Understanding a Very Simple Two-Diode Mixer

 

Take a look at the simple little mixer above.  I think I first saw it in SPRAT.  Thinking that it was really just a simplified version of the two diode Doug DeMaw mixer that I had been using for years, I couple of years ago I built it into a little Direct Conversion receiver.  It worked great.  But later, I began to have doubts about it.  In the words of young James Clerk Maxwell, I started to wonder about "the particular go of it." 

You see, the way the DeMaw mixer is set up,  both of the diodes are simultaneously on and off.  This has the effect of "chopping up" the incoming RF at a rate set by the VFO frequency.  Boom.  Fournier.  Mixing.  Great.  

But look at the mixer at the top of this post.  Here the VFO signal is coming in on the wiper of the 1k pot. The same signal is hitting both diodes at the same time.  The diodes are not being fed differentially.  So D1 and D2 are NOT both simultaneously tuning on and off.  Instead, when the wiper goes positive, D2 turns on while D1 is off.  On negative swings of the voltage at the wiper, D1 turns on while D2 is off.  For me, this made it a "mystery mixer." 

This reminded me of the sub-harmonic DC receiver I built earlier in the year:  The VFO runs at half the operating frequency, but the diodes are set up to switch on and sample the RF TWICE each VFO cycle.  This is the equivalent of having the VFO at the operating frequency.  

Could it be that this was just a sub-harmonic mixer with the VFO at the operating frequency? (I should note that Doug DeMaw published a design that actually made this mistake.  See:  https://soldersmoke.blogspot.com/2011/07/doug-demay-and-polyakov.html ) I knew that this would sort of work, but it would not work very well.  And the mystery mixer seemed to work very well.  Hmmm. 

I was loaning the DC receiver with the mystery mixer in it to a local high school.  I worried that I was loaning them something that I didn't really understand. I remembered that I'd been trying to figure out this mixer since early 2021:  https://soldersmoke.blogspot.com/2021/02/some-thoughts-on-singly-balanced-mixers.html  

Our beloved book, Solid State Design for the Radio Amateur (SSDRA) has an explanation of this circuit on page 74.  But this explanation didn't seen to work for me.  Check it out. YMMV. 

Bottom line:  I still couldn't figure this circuit out, so left it alone for while.  

The other day I woke up and looked at it with fresh eyes.  Suddenly it hit me.  Although the VFO was hitting the diodes in the same non-differential way as is done in the sub-harmonic mixer,  the RF (signal) is entering the mixer in a differential way.  This means that the two diodes are taking turns sampling the upper side of L2, then bottom side of L2, via L1 and L2.  This results in a complex repeating waveform that is similar to that of diode ring mixer.  Within that complex repeating waveform, there are sum and difference frequencies. I did some noodling on this: 

The key difference between this mixer and the sub-harmonic mixer is the way L2 is positioned:  In the sub-harmonic mixer, there is no differential feed of the RF.  Both diodes get the same polarity of RF.  The VFO switches on D1, then D2.  The RF is sampled at twice the VFO frequency.    But in the mystery mixer that had me scratching my head, the RF is fed to the diodes in differential form.  So while the diodes here are -- as in the sub-harmonic mixer -- being switched on and off sequentially, they are taking turns sampling the top and the bottom of L2.  That provides the complex repeating waveform that we need to get the sum and difference frequencies.  In a DC receiver the difference frequency is audio. 

What do you guys think?  Do I have this right?  How would you characterize this mixer:  Is it multiplying by 1 and 0?  Or is it multiplying by 1 and -1? 

This would be good mixer for a school project.  It is simpler than a mixer with a tri-filar toroid. 

Help SolderSmoke! Playlists for SolderSmoke YouTube Videos

In response to popular demand, I am putting up YouTube Playlists for SolderSmoke videos.  

Most of these lists are rig or project specific.  For example, my Hammarlund HQ-100 has its own playlist. 

But there is also one MASSIVE playlist with about 214 YouTube videos.   This one is especially good if you just want to keep SolderSmoke videos playing in the background as you work on rigs in your hamshack.  This video will also cause a big increase in the "SolderSmoke hours watched" metric of YouTube. 

