Category: mixer theory

SolderSmoke Podcast #73 Jan 2, 2008 — AA1TJ Circuits and Poetry, Mixers, CW, Straight Key Night at WA6ARA, Boatanchors in South Africa with ZS6ADY (Part 1)

This is the first in a series of four podcast that include Echolink conversations with Andy ZS6ADY about old tube radios (boatanchors) in South Africa. Click on the YouTube link above to listen.

January 2, 2008 SPECIAL NEW YEAR’S EDITION AA1TJ’s circuitry and poetry. Homemade tubes. Book Review “Early Radio” by Peter Jensen. The Vatican’s antennas. Google Earth flight simulator. Mixer madness continues (now in LTSpice). Mars-asteroid collision? Bollywood: The BITX-20 connection. BANDSWEEP: Straight Key Night at WA6ARA. ECHO-GUEST: Andy, ZS6ADY, South African Boatanchor fan. MAILBAG: Jake N4UY(NOVA QRP), Steve G0FUE (Bath Build-a-thon), Nigel M0NDE

Vasily Ivanenko on Vladimir Polyakov’s Subharmonic Detector

Our old friend “Vasily” sent in a very insightful comment about the Polyakov receiver. It was so good that it merits a blog post of its own. Here it is. Thanks Vasily!

Vasily IvanenkoAugust 9, 2022 at 12:49 AM

Thanks Bill. My own experiments at HF with subharmonically pumped Schottky diode mixers show clearly that almost every mixer parameter we measure is worse than our classic balanced mixer topologies. Definitely 2LO-RF isolation was better than other unbalanced mixers without the need for a transformer.

I guess it’s appealing for low-complexity receiver builders. For zero IF receivers, I like and run my LO at 1/2 RF frequency and then use a doubler — that’s a great advantage for
a DC/ Zero-IF receiver and a built-in feature for the subharmonic mixer.

The SH mixer becomes quite appealing at SHF to mm-wave lengths where making a quiet, temp stable LO gets rather expensive and tricky.

Subharmonically pumped mixers can also work at odd integers if the mixer LO/RF drive is balanced and designed to produce distortion that for example, triples the LO frequency. Rohde & Schwarz had a 40.1 GHz spectrum analyzer with one — and if the LO was 13 GHz while the RF was 39.5 GHz, this gave an IF output of 500 MHz in 1 particular circuit. Really amazing design work. Here’s an interesting URL:

https://www.eravant.com/products/mixers/subharmonically-pumped-mixers

The SH mixer has been around for > 4 decades. The oldest SH mixer paper I’ve got in my library is from Schneider and Snell from 1975. I don’t think they invented the SH, but this pair helped popularize it for the world and design work continues today.I’ve seen optical SH mixers with I/Q outputs in research papers.

Here’s the abstract and citation:

Harmonically Pumped Stripline Down-Converter

M. V. Schneider, W. W. Snell
Published 1 March 1975
Physics, Engineering
IEEE Transactions on Microwave Theory and Techniques

A novel thin-film down-converter which is pumped at a submultiple of the local-oscillator frequency has given a conversion loss which is comparable to the performance of conventional balanced mixers. The converter consists of two stripline filters and two Schottky-barrier diodes which are shunt mounted in a strip transmission line. The conversion loss measured at a signal frequency of 3.5 GHz is 3.2 dB for a pump frequency of 1.7 GHz and 4.9 dB for a pump frequency of 0.85 GHz. The circuit looks attractive for use at millimeter-wave frequencies where stable pump sources with low FM noise are not readily available.

Best to you!

Polyakov Direct Conversion Receiver on 80 Meters (video)

In today’s episode I put the switch in the open position turning the receiver into an ordinary Direct Conversion receiver with a single diode as the detector. I find that it works pretty well on 80, but probably not as well as it does on 40 (where it is in full Polyakov mode). (Yesterday I demonstrated the receiver in action on 40 and provided details on the circuit. See: https://soldersmoke.blogspot.com/2022/08/polyakov-ra3aae-direct-conversion.html)

You will notice that when I throw the switch, but before I retune the input LC network, you can still hear the signal from the previous band. So when I have it in 40 and I throw the switch to open, you can still hear the 40 meter signal. Apparently one diode will (poorly) demodulate a signal with the VFO running at HALF the operating frequency. I saw this in the real world receiver and also saw it in an LTSpice simulation. In LTSpice the signal level drops significantly when I go to just one diode: From 50 mv peak to 15 mv peak, but it can still be heard. Something similar happens when I go from 80 to 40. When I close the switch and suddenly have two diodes and a 3.5 MHz VFO trying to demodulate the 80 meter signal, I can still hear the 80 meter signal, but it is much weaker and a lot more noise is getting through. Again, I saw this in the real world and in LTSpice. It looks as if with the two diodes, the 3.5 MHz signal is being sampled twice each VFO cycle. This may result in some output in the audio range. But again, it is much weaker.

SolderSmoke FDIM Interview with Keith W. Whites — Teaching Electronic Design to EE students using a QRP Transceiver designed by Wayne Burdick


When I first listened to Bob Crane’s interview at FDIM with Keith Whites, I thought of the book “The Electronics of Radio” out of CalTech by David Rutledge. Keith Whites told me that the difference between his effort at University of Kentucky was that Rutledge’s course was designed for freshmen at Cal Tech, while White’s course was aimed at Juniors and Seniors.

