As noted in a previous entry of this blog where I discussed the "Raspberry Kiwi" SDR - a (near) clone of the KiwiSDR - there is also the "FlyDog" receiver - yet another clone - that has made the rounds.  As with the Raspberry Kiwi, it would seem that the sources of this hardware are starting to dry up, but it's still worth taking a look at it.

I had temporary loan of a FlyDog SDR to do an evaluation, comparing it with the KiwiSDR - and here are results of those tests - and other observations.

Figure 1: The Flydog SDR.  On the left are the two "HF" ports and the port for the GPS antenna.  Note the "bodge" wires going through the shielded area in the upper left. The dark squares in the center and to its right are the A/D converter and the FPGA.  The piece of aluminum attached to the oscillator is visible below the A/D converter. Click on the image for a larger version.

How is this different from the Raspberry Kiwi?

Because of its common lineage, the FlyDog SDR is very similar to the Raspberry Kiwi SDR - including the use of the same Linear Technologies 16 bit A/D converter - and unlike the Raspberry SDR that I reviewed before, it seems to report a serial number, albeit in a far different range (in the 8000s) than the "real" KiwiSDRs which seem to be numbered, perhaps, into the 4000s.

The most obvious difference between the FlyDog and the original KiwiSDR (and the Raspberry Kiwi) is the addition of of a second HF port - which means that there is one for "up to 30 MHz" and another that is used for "up to 50 MHz" - and therein lies a serious problem, discussed below.

Interestingly, the FlyDog SDR has some "bodge" wires connecting the EEPROM's leads to the bus - and, unfortunately, these wires, connected to the digital bus, appear to run right through the HF input section, under the shield!  Interestingly, these wires might escape initial notice because they were handily covered with "inspection" stickers. (Yes, there were two stickers covering each other - which was suspicious in its own right!)  To be fair, there's no obvious digital "noise" as a result of the unfortunate routing of these bodge wires.

Why does it exist?

One would be within reason to ask why the FlyDog exists in the first place - but this isn't quite clear.  I'm guessing that part of this was the challenge/desire to offer a device for a the more common, less-expensive and arguably more capable Raspberry Pi (particularly the Pi 4) - but this is only a guess.

Another reason would have been to improve the performance of the receiver over the KiwiSDR by using a 16 bit A/D converter - running at a higher sampling rate - to both improve dynamic range and frequency coverage - this, offering usable performance up through the 6 meter amateur band.  

Unfortunately, the Flydog does neither of these very well - the dynamic range problem being the same as the Raspberry Kiwi in the linked article compounded by the amplitude response variances, choice of amplifier device and frequency stability issues discussed later on.

Observations:

Getting immediately to one of the aspects of this receiver, I'll discuss the two HF ports. Their basic nature can be stated in two words:  Badly implemented.

When I first saw the FlyDog online with its two HF ports, I wondered "I wonder how they selected between the two ports - with a small relay, PIN diodes, or some sort of analog MUX switch, via hardware?" - but the answer is neither:  The two ports are simply "banged" together at a common point.

When I heard this, I was surprised - not because of its simplicity, but because it's such a terrible idea.  

As a few moments with a circuit simulator would show you, simply paralleling two L/C networks that cover overlapping frequency ranges does not result in a combined network sharing the features/properties of the two, but a terrible, interacting mess with wildly varying impedances and the potential for huge variations of insertion loss.

The result of this is that the 30 MHz input is, for all practical purposes, unusable, and its existence seriously compromises the performance of the other (0-50 MHz) port.  Additionally, if one checks the band-pass response of the receiver using a calibrated signal generator against the S-meter reading, you will soon realize that the resulting frequency response across the HF spectrum is anything but flat.

For example, one will see a "dip" in response (e.g. excess loss) around 10 MHz on the order of 20 dB if you put a signal into the 50 MHz port, effectively making it (more or less) unusable for the 30 meter amateur band and the 31 meter shortwave broadcast band.  Again, there is nothing specifically wrong with the low-pass filter networks themselves - just the way that they were implemented:  You can have only one such network connected to the receiver's preamplifier input at a time without some serious interaction!

