With ARRL Field Day around the corner, it is the time of year where amateur radio operators far and wide wonder if they are going to be stuck having their QSOs wiped out every time their neighbor keys up the microphone. Interference between stations in a multi-transmitter field day operation can be the norm if you didn’t think to use band pass filters.

So out my stash of little gray metal boxes came, and I began checking their VSWRs for a down-‘n’-dirty pre-Field Day check-a-roo…

I don’t love the VSWR trace of this 6M filter, but it will probably suffice for Field Day, where I plan on setting up a 6M 4-element beam, and operating largely on FT8 to try to intercept the “Alpha” stations that are trying to rack up the “Free VHF station” points. That, and when else do I get to put up my 6M yagi???? FT8 is operated on 50.313 MHz which should have a VSWR under 1.2.

These Array Solutions elliptic filters have a beautiful looking VSWR. I really wish I had spent my ham bucks acquiring a full set of these. Apparently Array Solutions is not making them anymore, but a company called Hamation is? Oh, and for anyone not familiar with the RigExpert Antenna Analyzer (1-port VNA), the blue portion of the display indicates the ham band with frequency along the horizontal access and VSWR on the vertical access. Keep in mind that the VSWR we want is as close to 1 as possible!

Now 12M is a WARC band of course, meaning you cannot use it for contesting. In general, it is second only to 60M as my least used band. But, boy, that band pass filter looks great!

I expect 15M to be hopping on Field Day. I am glad this filter looks good.

Another WARC band, i.e. Field Day no-go… But a good looking filter!

Now 20M. Let’s just say I am not at all happy with this filter. Granted, it has probably been heavily abused over its several years now with me. Dunestar has gone out of business since August of 2023. Their original owner became a silent key right around the time that I purchased this set. I decided to try out Morgan Systems Surestop bandpass filters for 20M and 40M for this year’s Field Day. You’ll notice the 40M filter looks reasonable, but when actually under use, the VSWR seen at the transceiver is often high. And we can’t be without a highly functional 20M and 40M stations when it comes to Field Day operations. We will see how the Surestop filters behave…

The 30M filter looks superb! Of course, there is no operating 30M on Field Day.

The 40M filter looks a bit janky. Technically, it should function okay. But like I mentioned, this filter often creates a high SWR at the transceiver. I have a replacement here for it now.

Ugghh. The 80M filter is downright scary looking. I probably should have replaced it when I had a chance.

The top band filter isn’t great. What else can I say? I am not sure I ever even used this filter on 160M. I do think I will slowly start replacing my filters with one of the other manufacturers with time. Although I am grateful to have been able to get a set of Dunestar filters, especially since they provided a boatload of good multi-operator experiences over the years, the older and wiser me wishes I had put my money elsewhere.

Here are the “guts” of one of the Array Solutions 3rd order elliptic filters. Note the interesting use of a hot glue like substance to hold the windings in place, the beefy size of the enameled wire, and the use of ceramic capacitors. Silver-Mica capacitors are often recommended for use in band pass filters

…and a representative schematic from this excellent LC Filter Design calculator by Marki Microwave…

Now we can contrast the design and construction of the Array Solutions band pass filter with the 2nd order Dunestar bandpass filter (below). This design consists of two airwound coils and capacitors mirroring and shielded from each other on the input and output side.

Every time I get around to thinking about, testing, and opening up my band pass filters, I can’t help but think: It would be so much better to make these myself. For some reason, this does not seem to be an area that has been overly tackled by hams. In fact, there is really only one prevailing design by Lew Gordon K4VX, a 3rd order Butterworth filter, that is well-described and seems easy-ish to reproduce by the average everyday ham (i.e., one that does not design RF products for a living). The W3NQN band pass filter design article is a much more complex document to follow.

I have dabbled in making band pass filters before, but have found myself hindered by the testing process. I since learned to use the “low Z” setting of my oscilloscope. So, once I again, I found myself constructing an ugly little device, this time a low-power 160M version of K4VX’s Butterworth filter. The schematic, construction, and component values are all documented in the article. This is nothing more than a capacitor (~4000 pF) and inductor (~2.2 µH) connected in parallel on the left hand side as well as a capacitor (~4000 pF) and inductor (~2.2 µH) connected in parallel on the right hand side, with another capacitor (~400 pF) and inductor (22 µH) in series in the middle connecting the two sides. I just soldered everything together and attached it across VHF connectors.

And although the Marki Microwave design tool proposes different capacitor and inductor values for its version of the 160M 3rd order Butterworth band pass filter, you can still get an idea of what the schematic, and scatter plot parameters (insertion loss and return loss) of the filter should look like.

The first test I performed with the band pass filter was pass a sine wave through it from below the 160M band (which spans from 1.8 MHz to 2 MHz). I started with 500 kHz and passed the signal into my oscilloscope, making sure to turn on the low impedance (50 ohm) setting.

I did indeed have a fairly weak signal.

When I increased the signal generator frequency so that the waveform outputted was within the pass band of the filter (1.8 MHz), the oscilloscope showed a much larger voltage. Keep in mind that it is Channel 2 (“CH2”, the bottom box!), that you want to be looking at on the signal generator if you are following along with the pictures.

There is no change to the oscilloscope settings between the 1.8 MHz input (below) and the 500 kHz input (earlier). Clearly the voltage recovered at the 1.8 MHz setting is much larger.

Now to take a look at the NanoVNA results. The filter was simply placed between port 0 and port 1 of the NanoVNA. The vertical gray bar represents the frequency range of the 160M ham band. The filter I constructed did not use components of the exact values recommended in the K4VX article, thus the reason the filter performs at a lower frequency than expected.

Regardless, you can see below that the shape of the S11 (return loss) and S21 (insertion loss) parameters are very similar to that predicted by the Marki calculator. My filter is below:

And, again, the S11 and S21 parameters as predicted by the Marki calculator:

Well, there you have it. Band pass filter season! Field Day is almost here, and we are going to go with what we have. However, my mind has been spinning around the idea of constructing my own band pass filters so that I can more easily fix and replace the rather fragile devices as needed. And although this was a tiny little experiment, I think it shows that these band pass filter designs are indeed reproducible with accuracy. Will a KM1NDY band pass filter design show up here in the near future?! The Magic 8 Ball says “Reply Hazy. Try Again Later”!

Catchya on the flippity flip!

KM1NDY

 

"I have discovered spurs in the output of my transmitter.  They are 60 db down, but I still can't stop thinking about them.  What should I do?"

I can't help thinking that if Jean Shepherd had access to something like this, his Heising modulator trouble might not have spoiled his date with the girl from his school.  

What do you guys think about the Woebot?  

Ham Radio With K0PIR

Big surprise after opening the Elecraft K3S! I bought this rig used off of Schulman Auctions. I love this radio, but I find it is not as easy to use or configure as the Icom 7300. Let me tell you...

The post Elecraft K3S Big Surprise! appeared first on Ham Radio with K0PIR - Icom 7300 and 7610 SDR Transceivers and now Elecraft!.

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.)