Here are the Playlists (more will be added over the next few weeks): 

(20) SolderSmoke - YouTube

Here is the big Playlist with 214 SolderSmoke videos: 

(20) Polyakov Direct Conversion Receiver on 80 meters - YouTube

What Coil for the Polyakov Input Circuit? How to calculate a coil value for resonance.

So,  what is the value for L1 and L2?   What coil should I use?  

Michael AG5VG had that question.  And so did I when I built this receiver.  See below for the process I used in answering this question. 

On Tuesday, August 9, 2022 at 10:53:32 PM EDT, Michael S  wrote:

Good Evening Bill,

My name is Michael and I really enjoy your podcast with Pete. I have also spoken with him in regards to the design of a 20M bandpass filter I made for a homebrew rig.  I am currently in the process of making a 20 meter DSB - SC type. Thank you for all the information that you speak and teach about during your podcast. I also enjoy the humor. It's great.

The Polyakov is a simple DC receiver and it amazes me and how the sound quality is. My question is, what is the turns on the toroid for the antenna primary side and the radio secondary and how did you figure out the turns because looking at the schematic it doesn't give that information that I can see. Also how you resonated it with the variable capacitor that looks like a 365pf air variable. 

Thank you for your time and keep up the great work on the podcast and the content on YouTube.

73s,

Michael

AG5VG

My response: 

Good questions Michael.   When I saw the SPRAT article I too was struck by the fact that it didn't give a value for the coil.  But DK2RS did have a large value variable capacitor... And he was billing this as a dual-band (80-40) rig.  So I figured he wanted that LC circuit to resonate as low as 3.5 MHZ and as high as 7.3 MHZ.  So, with a variable cap that goes up to 350 pf, what value L should I use?   I started by calculating the resonant frequency of the frequency mid-way point: 5.1 MHz.   I figured the variable cap should be around 162 pf at the mid-way point.  At this point I went to the on-line resonant frequency calculator: https://www.1728.org/resfreq.htm (a REALLY useful site!).   This site revealed I needed a coil of about 6 uH.  This put me in the ballpark.   But then -- with the site -- I tested it with the values of the variable cap I had on hand.  Mine was 23pf to 372 pf.  (you really need an LC meter to do this kind of thing). 

Again at the resonant freq calculation site:  23 pf and 6uH = 13.5 MHz           372 pf and 6 uH = 3.3688 MHz

This would have been OK, but I wanted to move the frequency range down a bit, so I tried. 6.5 uH 

23 pf and 6.5 uH = 13 MHz                 372 and 6.5uH = 3.23 MHz

Now, how many turns?  First look at the overall coil -- don't worry about taps at this point.    I use the Toroid Turns Calculator: http://toroids.info/

 Start by asking yourself "What core do I have on-hand?    Let's say you have a T-50-2 (red/clear).   The calculator shows you need about 36 turns.  Do-able, but physically kind of tight.  

I found a big core in my junk box.  A T-106-2.  The calculator showed I'd need about 22 turns on this core. It was much easier to get these turns on the larger core.  

You have to measure the core after you wind it to make sure you are at the desired inductance.   One side of the main coil went to ground, the other side to the top of the variable cap. 

 Now for the taps and secondaries:  The schematic shows a tap.  This is usually about 1/4 of the number of turns up from ground.   I picked about 5 turns, and wound a little tap in there at that point -- that tap went to the antenna.  You also have a secondary coil --no value is given, but based on experience I guessed around 5 turns -- I wound these turns on top of the primary one lead went to ground, the other went to the diodes and the switch. 

 The last thing to do is to see if the circuit resonates on both bands that you want to receive.  You can do this with a signal generator, or with the band noise:  Hook up an 80 meter antenna.  Put the cap closer to its max value and tune the cap -- can you hear band noise?  Or can you hear (or see on a 'scope) a signal at 3.5 MHz?   You should be able to peak it with the main cap.  Try to do the same thing on 40 meters -- here the variable cap should be closer to minimum capacity. 

 That's it.  That's how I did it.   You can do it too!   Good luck with the Polyakov.  

 One hint:  Building the VFO is the hard part.  You can get started by using a signal generator in place of the VFO.  Just make sure you have the level right -- around 620 mV input.  