I told Keith Whites that I had struggled to understand the Gilbert Cell and the NE602, the device that lies at the heart of the rig used in both courses: The NE-602 Gilbert Cell Mixer used in Wayne Burdick’s NORCAL 40A. Here is how I came to understand the device: https://soldersmoke.blogspot.com/2021/11/how-to-understand-ne-602-and-gilbert.html

Here is Bob Crane’s interview: http://soldersmoke.com/2022 Whites.mp3

Here the slides that Keith used at FDIM: http://soldersmoke.com/2022 Teaching NorCal40A.pdf

Keith’s students obviously got a lot out of this course. Keith has kindly offered to make his course notes available to those who need them.

Thanks to Bob Crane, Keith Whites, David Rutledge and Wayne Burdick.

Putting a Barebones Superhet on 17 Meters with an NE602 Converter (Video)

Armed now with a NanoVNA, I took a look at the passband of the 5 MHz filter in my Barebones Superhet (BBRX) W4OP built it on a Circuit Board Specialist Board. He put a 5 MHz CW filter in there; I broadened the passband for phone by changing the values of the capacitors. Here is what the passband now looks like in the NanoVNA:
This is what DeMaw would call an “LSB filter.” You would get much better opposite sideband rejection by using it with an LSB signal, placing the BFO/Carrier Oscillator slightly above the passband, in this case near 5.002 MHz.

When I first built the down converter to get the 18.150 MHz signal down to the 7 MHz range (where I had the receiver running) I used an 11 MHz crystal for the NE602’s local oscillator. But this created a big problem: 18.150 – 11 = 7.150 MHz. That is in the 40 meter band, but note: NO SIDEBAND INVERSION. Then in the BBRX 7.150 MHz – 2.150 MHz = 5 MHz (the filter frequency) but again: NO SIDEBAND INVERSION. The signal started as a USB signal and remained a USB signal.

I briefly tried shifting the BFO frequency to the other side of the filter passband. If I could get it to around 4.985 MHz, it might work, but because the filter passband was so large, and because the crystal frequency was so low, I was unable to shift the crystal frequency that far. In any case the results would have been less than ideal because of the “LSB” shape of the filter. Back to the drawing board.

I decided to cause one sideband inversion.

At first I put a 25.175 MHz crystal module in my down converter. This shifted the 17 meter phone band down to the 40 meter CW band. It worked, but I cold hear strong 40 meter CW signals being picked up by the wiring of the receiver (the box is plastic!). I went back to the module jar in search of frequency that would move 17 meter phone to the 40 meter area (so I would not have to re-build the BBRX front end) but outside the actual 40 meter band.
I ended up using a 25 MHz crystal in the down converter. 25 MHz – 18.150 MHz = 6.85 MHz WITH SIDEBAND INVERSION. After checking on the NA5B Web SDR to see that there are no strong signals in the 6.835 to 6.89 MHz range, I retuned the output circuit on the converter and tweaked the input capacitor on the Barebones. I shifted the VFO frequency down to 1.835 to 1.89 MHz and put the BFO at 5.002 MHz. The receiver was inhaling on 17 meter SSB.

One more change to the BBRX: in his June 1982 QST article, DeMaw warned that trying to get speaker level audio out of the 741 op amp that he used would result in audio distortion. And it did. So I put one of those little LM386 boards I have been using into the BBRX box. I just ran audio in from the wiper of the AF gain pot. It sounds good.

In effect this is my first double-conversion receiver. I usually prefer single conversion, but this project has highlighted for me one of the advantages of double conversion for someone like me who eschews digital VFOs: Starting with a crystal filter at 5 MHz, with double conversion I could keep the frequency of the LC VFO low enough to ensure frequency stability. That would have been impossible with a 5 MHz IF in a single conversion 17 meter rig. But if I were starting from scratch for a 17 meter rig, I could stick with single conversion by building the filter at 20 MHz, keeping the VFO in the manageable 2 MHz range.

Now, on to the SSB transmitter. The Swan 240 dual crystal lattice filter from the early 1960s needs some impedance matching.

How To Understand the NE-602 and the Gilbert Cell Mixer

I think the key to understanding the Gilbert Cell Double Balanced mixer is to separate out the three tasks that this device completes, and consider them one at a time, using different diagrams:

1) It mixes two signals to produce sum and difference outputs.
2) It balances out the RF input.
3) It balances out the LO input.
Task 1 — Mixing
The Gilbert cell is like the diode ring mixer in that it switches the polarity of the input signal at a rate set by the Local Oscillator. Another way of saying this is that the mixer multiplies the input signal by 1 and by -1.

Steve Long of the University of California described the essence of this mixing this way (using the diagram above):
An ideal double balanced mixer simply consists of a switch driven by the local oscillator that reverses the polarity of the RF input at the LO frequency. http://literature.cdn.keysight.com/litweb/pdf/5989-9103EN.pdf

In an effort to see this for myself, I drew (noodled!) this diagram:
There are four transistors — two differential pairs with RF coming into the bases of the pairs.
The LO is a square wave. The LO alternately turns on transistors 1 and 4, then 2 and 3. When 1 and 4 are on, we are in period 1 — here there is no switching of polarity. Portions of the RF waveform are passed to the outputs. But when the LO turns on transistors 2 and 3, portions of the RF wave form are “crossed over” to the opposite output. Polarity is reversed. We see this in period number 2.