Work-around:

Having established that, out-of-the-box, that the FlyDog has some serious issues when used as intended on HF, one might be wondering what can be done about it - and there are two things that may be done immediately:

• Do microsurgery and disconnect one of the HF input ports.  If you have the skills to do so, the shield over the HF filter may be unsoldered/removed and the circuit reverse-engineered enough to determine which component(s) belong to the 30 MHz and 50 MHz signal paths - and then remove those component(s).  If you wish to retain 6 meter capability, disconnect the 30 MHz port.  Clearly, this isn't for everyone!
  

• Terminate the unused port.  A less-effective - but likely workable alternative - would be to attach a 50 ohm load to the unused port.  On-bench testing indicated that this seemed to work best when the 50 MHz port was used for signal input and the 30 MHz port was connected to a 50 ohm load:  The frequency of the most offensive "null" at about 10 MHz shifted down by a bit more than 1 MHz into the 9 MHz range and reduced in depth, allowing still-usable response (down by only a few dB) at 10 MHz, and generally flattening response across the HF spectrum:  Still not perfect, but likely to be adequate for most users.  (In testing, the 30 MHz port was also shorted, but with poorer results than when terminated.) 

In almost every case, the performance (e.g. sensitivity) was better on the 50 MHz port than the 30 MHz port, so I'm at a loss to find a "use case" where its use might be better - except for a situation where its lower performance was outweighed by its reduced FM broadcast band rejection.

This issue - which is shared with the RaspberryKiwi SDR - is that the low-pass filter (on the 50 MHz port) is insufficient to prevent the incursion of aliases of even moderately strong FM broadcast signals which appear across the HF spectrum as broad (hundreds of kHz wide) swaths of noise with a hint of distorted speech or music.  This is easily solved with an FM broadcast band filter (NooElec and RTL-SDR blog sell suitable devices) - and it is likely to be a necessity.

Other differences:

• Lower gain on the FlyDog SDR:  Another difference between the FlyDog and KiwiSDR is the RF preamplifier.  On the KiwiSDR and Raspberry Kiwi, a 20 dB gain amplifier (the LTC6401-20) is used, but a 14 dB gain amplifier (LTC6400-14) is used instead - a gain reduction of about 6 dB, or one S-unit - and the effects of this are evident in the performance as described below.  Was this intentional, a mistake, or was it because the 14 dB version was cheaper/more available?
  

Clock (in)stability:

The Flydog SDR uses a 125 MHz oscillator to clock the receiver (A/D converter) - but there is a problem reported by some users:  It's a terrible oscillator - and it's bad enough that it is UNSUITABLE for almost any digital modes - particularly WSPR, FT-8, and FT-4 - to name but a few unless the unit is in still air and in an enclosure that is very temperature-stable.

Figure 2: Stability of the "stock" oscillator in the Flydog at 125 MHz in "still" air, on the workbench.  The amount of drift - which is proportional to the receive frequency - makes it marginally usable for digital modes and is too fast/extreme to be GPS-corrected. Click on the image for a larger version.

Figure 2, above, is an audio plot from a receiver (a Yaesu FT-817) loosely coupled and tuned to the 125 MHz oscillator on the Flydog's receive board:  Due to the loose coupling (electrical and acoustic), other signals/noises are present in the plot that are not actually from the Flydog.  The horizontal scale near the top has 10 Hz minor divisions and the red has marks along the left side of the waterfall represent 10 seconds.

From this plot we can see over the course of about half a minute the Flydog's main receiver clock moved well over 50 Hz, representing 5 Hz at 12.5 MHz or 1 Hz at 2.5 MHz.  With this type of instability, it is probably unusable for WSPR on any band above 160 meters much of the time - and it is likely only marginally usable on that band as WSPR can tolerate only a slight amount of drift, and that's only if its change occurs in about the same time frame as the 2 minute WSPR cycle.  The drift depicted above would cause a change of 1 Hz or more on bands 20 meters and above within the period of just a few WSPR - or FT8 - symbols, rendering it uncopiable.

"The Flydog has GPS frequency correction - won't this work?"

