Digi-Key (link here ).  This material has roughly half  the thermal resistance (e.g. better thermal conductivity) of other common pad materials and it is suitably "squishy" in that it will form around components and help fill small voids as it does so.

Unfortunately, this material is somewhat expensive in that it's available only as a rather large piece - about $32 (at the time of posting - not including shipping) for one that is 22.8x22.8cm square - but this will modify several RX-888s - but even at the price of $32, it's still a reasonable price to pay for improved reliability of a $150-$200 device!  If you do this, it's recommended that you work with other RX-888 owners to split the cost of the pad - but be sure to keep the pad - or any pieces that you cut from it - in a zip-bag or clean plastic cling film to prevent its surface from being contaminated with dirt and dust.  If you post/mail this pad material to someone else, be sure to protect it between two pieces of cardboard to prevent it from being mangled.

Note:  Others have obtained 5mm thick thermal pad material from other sources (e.g. Amazon) and while it likely does not have as low thermal resistance as the Laird product mentioned, reports indicate that it works adequately - most likely a result of the larger size of this pad compared to the original, drawing heat away from the entire bottom surface of the board.

Figure 7: The new pad, installed, as viewed from the end with the USB connector, near the ADC and FX3 USB interface chip. Click on the image for a larger version.

A rectangular piece of thermal pad 45mm x 65mm will cover the bottom of the board where there are heat-generating components and ensure superior heat transfer to the case.  Since this material is a bit "sticky", it may be a bit difficult to get it installed as it will be resistant to sliding, but a very light coating of white heat-sink grease on the side of the pad facing the heat sink material will provide sufficient lubrication to allow it to slide as the board is inserted along its mounting rails.

Comment:  This process is fairly messy, so if you plan to add a connector for an external clock input, I would suggest that you do so at the time that you install the new pad as you will probably not to repeat the process unnecessarily.

Remount the heat sinks.

As noted earlier, the four heat sinks (two on the "bottom" side opposite the label and one on each side) are held on by double-sided paper tape.  It is recommended that these be removed - along with any tape residue (best done with paint thinner and/or alcohol) - and be reattached with thermal epoxy.

Figure 8: An RX888 (Mk2) in the process of gluing on the side heat sinks, using a vise for clamping.  Alternatively, weight may be placed on the heat sink(s) while the epoxy cures to compress it and squeeze out excess - but note that until it cures that the heat sinks may slide slowly out of position if one isn't careful. Click on the image for a larger version.

As noted previously, the heat sinks do not fit flat with each other so  it would be a good idea to assure that the surfaces are reasonably to maximize thermal conductivity by drawing the case and the mating surfaces of the heat sinks across 800-grid sandpaper (using a flat piece of metal or glass as a substrate) - taking care to prevent metal particles from getting onto the board or inside the case:  It would be best to remove the board and do this prior to the installation of the new thermal pad and wash any such particles from the case before reassembly.

Once the mating surfaces have been flattened and cleaned, using thermal epoxy (or the gray "JB-Weld") reattach the heat sinks one-at-a-time - preferably by compressing them in a vice or with a clamp to squeeze out as much adhesive as possible.

It's worth noting that even if you don't go through the trouble of flattening the heat sink and the surface of the case, the use of a void-filling adhesive will certainly offer far more efficient thermal transfer than  the original double-sided paper sticky tape along with it s rather large air gap between the two surfaces.

Out of curiosity I measured the difference in temperature between the heat sinks stuck on with double-sided tape and the exposed portion of the case right next to the heat sink and it was found to be about 3-5F (1.7-2.8C) - surprisingly good, actually.

Before and after thermal measurements

Figure 9: Two RX888 Mk2's with reattached heat sinks, ready for a  bit of clean-up and final assembly. Click on the image for a larger version.

Using a thermal infrared camera and verifying with a thermocouple, temperature measurements were made of various components with an RX-888 operating at 130 Msps at an ambient temperature of 74F (23C) after 10 minutes of operation.  The readings were as follows:

With the original thermal pad, end plates removed - heat sink cooling by convection only:

ADC:  175F (79C)

FX3 (USB interface): 155F (68C)

Capacitor near 3.3 volt regulator:  145F (63C)

3.3V Regulator:  170F (77C)

1.8V Regulator:  178F (81C)

 

With Laird 45mm X 65mm pad - heat sink cooling by convection only:

ADC: 145F (63C)

FX3: 130F (54C)

Capacitor near 3.3 volt regulator:  125F (52C)

3.3V Regulator:  145F (63C)

1.8V Regulator:  150F (66C)

Note:  There is another capacitor near the 1.8 volt regulator, but it is temperature cannot be readily measured while the board was installed in the case, but other measurements made outside the case indicates that its temperature was at least as high as that of the capacitor near the 3.3 volt regulator.

Results and comments:

The replacement of the original thermal pad with one that is 45mm X 65mm in size to cover the bottom of the board where there are active components has resulted in a very significant heat reduction:  As with all electronics, reducing the temperature of the components will increase the operational lifetime.

Considering that one can use - as a guideline - the temperature rise above ambient, we can make some estimations as to what will happen if the modified RX-888 (Mk2) is operated at a higher temperature.  

For example, if we consider 212F (100C) to be the maximum allowed case temperature of any of the components, we can see that with the original thermal pad, this limit would occur with the ADC converter at an ambient temperature of around 111F (44C) - a temperature that one could reasonably expect during the summer in a room without air conditioning.  In contrast, with the larger pad the ADC's temperature would likely be closer to 185F (85) in the same environment.

With a small amount of air moving across the heat sinks, their temperature rise would also be lower, further reducing internal temperature - and even though it isn't strictly necessary, it wouldn't hurt to use a small fan - even on a modified RX-888 (Mk2) to cool it even more, and feel confident that it will still survive should that fan fail.

Finally, I would again remind the reader that I consider the RX-888 (Mk2) to be an excellent-performing and extraordinarily flexible device and well worth extra trouble to make it better!

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This page stolen from ka7oei.blogspot.com

 

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