 Good luck -- Let us know if you have trouble. And please let us know how the project goes.   

 73    Bill    N2CQR  

20 meter rig built by AG5VG

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.

Polyakov (RA3AAE) Direct Conversion Receiver: 40 meter DC RX with VFO at 3.5 - 3.6 MHz (with video)

I've been reading about Polyakov (or "sub-harmonic") Detectors for a long time: 

https://soldersmoke.blogspot.com/search/label/Polyakov--Vladimir

But until now, I never built one.  Recently,  Dean KK4DAS and the Vienna Wireless Makers group have been building a Direct Conversion receiver.  Their receiver uses an Si5351 as the VFO, but of course Dean and I have decided to try to do things the hard way by building non-digital VFOs.  At first we just came to the conclusion that my earlier Ceramic Resonator VFO wasn't much good (it drifted too much).  This led us into standard Colpitts and Armstrong VFOs, and the fascinating world of temperature compensation.  Then I remembered the Polyakov circuit -- this would allow us to use a 3.5 MHz VFO on the 7 MHz band.  Lower frequency VFOs are easier to stabilize, so I started building my first Polyakov receiver.  You can see the results (on 40 meters) in the video above. 

I started working with a circuit from SPRAT 110 (Spring 2002). Rudi Burse DK2RS built a Polyakov receiver for 80 and 40 that he called the Lauser Plus.  (Lauser means "young rascal" or "imp" in German.) For the AF amplifier, I just attached one of those cheap LM386 boards that you can get on the internet.  With it, I sometimes use some old Iphone headphones, or an amplified computer speaker. 

The Polyakov mixer is a "switching mixer."  The book excerpt below shows how I understand these circuits.  The enlightenment came from the Summer 1999 issue of SPRAT (click on the excerpt for an easier read): 

Leon's circuit shows us how a simple switching circuit in which the switches are controlled by the VFO can result in an output that has the sum and difference components. That is the hallmark (and most useful part) of real mixing.  Remember -- we say that mixing happens in non-linear circuits when the passage of one signal depends on what is happening with the other signal.  A switch is as non-linear as you can get! And that switch is being controlled by the VFO.  

In a Direct Conversion receiver we usually run the VFO at the operating frequency. This results in audio just above and just below the operating frequency. 

The Polyakov Direct Conversion circuit is a bit different.  It has the switches (the diodes)  turned on twice each cycle:  When the VFO voltage goes to a positive peak, this turns on one of the diodes.  When the VFO goes to a negative peak, this turns on the other diode.   So in effect the switch is being turned on TWICE each cycle.  So with the Polyakov you run the VFO at HALF the operating frequency.  For a DC receiver designed to run around 7.060 MHz, you build a VFO at around 3.53 MHz.  This has some immediate advantages.  My favorite is that it is easier to get a VFO stable at a lower frequency.  It is easier to stabilize a VFO at 3.53 MHz than it is at 7.060 MHz. 

When you open that SW 1 switch in the Lauser Plus, you no longer have a Polyakov mixer.  Now you just have a diode mixer.   It will be opening and closing once each cycle at the VFO frequency.  DK2RS used this to cover not only the 40 meter band (in Polyakov mode) but also the 80 meter band (in single diode detector mode).  That is why DK2RS has that big variable capacitor in the input circuit -- that LC circuit needs to tune all the way down to 3.5 MHz and all the way up to around 7.3 MHz.  (I used a coil of about 6.5 uH to do this.) 

With just one diode and operating at 80 meters, it works, but not as well as it does in the Polyakov mode on 40.  I can pick up 80 meter signals, but in this mode there seems to be more of an "AM breakthrough" problem. "Experimental Methods in RF Design" on page 8.11  describes what is going on (the last sentence is most relevant here): 

Here are some very good links with information on the Polyakov receiver: 

LA8AK on Polyakov: http://noding.com/la8ak/c21.htm

LA8AK's Home Page: http://www.agder.net/la8ak/index1.htm

LA8AK SK: http://www.agder.net/la8ak/   Almost seventeen years after his death he continues to help his fellow radio amateurs through his web sites.  TNX OM!  FB! 