Unfortunately not - this drift is way too fast for that to possibly work as the GPS frequency correction works over periods of seconds. 

What to do?

While replacing the 125 MHz clock oscillator with another device (I would suggest a crystal-based oscillator rather than a MEMs-based unit owing to the former's lower jitter) or apply a stabilized, external source (e.g. a Leo Bodnar GPS-stablized signal source) are the best options, one can do a few things "on the cheap" to tame it down a bit.

While on the workbench, I determined that this instability appeared to be (pretty much) entirely temperature-related, so two strategies could be employed:

• Increase the thermal mass of the oscillator.  With more mass, the frequency drift would be slowed - and if we can slow it down enough, large, fast swings might be damped enough to allow the GPS frequency correction to compensate.  With a slow enough drift, the WSPR or FT-8 decoders may even be able to cope without GPS correction.
  

• Thermally isolate the oscillator.  Because it's soldered to the board, this is slightly difficult so our goal would be to thermally isolate the mass attached to the oscillator.
  

To test this idea I added thermal mass:  I epoxied a small (12x15mm) piece of 1.5mm thick aluminum to the top of the oscillator itself.  The dimensions were chosen to overlap the top of the oscillator while not covering the nearby voltage regulator, FPGA or A/D converter and the thickness happens to be that of a scrap piece of aluminum out of which I cut the piece:  Slightly thicker would be even better - as would it being copper.

The epoxy that I used was "JB Weld" - a metal-filled epoxy with reasonable thermal conductivity, but "normal" clear epoxy would probably have been fine:  Cyanoacrylate ("CA" or "Super" glue) is NOT recommended as it is neither a good void filler or thermal conductor.

Comment:  If one wishes to remove a glued-on piece of metal from the oscillator during experimentation, do not attempt to remove it physically as this would likely tear it from and damaging the circuit board, but slowly heat it with a soldering iron:  The adhesive should give way long before the solder melts.

The "thermal isolation" part was easy:  A small piece of foam was cut to cover the piece of aluminum - taking care to avoid covering either the FPGA or the A/D converter, but because it doesn't produce much heat - and is soldered to the board itself - the piece of foam also covered the voltage regulator.

The result of these two actions may be seen in the plot below:

Figure 3: The stability of the oscillator after the addition of the thermal mass and foam.  Still not great, but more likely to be usable.  (The signal around 680-700 Hz is the one of interest.) Click on the image for a larger version.

Figure 1: The older DL-1000 (top) and the newer AD-1000, both after modification. Click on the image for a larger version.

Comment: 

There is a follow-up of this article where an inexpensive PT2399-based reverb board is analyzed and converted into a delay board suitable for repeater use:   Using an inexpensive PT2399 music reverb/effects board as an audio delay - LINK

A few weeks ago I was helping one of the local ham clubs go through their repeaters, the main goal being to equalize audio levels between the input and output to make them as "transparent" as possible - pretty much a matter of adjusting the gain and deviation appropriately, using test equipment.  Another task was to determine the causes of noises in the audio paths and other anomalies which were apparent to a degree at all of the sites.

All of the repeater sites in question use CAT-1000 repeater controllers equipped with audio delay boards to help suppress the "squelch noise" and to ameliorate the delay resulting from the slow response of a subaudible tone decoder.  Between the sites, I ran across the older DL-1000 and the newer AD-1000 - but all of these boards had "strange" issues.

The DL-1000:

This board uses the MX609 CVSD codec chip which turns audio into a single-bit serial stream at 64 kbps using a 4-bit encoding algorithm, which is then fed into a CY7C187-15 64k x 1 bit RAM, the "old" audio data being read from the RAM and converted back to audio just before the "new" data is written..  To adjust the amount of delay in a binary-weighted fashion, a set of DIP switches are used to select how much of this RAM is used by enabling/disabling the higher-order address bits.

The problem:

It was noticed that the audio from the repeater had a bit of an odd background noise - almost a squeal, much like an amplifier stage that is on the verge of oscillation.  For the most part, this odd audio property went unnoticed, but if an "A/B" comparison was done between the audio input and output - or if one inputted a full-quieting, unmodulated carrier and listened carefully on a radio to the output of the repeater, this strange distortion could be heard.