Take a look at the resulting output waveforms. This is the same waveform we see coming out of a diode ring mixer. I really like this drawing because in that complex waveform you can actually see the sum and difference frequencies:
I could see this diode ring waveform myself on my oscilloscope:
https://soldersmoke.blogspot.com/2020/11/diode-ring-magic.html
TASK 2 — Balancing Out the RF Input

In a diode ring, and in other diode mixers, the balancing out of the input signals really takes place in the trifilar toroidal coils that are part of the circuit. Barrie Gilbert needed an integrated circuit mixer that did not use coils.
Again referring to the above diagram, Steve Long of the University of California put it this way:

The ideal balanced structure above cancels any output at the RF input frequency since it will average to zero.

To fully understand this I find it helps to look at the Gilbert cell circuit drawn in a different way. Here is a drawing from Alan Wolke W2AEW that I found very helpful. It comes from his excellent YouTube video: https://www.youtube.com/watch?v=7nmmb0pqTU0
Suppose the RF waveform at I1 is causing the current through R1 and R2 to increase. At the same time, the opposite phase current through I2 will be causing the current through R1 and R2 to DECREASE. So there is no net effect of the RF signal at the output. The RF is balanced out.
TASK 3 – Balancing Out the Local Oscillator Signal
Here too I used my own drawing, and was guided by the words of Steve Long:

It also cancels out any LO frequency component since we are taking the IF output as a differential signal and the LO shows up as common mode.

The important thing to realize here is which transistors are being turned on and off by the local oscillator signal. On one half cycle of the LO, transistors 1 and 4 are on. So the LO signal at the LO frequency are both pulling the same amount of LO frequency current through the resistors. So you have the same change in voltage at the output terminals. And the output terminals are differential. The LO signal results in no voltage difference between the terminals. So the LO frequency is balanced out.

The same thing happens on the following half of the LO cycle. Here, transistors 2 and 3 are turned on. Again, both transistors pull the same amount of LO frequency current through the resistors. There is no differential voltage. So no LO frequency energy passes to the output. LO frequency is balanced out.
——————————–
I am surrounded by Gilbert Cell Mixers and I have been using them in my homebrew rigs for many years. I use them in up-converters for my RTL-SDR receivers. I have one in the downconverter for my 17 meter receiver and had one as the mixer in my first SSB transmitter. I built a 40 meter SSB transceiver with NE602s on either end of the crystal filter. Years ago, I built a DSB transceiver with several NE602s. My SST QRP CW transceiver is made with NE602s. I have on my bookshelf Rutledge’s book “The Electronics of Radio” that is all about the NORCAL 40 transceiver, built using NE602 chips. But until now I really didn’t know how these chips worked. Truth be told, for me they were mysterious little black boxes, and that bothered me. Now I feel a lot better about using these clever devices. I plan on stocking up on the old style (non-SMD) NE602s.

Apparently Barrie Gilbert rejected the idea that he invented the circuit that bears his name. It seems that Howard Jones first used this circuit in 1963, with Gilbert developing it independently (in an improved form) in 1967.

Barrie Gilbert was quite a guy, with electronic roots in the world of tinkering:

https://www.microwavejournal.com/articles/33425-living-and-learning-with-barrie-gilbert

https://vintagetek.org/barrie-gilbert/

W2EWL’s “Cheap and Easy SSB” Rig — And The LSB/USB Convention Myth

In March 1956 Tony Vitale published in QST an article about a “Cheap and Easy” SSB transmitter that he had built around the VFO in an ARC-5 Command Set transmitter. Vitale added a 9 MHz crystal-controlled oscillator, and around this built a simple phasing generator that produced SSB at 9 MHz. He then made excellent use of the ARC-5’s stable 5 – 5.5 MHz VFO. His rig covered both 75 meters and 20 meters. Here is the article:

http://nebula.wsimg.com/2b13ac174f7f2710ca2460f8cf7d6b8b?AccessKeyId=D18ED10DA019A4588B7B&disposition=0&alloworigin=1

Because it used the 9 and 5 frequency scheme, over the years many, many hams have come to think that Vitale’s rig is the source of the current “LSB below 10 MHz, USB above 10 MHz.” This is wrong. An example of this error popped up on YouTube just this week (the video is otherwise excellent):

First, Vitale’s rig had a phasing SSB generator. All you would need to switch from USB to LSB was a simple switch. And indeed Vitale’s rig had such a switch. Pictures of other Cheap and Easy transmitters all show an SSB selection switch. So with a flip of the switch you could have been on either USB or LSB on both 75 and 20. With this rig, you didn’t even need sideband inversion to get you to 75 LSB and 20 USB.