BH1RBG: https://sites.google.com/site/linuxdigitallab/rf-ham-radio/dc-polyakov-based-first-dc-receiver

Hack-A-Day on Polyakov: https://hackaday.com/2015/11/15/polyakov-direct-digital-synthesis-receiver/

I will post a video tomorrow showing the receiver in operation on 80 meters.  

Three cheers for Vlad Polyakov, RA3AAE

SolderSmoke Podcast #239: Hex DX, VFO Temp Comp, DC RX, Polyakov!, DX-100, Wireless Set, Farhan's "Daylight Again" HDR rig, MAILBAG

N2CQR Hex Beam Aimed at Europe

SolderSmoke #239 is available for download: 

http://soldersmoke.com/soldersmoke239.mp3

TRAVELOGUE: 

James Webb Space Telescope.  Mars returning to opposition in early December.   

BILL'S BENCH

Hex Beam K4KIO - on roof – TV Rotor – 20-17-12  Lots of fun.  Working Japan regularly, Australia, South Africa on long path 17,000 miles.  52 countries SSB since July 11.

VFOs and Temp stabilization.  Dean KK4DAS found my ceramic resonator VFO for DC receiver drifty. He was right.  So I built a real LC Colpitts VFO.  Got me into temp stabilization.  A new hobby!  An obsession.  HT-37 and Ht-32 parts. Ovens?  WU2D’s second VFO video.  Understanding thermal drift and how to address it. Split stator caps.  Cut and try.  

Built a Polyakov DC Receiver. https://soldersmoke.blogspot.com/2010/03/polyakov-plus-dual-band-receiver-with.html  Lauser Plus.  Lauser = Imp or Young Rascal!  DK2RS.  He used a ceramic Resonator VXO at 3.58 MHz.   Mine works great on 40 with VFO running 3.5 -- 3.65 MHz. See schematic below. 

On 40 AM with DX-100 and MMMRX.  DX-100 died.  12BY7 VFO buffer went bad.  How common is failure in this tube type?  Nice QSO with Tim WA1HLR about the DX-100.

Got my Dominican license:  HI7/N2CQR!  SSSS on the way.   Thanks to Radio Club Dominicano and INDOTEL.

Getting more active in the Vienna Wireless Society.  

BOOK REVIEW:  

"The History of the Universe in 21 Stars” by Giles Sparrow.  Written during the pandemic.  Published by Welbeck, in London. https://www.amazon.com/History-Universe-21-Stars-imposters/dp/1787394654  Also:  From “Atoms to Amperes” by F.A. Wilson available for download.  See blog.

SHAMELESS COMMERCE DIVISION:   

Todd K7TFC getting ready to launch “Mostly DIY RF.”   I used his TIA boards in my 1712 rig.  He will have boards like this and much more.  Stay tuned.

I need more viewers on YouTube.  They want 4,000 hours IN A CALENDAR YEAR!  Please watch!

FARHAN’S NEW “DAYLIGHT AGAIN” RIG.  Analog.  VFO.   Comments, observations. We need to get him on the podcast.  Maybe two shows: SDR and HDR. 

PETE'S BENCH

Time very limited. But still sharing lots of tribal wisdom.

Wireless set with tubes!

Tool recommendation – Air compressor

 MAILBAG:

Farhan VU2ESE – Speaking of big antennas “Whenever I look at the huge construction cranes in Hyderabad, I always think how one could make 160m, 4 element yagi using it as a boom..”

Todd K7TFC in Spain, spotting Log Periodics in Madrid.

Andreas DL1AJG:  Can Biologists fix Radios?

Janis AB2RA Wireless Girl.  Expert on Hammarlunds.  And was my first contact with the Tuna Tin 2. She too was HB!

Peter Parker VK3YE on Owen Duffy VK1OD

Lex PH2LB on homebrew radio

Would this really be homebrew?  Mail from H-A-D article on FM receiver

F4IET a DSB rig from France

Ciprian got his ticket YO6DXE    

Josh G3MOT sent us a good video about the Vanguard satellite and IGY.

Dave Wilcox K8WPE bought Chuck Penson’s Heathkit book.