Figure 2: The location of C5 on the DL-1000.  A 0.56 uF capacitor was used to replace the original 0.1 (I had more of those than I had 0.47's) and either one would probably have been fome As noted below, I added another to the bottom of the board. Click on the image for a larger version.

This issue was most apparent when a 1 kHz tone was modulated on a test carrier and strange mixing products could be heard in the form of a definite "warble" or "rumble" in the background, superimposed on the tone. Wielding an oscilloscope, it was apparent that there was a low-frequency "hitchhiker" on the sine wave coming out of the delay board that wasn't present on the input - probably the frequency of the low-level "squeal" mixing with the 1 kHz tone.  Because of the late hour - and because we were standing in a cold building atop a mountain ridge - we didn't really have time to do a full diagnosis, so we simply pulled the board, bypassing the delay audio pins with a jumper.

On the workbench, using a signal tracer, I observed the strange "almost oscillation" on pin 10 of the MX609 - the audio input - but not on pin 7 of U7B, the op-amp driver.  This implied that there was something amiss with the coupling capacitor - a 0.1uF plastic unit, C5, but because these capacitors almost never fail, particularly with low-level audio circuits, I suspected something fishy and checked the MX609's data sheet and noted that it said "The source impedance should be less than 100 ohms.  Output channel noise levels will improve with an even lower impedance."  What struck me was that with a coupling capacitor of just 0.1uF, this 100 ohm impedance recommendation would be violated at frequencies below 16 kHz - hardly adequate for voice frequencies!

Figure 3: The added 2.2uF tantalum capacitor on the bottom of the board across C5.  The positive side goes toward the MX609, which is on the right. Click on the image for a larger version.

Initially, I bridged C5 with another 0.1uF plastic unit and the audible squealing almost completely disappeared.  I then bridged C5 it with a 0.47uF capacitor which squashed the squealing sound and moved the 100 ohm point to around 4 kHz, so I replaced C5 with a 0.56uF capacitor - mainly because I had more of those than small 0.47uF units.

Not entirely satisfied, I bridged C5 with a 10uF electrolytic capacitor, moving the 100 ohm impedance point down to around 160 Hz - a frequency that is below the nominal frequency response of the audio channel - and it caused a minor, but obvious quieting of the remaining noise, particularly at very low audio frequencies (e.g. the "hiss" sounded distinctly "smoother".)   Because I had plenty of them on-hand, I settled on a 2.2 uF tantalum capacitor (100 ohms at 723 Hz) - the positive side toward U2 and tacked to the bottom of side of the board - which gave a result audibly indistinguishable from 10 uF.  In this location, a good-quality electrolytic of 6.3 volts or higher would probably work as well, but for small-signal applications like this a tantalum is an excellent choice, particularly in harsh temperature environments.

At this point I'll note that any added capacitance should NOT be done with ceramic units.  Typical ceramic capacitors in the 0.1uF range or higher are of the "Z5U" type or similar and their capacitance changes wildly with temperature meaning that extremes may cause the added capacitance to effectively "go away" and the squealing noise may return under those conditions.  Incidentally, these types of ceramic capacitors can also be microphonic, but unless you have strapped your repeater controller to an engine, that's probably not important.

Were I to do this to another board I would simply tack a small tantalum (or electrolytic) capacitor - anything from 1 to 10 uF, rated for 6 volts or more - on the bottom side of the board, across the still-installed, original C5 (as depicted in Figure 3) with the positive side of the capacitor toward U2, the MX609.

Note: 

One of the repeater sites also had a "DL-1000A" delay board - apparently a later revision of the DL-1000.  A very slight amount of the "almost oscillation" was noted on the audio output of this delay board, too, but between its low level and having limited time on site, we didn't investigate further. 

This board appears to be similar to the DL-1000 in that it has many of the same chips - including the CY7187 RAM, but it doesn't have a socketed MX609 on the top of the board, and likely a surface-mount codec on the bottom.  It is unknown if this is a revision of the original DL-1000 or closer to the DL-1000C which has a TP4057 - a codec functionally similar to the MX609.