Second, even if hams somehow became so frugal that they wanted to save the expense of the switch, leaving the switch out (as suggested above) would NOT yield the desired “75 LSB 20 USB” that the urban legend claims that W2EWL. As we have been pointing out, a 9 MHz SSB generator paired with a 5 MHz VFO (as in the Vitale rig) will NOT — through sideband inversion — yield LSB on one band but USB on the other.

W2EWL’s rig could not have been the source of the LSB/USB convention. I still don’t know where the convention came from. I am still looking for the source.

But leaving the LSB/USB convention issue aside, Tony Vitale’s rig is an excellent example of early SSB homebrewing, and of a very clever use of war surplus material. In the January 1992 issue of Electric Radio magazine, Jim Musgrove K5BZH writes of his conversations with Vitale about the Cheap and Easy SSB. Tony told Jim that this rig came about because the Central Electronics exciters required an external VFO — they recommended a modified BC458. B&W had recently come out with a phase shift network. Vitale went ahead and built the whole rig inside a BC458 box. FB Tony!

In the December 1991 Electric Radio, Jim K5BZH reports that Tony was recruited into the ranks of SSBers when he watched a demonstration of SSB by Bob Ehrlich W2NJR in November 1950. Tony very quickly started churning out SSB rigs. His daughter Trish Taglairino recounted that when her father had “done something great again” there would be a parade of hams to the basement shack. About 30 guys showed up when Tony put his first SSB rig on the air — they sent out for beer.

Thanks to Jim for preserving so much SSB history.

QST Recognized Error on Sideband Inversion, But Continued to Make the Same Mistake

I don’t really know if this is good news or bad news. It’s good that in November 1985 they recognized the error, but then they allowed the same error to be repeated by the same author in the 1989 article “A Four-Stage 75-Meter SSB Superhet,” and again in 1990 in W1FB’s Design Notebook. It also made it into the 2002 ARRL Handbook.

Thanks to Chuck WB9KZY for alerting us to this Feedback piece.

A Video Series on the Mythbuster 75/20 Rig — Video #1

I am happy to report great progress on the Mythbuster project. I have the receiver working on both 75/80 and 20 meters. And it in fact inverts the 75 meter LSB signals, turning them into 5.2 MHz USB signals for passage through my 5.2 MHz USB filter/BFO combo. No switching or shifting of the BFO is needed.

I am following Farhan’s BITX20 advice — I have paused in the construction and am enjoying the receiver that I have built. I’ll build the transmit circuitry later.

Inspired by Frank Jones (you really should be reading the FMLA articles) I have this rig prototyped “Al Fresco” on a pine board that I found discarded on a neighbors front stoop.

There is no RF amplifier in this rig. Following the advice of multiple receiver gurus, I ran the BP filters right into the ADE-1 diode ring mixer. I have the TIA amps set at about 24 dbm. There is a lot of audio gain from the LM386 and the audio pre-amp. This seems to be enough, even on 20. I hear the band noise when I connect the antenna on both 75 and 20.

Here is the first video in the series. I’m posting them first on Patreon, then, a few days later, here and on the YouTube channel.

Farhan’s sBITX — Combining SDR with the Traditional Superhet

Here is Farhan’s amazing presentation to the virtual 2021 FDIM event. There is a lot of tribal knowledge in this video. Lots of old and new technology. I was especially intrigued by Chris Trask’s Kiss mixer. Farhan’s discussion of simple Arduino-based speech equalization and compression made me think that I have work to do in this area. And of course, Farhan’s whole discussion of how to bring SDR into — literally into — the circuitry of a uBITX is really cool and very educational.

The SST QRP Transceiver

Click on the schematic for a better view
Bob KD4EBM recently sent me an amazing package of radio goodies. Included was a little metal box not much larger than a deck of cards. It is a 20 meter SST transceiver designed by Wayne Burdick N6KR during the late 1990s. This transceiver is built around three NE602 Gilbert Cell mixer chips. It arrived in my shack as I was struggling to understand the Gilbert Cell. TRGHS. It also put me back on the path of QRP CW righteousness. Thanks Bob. Thanks Wayne.

I e-mailed Wayne Burdick (now of Elecraft fame) to tell him I was now using the rig he had designed so long ago. Wayne e-mailed back, saying that the SST was the smallest “real” radio that he had ever designed. SST stands for Simple Superhet Transceiver.

I’ve been using the SST every day for the last week or so. It is a pleasure to operate. I’m using it with the key from India that Farhan brought for me. It is truly QSK — the receiver stays on when I transmit. I’ve never used a QSK rig before and I can now see the big advantage that this provides: When I am responding to a CQ, I can immediately hear if the other guy put out another CQ or respond to someone else — I can stop calling at that point. My first contact with it was with F6EJN. Again, TRGHS.

I made two small mods to the SST: I added 1 uH to the RFC in the VXO; it now tunes 14.053 — 14.063. And I took out a noise blanker that had been installed. Removing the noise blanker left an ugly hole in the front panel which I promptly filled with a completely cosmetic machine screw.

Here’s the manual:

https://qrpbuilder.com/wp-content/uploads/2017/04/sst_manual_042217.pdf

HRDX Interviews Paul Taylor VK3HN

Wow, Paul Taylor, VK3HN is working on homebrew rig #11. FB.

This interview was quite thought-provoking.