Rogier -- So many great articles and links from PA1ZZ

Bill AH6FC  Aloha. Retiring.  Wants to build.  Mahalo!

Grayson KJ7UM  Working on an Si5351.  Gasp.

Mike KE0TPE viewing YouTube while monitoring 6 meters.   He will have a lot of time to watch!

Chris KD4PBJ spotted Don KM4UDX from VWS FB

Mark WB8YMV building a superhet.  Having trouble with 455 kc IF can filter.

Walter KA4KXX Great comment on the Daylight Again rig. 

Ramakrishnan Now VU2JXN was VU3RDD.  Found lost Kindle with SolderSmoke book on it. Building SDR rig from junk box.  Trouble with the LM386. 

Pete, Farhan and Tony:  Shelves of Shame

Daylight Again by Farhan

The Polyakov receiver I built yesterday (from SPRAT 110, 2002!)

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. 

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.  

A Phasing Receiver from Montenegro (Video)

Thanks to Peter Parker VK3YE for alerting us to this beautiful receiver from Bore 4O6Z in Montenegro.  This may be our first ever report on a Montenegrin rig.  And it is a thing of beauty. Bore says it is based on a phasing circuit by homebrew legend V. Polyakov, RA3AAA.

Here is 4O6Z up on the tower:

AA1TJ Crosses the Pond with 10 milliwatts

From a Facebook Post by Mike, AA1TJ:

Made 7 contacts with this tiny transceiver on 20m CW today with an RF output power of 10milliWatts. Five were stations in Georgia (GA QSO Party). The 6th was a regular QSO with a guy in Mississippi.
I answered a DX station calling CQ at 2230Z. Hearing nothing in response, I sent my call sign a half-dozen times anyway. More silence. As I was reaching for the knob to QSY he suddenly returned my call! ...Carlos, CT1BQH northeast of Lisbon, Portugal (that's him in the second photo). I was only 329 on his end but we kept it going for three minutes!

FYI: the transmitter (top circuit board) begins with a 3.58MHz ceramic resonator VXO (a 2N706 from the early 60's). That drives a push-push frequency doubler built around another 1960's-vintage, 2N2644 (obsolete stock from atop Mt. Mansfield, kindly given to me by Rich at Vermont Public Television). On receive, the 7MHz energy is routed via a DPDT relay (the orange rectangle) to the sub-harmonic (Polyakov) mixer located on the lower board. One stage of AF amplification is provided by a 2N333 that came off the GE assembly line in November of 1958. The DPDT relay is keyed directly. On transmit the 7MHz energy feeds a second push-push frequency doubler to produce 10mW at 14MHz (all spurs -35dBc, or less, with only the output resonator). The relay also switches the antenna between the transmitter and receiver.
Gosh, that was fun!

CT1BQH

Our book: "SolderSmoke -- Global Adventures in Wireless Electronics" http://soldersmoke.com/book.htm Our coffee mugs, T-Shirts, bumper stickers: http://www.cafepress.com/SolderSmoke Our Book Store: http://astore.amazon.com/contracross-20

Steve "Snort Rosin" Smith: Silver-Tongued Devil! Dual-band "Vlad the Inhaler" RX

You'll see in the comments attached to my last blog post that our man on the left coast, Steve Smith, gave that cute little Doug DeMaw/Vlad Polyakov receiver a name that might set American-Russian ham relations back a bit: He called it "Vlad The Inhaler." Good one Steve! (But you might want to stay out of the diplo game!)

It occurred to me that with the installation of one little switch in the diode part of the circuit, we could turn this into a dual-band RX. Take a look here:

http://soldersmoke.blogspot.com/2010/03/polyakov-plus-dual-band-receiver-with.html

Check out "SolderSmoke -- Global Adventures in Wireless Electronics"
http://soldersmoke.com/book.htm

Doug Demaw and Vlad Polyakov

I was looking through W1FB's Design Notebook the other day and I came across the above schematic for a direct conversion receiver (page 111). Note the Polyakov detector. One strange thing though: Doug was running the oscillator at the operating frequency. I thought the big advantage of the Polyakov design was that you ran the oscillator at half the operating frequency (that's why it is sometimes called a "subharmonic" detector). Any ideas on why Doug did it this way?