The question arises as to why this modification might be necessary?   Clearly, the designers of this board didn't pay close enough attention to the data sheet of the MX609 codec otherwise they would have probably fitted C5 with a larger value - 0.47 or 1 uF would have probably been "good enough".  I suspect that there are enough variations of the MX609 - and that the level of this instability - is low enough that it would largely go unnoticed by most, but to my critical ears it was quite apparent when an A/B comparison was done when the repeater was passing a full-quieting, unmodulated carrier and made very apparent when a 1 kHz tone was applied.

* * * * * * * * * * * * * * *

The AD-1000:

This is a newer variant of the delay board that includes audio gating and it uses a PT2399, a chip commonly used for audio echo/delay effects in guitars pedals and other musical instrument accessories as it has an integrated audio delay chip that includes 44 kbits of internal RAM.

The problems:

This delay board had two problems:  An obvious audio "squeal", very similar to that on the older DL-1000, but extremely audible, but there was a less obvious problem - something that sounded like "wow" and flutter of an old record on a broken turntable in that the pitch of the audio through the repeater would warble randomly.  This problem wasn't immediately obvious on speech, but this pitch variation pretty much corrupted any DTMF signalling that one attempted to pass through the system, making the remote control of links and other repeater functions difficult.

RF Susceptibility:

Figure 4: The top of the modified AD-1000 board where the added 1k resistor is shown between C11/R13 and pin 2 of the connector, the board trace being severed. Near the upper-right is R14, replaced with a 10 ohm resistor, but simply jumpering this resistor with a blob of solder would likely have been fine. Click on the image for a larger version.

Tracing the audio input, it passes through C1, a decoupling capacitor, and then R2, a 10k resistor - and this type of series resistance generally provides pretty good resistance to RF ingress, mainly because a 10k resistor like this has several k-ohms of impedance - even at VHF frequencies, which is far higher impedance than any piece of ferrite material could provide!

The audio output was another story:  R13, another 10k resistor, is across the output to discharge any DC that might be there, but the audio then goes through C11, directly to pin 1 of U2, the output of an op-amp.  While this may be common practice under "normal" textbook circumstances, sending the audio out from an op-amp into a "hostile" environment must be done with care:  The coupling capacitor will simply pass any stray RF - such as that from a transmitter - into the op amp's circuitry, where it can cause havoc by interfering/biasing various junctions and upsetting circuit balance.  Additionally, having just a capacitor on the output of an op amp can be a hazard if there also happens to be an external RF decoupling capacitor - or simply a lot of stray capacitance (such as a long audio cable) as this can lead to amplifier instability - all issues that anyone who has ever designed with an op amp should know!

Figure 5: The added 1000pF cap on the audio gating lead. A surface-mount capacitor is shown, soldered to the ground plane on the bottom of the board, but a small disk- ceramic of between 470 and 1000 pF would likely be fine. Click on the image for a larger version.

Although not expected to be a problem, a 1000pF chip cap was also installed between the COS (audio gate) pin (pin 5) and ground - just in case RF was propagating into the audio path via this control line - this modification being depicted in Figure 5.

Of course, it will take another site visit to reinstall the board to determine if it is still being affected by the RF field and take any further action.

And no, the irony of a repeater's audio circuitry being adversely affected by RF is not lost on me!

 The "wow" issue:

On the bench I recreated the "wow" problem by feeding a tone into the board, causing the pitch to "bend" briefly as the level was changed, indicating that the clock oscillator for the delay was unstable as the sample frequency was changing between the time the audio entered and exited the RAM in the delay chip.  Consulting the data sheet for the PT2399 I noted that its operating voltage was nominally 5 volts, with a minimum of 4.5 volts - but the chip was being supplied with about 3.4 volts - and this changed slightly as the audio level changed.  Doing a bit of reverse-engineering, I noted that U4, a 78L05, provided 5 volts to the unit, but the power for U2, the op amp and U3, the PT2399, was supplied via R14 - a 100 ohm series resistor:  With a nominal current consumption of the PT2399 alone being around 15 milliamps, this explained the 1.6 volt drop.