— I agree with Paul about the importance of not being dogmatic about
always staying under 5 watts. It sounds like Paul is having fun with his
100 watt SOTA project.

— It was great to hear that Leon VK2DOB is still active in ham radio and running a QRP company in VK. FB. An article by Leon on CMOS mixers in the summer 1999 issue of SPRAT played a key role in my understanding how switching mixers really work. I put Leon’s diagram in my book SolderSmoke — Global Adventures in Wireless Electronics.

— On blowing up the finals in simple HB gear. The first real transmitter that I built was the VXO-controlled 6 watter from QRP Classics by the ARRL. It had a 36 volt Zener diode across the collectors of the final. This was to prevent the kind of final destruction Paul suffered up on that summit: “D2 is used to clamp the collector voltage waveform to protect the output transistors if the transmitter is operated into an open circuit or high SWR antenna system.” Maybe we should revive the use of that simple SWR protection circuit, especially for SOTA rigs.

AA7EE Casually Kills a Direct Conversion Receiver, then Coldly Discards a Diode Ring Mixer

I was really glad to see that Dave AA7EE has — after a long absence — posted another article on his blog. The article has some great personal reminiscences about his involvement with direct conversion receivers. Here is one passage:

I spent many happy hours tuning around and listening on 80M with the DSB80. It was this first experience that cemented my affinity for direct conversion receivers built with commercially available diode ring mixer packages. It just seemed so simple – you squirt RF into one port, a VFO into the other, and (after passing the result through a diplexer) amplify the heck out of the result. The seeming simplicity of the process of converting RF directly to baseband audio has held great appeal for me ever since. Unfortunately, that project didn’t survive. One day, in later adulthood, in my apartment in Hollywood, I reversed the polarity of the 12V DC supply and, discouraged at it’s subsequent refusal to work, tossed the whole thing away. Now, I cannot quite believe that I did that, but it was during a long period of inactivity on the ham bands, and complete lack of interest. If only I could go back, and not have thrown it into the dumpster of my apartment building! Hollywood is ridden with recent notable history. My little double sideband transceiver met it’s unfortunate end just 100 feet from the spot where Bobby Fuller, of The Bobby Fuller Four, was found dead in his car, in 1966, the subject of a still unsolved mystery to this day. The death of my little DSB rig was a lot less mysterious. To think that I heartlessly tossed an SBL-1 mixer into a dumpster, is a mark of how far I had strayed from my homebrewing roots, forged in a little village in England. Now, a few years later, in a city known for it’s sin and excess, I had cruelly ended the life of a stout and honest diode ring mixer. I suppose I should spare a thought for the polyvaricon but, well, you know – it was a polyvaricon!

https://aa7ee.wordpress.com/2021/03/04/the-ve7bpo-direct-conversion-receiver-mainframe/

Phasors and the Propeller Analogy from Walla Walla University

We covered this excellent and very illuminating work before. As a follow-up, student Konrad McClure was kind enough to send me this video, which goes the extra mile with the propeller analogy.

For me, the most interesting aspect of this is that it provides an explanation of the phase differences between upper and lower sidebands. I need to study more about aliasing and the Nyquist criteria.

Check out the video. It get us a lot closer the an intuitive understanding; math often falls short in this area.

Thanks Konrad!

Please send feedback to Konrad via the comment box below.

Some Thoughts on Singly Balanced Mixers with Two Diodes and One Transformer

In 2001, out it in the Azores, I built a 17 meter version of Doug DeMaw’s Double Sideband transmitter (“Go QRP with Double Sideband” CQ Magazine, February 1997). I struggled to understand the balanced modulator — how it mixed, balanced, and how it produced DSB. I later presented my understanding of the circuit in my book “SolderSmoke — Global Adventures in Wireless Electronics” pages 132-137. In essence, I figured out that you had to think of the balancing and the mixing as two separate operations: The transformer provided the balance that eliminated the carrier (the LO signal) while the diodes presented the two signals (audio from the mic amp and LO from the VFO) with a highly non-linear path. The LO was successively turning on both diodes then turning off both diodes. The audio signal was being “chopped” at the rate of the LO. This produced a complex waveform that contained sum and difference frequencies — the upper and lower sidebands. The carrier was balanced out by the transformer because the two outputs of the transformer were always of opposite polarity, and they were joined together at the output of the mixer.

Fast forward to 2013. I built a 17 meter version of Farhan’s famous BITX 20 rig. Above you can see the balanced modulator stage, which also serves as the product detector. As you can see, it is essentially the same circuit as the one used by Doug DeMaw in his DSB rig.

In 2018 I built a simple direct conversion receiver for my nephew. For the mixer I used what I considered to be just a cut-down version of the circuit used by DeMaw and Farhan. I got the idea for this from Olivier F5LVG and his RX-20 receiver from SPRAT. It had the RF signal coming in on L1 and the VFO signal coming in to the wiper of the 1 k pot. But with this arrangement, the diodes were NOT both being turned off on half the VFO cycle, then both being turned on during the other half. Instead, as the VFO signal swung positive, D2 would conduct and D1 would shut down. When the VFO signal swung negative, D1 would conduct and D2 would shut down. It worked, but the diodes were being switched in a very different way than they had been in the DeMaw and Farhan circuits. If you have the strong LO signal going in on L1, BOTH diodes conduct, then BOTH don’t conduct. But if you have the LO going in through the pot, one diode conducts while the other does not conduct.

After I concluded that the BJT product detector circuit in the HA-600A was causing distorted SSB and CW reception, I tried the old DeMaw/Farhan circuit, this time in product detector mode. See above. This worked better, but I realized that this configuration was balancing out the BFO signal, and not the IF signal. My problem with the original product detector had been that IF signal was getting simultaneous envelope detection AND product detection. So I decided to just switch the inputs and put the IF signal into L1 (where it would be balanced) and the BFO into R1/R2 (the 100 ohm pot).


This seemed like it would reduce the envelope detection problem, right? I mean, L1 is the balanced input, right? But I wonder if we need to consider how the diodes were being switched in this arrangement. Instead of having both conducting and then both not conducting, in this arrangement one would be conducting during half the BFO’s cycle, while the other was not. That means that at any given moment, the two output sides of the transformer would be looking into very different loads — hardly a condition conducive to balance. But I used LTSpice to look at the audio output under the two different port arrangements. Sherwood advised looking at the output of the product detector with the BFO turned off –there should be no output with the BFO off. And indeed, putting the IF signal into L1 and the BFO into the R1/R2 pot resulted in less of the distortion causing envelope detection. The way the diodes were being switched didn’t seem to adversely affect the balancing out of the IF signal. I am not sure why this doesn’t seem to cause trouble.

There was, however, another problem with the use of this circuit in the Lafayette HA-600A: port isolation. The BFO signal was getting back into the IF signal input on L1. I could see it on the S-meter. This was worrisome not only because of the S-meter, but also because the same circuit was driving the receiver’s AGC — in effect, the BFO was turning the gain down. Theoretically, this should not have been happening. Look at the transformer. the BFO currents going through L2 and L3 should be of opposite polarities and should be cancelling each other out in L1. But obviously this was not happening. Perhaps this was the result of the sequential way the diode are switching in this arrangement. On the bench, if I put the BFO into L1, I saw very little BFO signal at the R1/R2 junction. If I put the BFO signal into the R1/R2 junction, I was a lot of BFO signal at the top of L1. And that is what I saw on my S-meter when this circuit was used in the HA-600A.

On the bench, if I turned off the BFO and put an AM modulated signal into the junction of R1/R2, I can see audio getting through once the input signal reaches 1 volt peak. I do NOT see that kind of “breakthrough” envelope detection when (with the BFO off) I put a modulated signal into L1. So the singly balanced circuit is doing that it is supposed to do — it is balancing out the the signal going into L1.

So it seemed that with the singly balanced circuit I would have to choose: suffer from the poor port isolation or AM breakthrough. Clearly it was time to go for a doubly balanced circuit. And that is what I did.

Finally, I took a look at another two diode detector, the Polyakov or “subharmonic” detector. This is a really interesting circuit that can teach us a lot about how mixers work. Here you can run the local oscillator at 1/2 the signal frequency. With two diodes back to back, the incoming signal is being sampled TWICE during each cycle of the local oscillator. That is equivalent to having the signal sampled at twice the local oscillator frequency. This circuit allows you to run the oscillator at a much lower frequency — this could allow much greater oscillator stability. In the circuit above, with both diodes connected, a 7 MHz incoming signal would produce a 2 kHz tone.

Another big plus of this circuit comes if you take D1 out of the circuit (as shown). In this configuration the circuit becomes a normal diode detector. Here it will receive a signal at 3.5 MHz, converting that signal into a 1 kHz audio tone. So you can get a direct conversion receiver for 40 and 80 meters fairly easily.

So Many Wonderful Things on W7ZOI’s Site

There he is. Wes Hayward, W7ZOI in 1957. I had never seen this picture before. I found it on Wes’s recently updated “shackviews” web page: http://w7zoi.net/shackviews.html .

There are so many treasures on that page, and on all the other portions of Wes’s site.

Some highlights for me:

— Wes’s description of the station in the above picture.

— On his page about Doug DeMaw, Wes mentions that after Doug edited Wes’s 1968 article about direct conversion receivers, Doug built some himself, experimenting with different product detector circuits. Having used Doug’s mixer circuit in many of my rigs, and having recently experimented with different product detectors for my HA-600A, I kind of felt like Doug was watching over my shoulder, guiding me along as I experimented.

— Wes’s use of a digital Rigol oscilloscope. Makes me feel better about giving up on my Tek 465.

— The page about Farhan’s visit to Wes, and the awesome gathering of homebrew Titans that ensued…

— Wes’s meeting with Chuck Adams.

Thanks Wes. Happy New Year and best of luck in 2021!

SolderSmoke Podcast #227: Solar System, SDR, Simple SSB, HA-600A, BITX17, Nesting Moxons? Mailbag

SolderSmoke Podcast #227 is available:


http://soldersmoke.com/soldersmoke227.mp3


Travelogue

Mars is moving away. Jupiter and Saturn close in the sky. And the Sun is back in action – Cycle 25 is underway. Also, the earliest sunset is behind us. Brighter days are ahead.


Book Review: “Conquering the Electron” With a quote from Nikola Tesla.