The output at resistor R14 is bypassed with C14, a 33 uF tantalum capacitor, likely to provide a "clean" 5 volt supply to decouple U14's supply from the rest of the circuit - but 100 ohms is clearly too much for 15 mA of current!  While testing, I bridged (shorted) R14 and the audio frequency shifting stopped with no obvious increase in background noise, so simply removing and shorting across R14 is likely to be an effective field repair, but because I had some on hand, I replaced R14 with a 10 ohm resistor as depicted in Figure 4 and the resulting voltage drop is only a bit more than 100 millivolts, but retaining a modicum of power supply decoupling and maintaining stability of the delay line.

Figure 6: Schematic of the AD-1000, drawn by inspection and with the aid of the PT2399 data sheet. Click on the image for a larger version.

Figure 6, above, is a schematic drawn by inspection of an AD-1000 board with parts values supplied by the manual for the AD-1000.  As for a circuit description, the implementation of the PT2399 delay chip is straight from the data sheet, adding a dual op-amp (U2) for both input and output audio buffering and  U1, a 4053 MUX, along with Q1 and components, were added to implement an audio gate triggered by the COS line.

As can be seen, all active circuits - the op-amp, the mux chip and delay line - are powered via R14 and suffer the aforementioned voltage drop, explaining why the the supply voltage to U3 varied with audio content, causing instability in audio frequencies and difficulty in decoding DTMF tones passed through this board - and why, if you have one of these boards, you should make the recommended change to R14!

Conclusion:

What about the "wow" issue?  I'm really surprised that the value of R14 was chosen so badly.  Giving the designers the benefit of the doubt, I'll ignore the possibility of inattention and chalk this mistake, instead, to accidentally using a 100 ohm resistor instead of a 10 ohms resistor - something that might have happened at the board assembly house rather than being part of the original design. 

After a bit of digging around online I found the manual for the AD-1000 (found here) which includes a parts list (but not a schematic) that shows a value of 100 ohms for R14, so no, the original designers got it wrong from the beginning!

While the RF susceptibility issue will have to wait until another trip to the site to determine if more mitigation (e.g. addition of ferrite beads on the leads, additional bypass capacitance, etc.) is required, the other major problems - the audio instability on the DL-1000 and the "wow" issue on the AD-1000 have been solved.

* * * * * * * * * * * * * * *

Comments about delay boards in general:

• Audio delay/effects boards using the PT2399 are common on EvilBay, so it would be trivial to retrofit an existing CAT controller with one of these inexpensive "audio effects" boards to add/replace a delay board - the only changes being a means of mechanically mounting the new board and, possibly, the need to regulate the controller's 12 volt supply down to whatever voltage the "new" board might require.  The AD-1000 has, unlike its predecessor, an audio mute pin which, if needed at all, could be accommodated by simple external circuitry.  Another blog post about using one of these audio delay/effects boards for repeater use will follow.
  

• In bench testing, the PT2399 delay board is very quiet compared the MX609 delay board - the former having a rated signal-noise ratio of around 90 dB (I could easily believe 70+ dB after listening) while the latter, being based on a lossy, single-bit codec, has a signal-noise ratio of around 45 dB - about the same as you'd get with a PCM audio signal path where 8 bit A/D and D/A converters were being used.
  

A signal/noise ratio of around 45 dB is on par with a "full quieting" signal on a typical narrowband FM communications radio link so the lower S/N ratio of the MX609 as compared with the PT2399 would likely go unnoticed.  Were I to implement a repeater system with these delay boards I would preferentially locate the MX609-based delay boards in locations where the noise contribution would be minimized (e.g. the input of the local repeater) while placing the quieter PT2399-based board in signal paths - such as a linked system - where one might end up with multiple, cascaded delay lines on link radios as the audio propagates through the system.  Practically speaking, it's likely that only the person with a combination of a critical ear and OCD is likely to even notice the difference!

This page stolen from ka7oei.blogspot.com

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