No real travel for us: Hunkered down. Lots of COVID cases around us. Friends, relatives, neighbors. Be careful. You don’t want to be make it through 10 months of pandemic only to get sick at the very end. SITS: Stay In The Shack.


Pete’s Bench and Tech Adventures:

Backpack SDR keithsdr@groups.io

Hermes Lite 2

Coaching SSB builders

G-QRP talk

A new source for 9 MHz crystal filters


Bill’s Bench:

Fixing the HA-600A Product Detector. Sherwood article advice. Diode Ring wins the day. Fixing a scratchy variable capacitor. Studying simple two diode singly balanced detectors. Polyakov. Getting San Jian frequency counter for it.

Fixing up the 17 meter BITX. Expanding the VXO coverage. Using it with NA5B’s KiwiSDR.


Resurrecting the 17 meter Moxon. But WHY can’t I nest the 17 meter Moxon inside a 20 meter Moxon? They do it with Hex beams. Why so hard with Moxons? DK7ZB has a design, but I’ve often heard that this combo is problematic. Any thoughts? I could just buy a 20/17 Hex-beam but this seems kind of heretical for a HB station.


Suddenly getting RFI on 40 meters. Every 50-60 Hz. Please tell me what you think this is (I played a recording).


MAILBAG:

Dean KK4DAS’s Furlough 40/20

Adam N0ZIB HB DC TCVR

Tony G4WIF G-QRP Vids. Video of George Dobbs.

Grayson KJ7UM Collecting Radioactive OA2s. Why?

Pete found W6BLZ Articles

Rogier KJ6ETL PA1ZZ lost his dog. And we lost ours.

Steve Silverman KB3SII — a nice old variable capacitor from Chelsea Radio Company.

Dave K8WPE thinks we already have a cult following.

Dan W4ERF paralleling amps to improve SNR.

Jim W8NSA — An old friend.

Pete Eaton WB9FLW The Arecibo collapse

John WB4GTW old friend… friend of:

Taylor N4TD HB2HB


And finally, we got lots of mail about our editorial. No surprise: Half supportive, half opposed. Obviously everyone is entitled to their opinion. And we are free to express ours. It’s a free country, and we want it to stay that way. That is why we spoke out.


Yesterday the Electoral College voted, finalizing the results. All Americans should be proud that the U.S. was able to carry out a free and fair national election with record turn out under difficult circumstances. And all loyal Americans should accept the results. That’s just the way it works in a democracy.


We are glad we said what we said. It would have been easier and more pleasant to just bury our heads in the sand and say nothing. But this was a critically important election and we felt obligated as Americans to speak out. We’d do it again. And in fact we reserve the right to speak out again if a similarly important issue arises.

Wrapping up the HA-600A Product Detector Project — Let’s Call Them “Crossed Diode Mixers” NOT “Diode Rings”

This has been a lot of fun and very educational. The problem I discovered in the Lafayette HA-600A product detector caused me to take a new look at how diode detectors really work. It also spurred me to make more use of LTSpice.

In the end, I went with a diode ring mixer. Part of this decision was just my amazement at how four diodes and a couple of transformers can manage to multiply an incoming signal by 1 and -1, and how this multiplication allows us to pull audio out of the mess.

But another part of the decision was port isolation: the diode ring mixer with four diodes and two transformers does keep the BFO signal from making its way back to into the IF chain. This helps prevent the BFO signal from activating the AGC circuitry, and from messing up the S-meter readings. LTSpice helped me confirm that this improvement was happening: in LTSpice I could look at how much BFO energy was making its way back to the IF input port on the diode ring mixer. LTSpice predicted very little, and this was confirmed in the real world circuit. (I will do another post on port isolation in simpler, singly balanced diode mixers.)

At first I did have to overcome some problems with the diode ring circuit. Mine seemed to perform poorly with strong signals: I’d hear some of the “simultaneous envelope and product detection” that started me down this path. I also noticed that with the diode ring, in the AM mode the receiver seemed to be less sensitive — it was as if the product detector circuit was loading down the AM detector.

One of the commenters — Christian — suggested putting some resistance into the input of the diode ring circuit. I put a 150 ohm pot across the input, after the blocking capacitor. The top of the pot goes to the capacitor, the bottom to ground and the wiper to the input of L1 in the diode ring circuit (you can see the circuit in the diagram above). With this pot I could set the input level such that even the strongest input signals did not cause the envelope detection that I’d heard earlier. Watching these input signals on the ‘scope, I think these problems arose when the IF signals rose above .7 volts and started turning on the diodes. Only the BFO signal should have been doing that. The pot eliminated this problem. The pot also seemed to solve the problem of the loading down of the AM detector.

With the pot, signals sounded much better, but I thought there was still room for improvement. I thought I could hear a bit of RF in the audio output. Perhaps some of the 455 kHz signal was making it into the AF amplifiers. I looked at the circuit that Wes Hayward had used after the SBL-1 that he used as product detector in his Progressive Receiver. It was very simple: a .01 uF cap and 50 ohm resistor to ground followed by an RF choke. I can’t be sure, but this seemed to help, and the SSB now sounds great.

A BETTER NAME?

One suggestion: We should stop calling the diode ring a diode ring. I think “crossed diode mixer” or something like that is more descriptive. This circuit works not because the diodes are in a ring, but because two of them are “crossed.” From now on I intend to BUILD this circuit with this crossed parts placement — this makes it easier to see how the circuit works, how it manages to multiply by -1, and to avoid putting any of the diodes in backwards.

I prefer the bottom diagram
A KNOWN PROBLEM?

I’m left wondering if the engineers who designed the HA-600A were aware of the shortcomings of the product detector. It is really strange that my receivers lacks a 12V line from the function switch to the product detector. And it is weirder still that the detector works (poorly) even with no power to the transistor. What happened there?

When you look at the HA-600A manual, you can see a hint that maybe they knew there was a problem. For CW and SSB, the manual recommends leaving the AF control at the quarter or halfway point, then controlling loudness with the RF gain control. This would have the effect of throttling back the RF gain (and the potential for product detector overload) when strong signals appear. MGC in addition to the AGC. Any memories or insights on this would be appreciated.

Diode Ring Magic

I continue to work on the product detector of my Lafayette HA-600A. This work has caused me to brush up on my understanding of how mixers really work.

I think one of the most interesting mixer circuits is the diode ring. With just four diodes and one or two transformers, this device manages to take an incoming signal and multiply it by either 1 or -1 depending on the polarity of the local oscillator signal. That is pretty amazing.

Alan Wolke W2AEW did an excellent video on this: https://www.youtube.com/watch?v=junuEwmQVQ8

Inspired by Alan, I took my most recent homebrew diode ring mixer (with transformers from Farhan, diodes from Jim W8NSA, and a PC board base from the CNC mill of Pete N6QW) and hooked it up to two signal generators and an oscilloscope. I had the local oscillator at 10 MHz and the signal oscillator at 7 MHz. You can see my results in the pictures (above and at the end). You can see the resulting difference frequency (3 MHz) in the broad up and down pattern. And you can see the sum frequency (17 MHz) signal in the faster oscillations. All you would need is some filtering to separate them out.
I really like the RSGB Handbook diagram (above). I think the bottom schematic with its crossed diodes really explains how the phase reversal takes place: when the LO turns on D1 and D3 (the horizontal ones), multiplication by 1 takes place. But when the LO turns on D2 and D4 (the crossed diodes), up goes to down and down to up, creating phase reversal, or, in math terms, multiplication by -1.

At a more basic level, mixing takes place whenever — in a non-linear circuit — one signal is controlling the gain or attenuation experienced by the other signal. A complex waveform results, a waveform that contains sum and difference products. A circuit like the diode ring, that alternately multiplies by 1 and -1, is non-linear in the extreme, and the multiplication is controlled by the LO. The results can be seen in the diagram’s complex waveforms, on Alan’s Tek ‘scope, and on my Rigol. And in those complex waveforms you can SEE the sum and difference frequencies. That is really cool.

A Diode Ring Product Detector for the HA-600A? Problems.

Pete advised me to try this a week or so ago, but it took me a while to follow through and try it out.

I got the two diode, one transformer product detector working well, but with it a new problem arose: 455 kHz energy from the BFO was leaking past the product detector back into the S-meter/AGC circuitry. This showed up in the form of a constant S-3 reading when I switched to SSB/CW. This was annoying.

I figured the problem was that the only signal really being balanced out was the IF signal going into L1 of the product detector. I took another shot at putting the BFO signal into this port, with the IF signal going into the unbalanced potentiometer port. This did indeed take care of the BFO leakage S-meter problem, but once again the SSB did not sound great — I think the old problem of simultaneous envelope and product detection returned.

This was obviously a port isolation problem. I remembered that the diode ring “doubly balanced” configuration has much better port isolation. So on Sunday morning I built one, first in LTSpice and then on the bench.

For the bench model I used some PC board pads out of Pete Juliano’s $250,000 CNC machine. For the toroids I used two trifilar coils wound by Farhan’s dedicated staff in Hyderabad. The diodes were sent to me by Jim W8NSA. So there was lots of soul in this new machine.

The circuit worked in LT Spice and at worked well when tested on my bench with my FeelTech (for the BFO) and HP8640B (for the IF signal) sig gens with my Rigol ‘scope watching for the audio out.

But I ran into some problems when I popped the new board in there in place of the old product detector: The 455 kc BFO leakage problem is gone and the S-meter is where it should be, but…

— I’m seeing a return of the old simultaneous envelope and product detection problem. SSB was sounding scratchy again and indeed, when I removed the BFO signal from the diode ring circuit I could hear SSB signals making it into the audio amplifiers. These signals sounded just like AM signals as heard through an envelope detector without a BFO.

— The diode ring circuit also had a very bad effect on how the HA-600A worked in AM mode. It seemed like the new circuit was loading down the diode AM demodulator. SW broadcast signals sounded awful in the AM mode until I disconnected the IF input to the diode ring circuit (this input is NOT switched — it is always connected, even in the AM mode).

So, for now, am back to using the two-diode, single transformer, singly balanced product detector with IF signal going to the balanced (L1) port and the BFO going in through the wiper of the 100 ohm pot.

Any suggestions on how to overcome the problems with the diode ring circuit?