Check out "SolderSmoke -- Global Adventures in Wireless Electronics"
http://soldersmoke.com/book.htm

SolderSmoke Podcast #133

http://soldersmoke.com/soldersmoke133.mp3

April 19, 2011

-- KindleSmoke
-- Amazon's Whispernet (not to be confused with WSPRnet)
-- April 1 and the Perils of Plausibility
-- The difference (significant!) between Cream of Tartar and Tartar Sauce
-- A short Italian lesson
-- Polyakov QRSS
-- Snort Rosin's Mighty Mite (NOTE: NO FILTER!)
-- Fly Fishing in Space: 2 Meter Packet Beacons and the International Space Station
-- Upside-down Amplifier
-- Rock and Roll and Ham Radio: The Bob Heil Story
-- The Shuttle Discovery Lands in Northern Virginia
-- MAILBAG

Check out the new Ikea Microphone!

The Polyakov QRSS Receiver

Of course, we like it because it is in an Altoid-like box. And because it is Direct Conversion. And because it is used for QRSS, with the output fed to an on-line grabber. But this one is EVEN BETTER because it uses a Polyakov detector!

Check out PA1GSJ's receiver: http://www.qsl.net/dl1gsj/html/qrssrx30.html

View the output (live): http://www.qsl.net/dl1gsj/qrss/

More on Polyakov detectors: http://soldersmoke.blogspot.com/search?q=polyakov

SolderSmoke Podcast #123

Listen at: http://www.soldersmoke.com

SolderSmoke #123
4/1/2010
Spring! Equinox! Sap in Vermont! Aurora in Norway! Birds singing in Rome!

SolderSmoke on Oprah. Program Schedule:http://www.soldersmoke.com/oprah.html

Special Report from MASSCON QRP Convention by Mark, NX1K
Propagation improvingWSPR as a personal sunspot detector
Building the W3PM WSPR rig
Colpitts oscillators
Polyakov's "Russian Mixer" BANDSWEEP: A low-frequency sweep by Chris, KD4PBJ
EMRFD on Direct Conversion and Direct Aversion
Faraday's toroid diagram
NA5N's ISBN
MAILBAG
SolderSmoke--THE BOOK:
http://stores.lulu.com/store.php?fAcctID=3999032
SolderSmoke--THE STORE:
http://www.cafepress.com/SolderSmoke

AA1TJ on Polyakov, Scrounging Parts from a Wehrmacht Tank, QRPp Success

Caro Bill, FB on your 20mW DSB WSPR DX! I've never actually been on the air during the upside of a solar cycle. This is wicked exciting! Thank you for the nice write-up on your Soldersmoke Blog, OM. I heard back from Fabio this morning; he seems to be as excited as I am (please see his message below). Italy is my fourth DX contact and third country with 10mW. Two weeks ago I worked F5NBX twice and FM5LD once. Again, all great fun... I'm glad to hear of your interest in the Vladimir Polyakov's mixer. I can attest to Vlad's claim about its resistance to SWBCI. So far, I've used silicon and germanium diodes, MOSFETS, and even a saturating inductor for the subharmonic, commutating switches. I haven't mentioned it till now, but leading up to my Gigi regenerodyne receiver, I built Polyakov reflexed 40m receiver that was lots of fun. The RF input signal first passed through a simple BPF on its way to the cathode of a triode, grounded-grid RF amplifer. The amplified signal next passed through the Polyakov anti-parallel diodes. The resulting AF signal was then re-injected into the grid of the same triode (now working as a common-cathode amplifier). In typical "reflex' fashion, the same tube amplifies first at RF and again at AF. Looking at my notebook entry, I was able to "plainly hear" a 1.0uVrms signal and in a week of operation I hadn't noticed any SWBC breakthrough. Actually, the subharmonic mixer was well-known by 1976, but I'd never seen it used by hams at HF until Polyakov popularized the idea. Our Eastern European comadres took a liking to it right away but it took some time before it finally caught on in the West. By the way, I had a short email exchange with Vladimir a few months ago. I'm pleased to report he's a really nice fellow. He happened to mention that his first amateur radio transmitter was built using parts that he salvaged from an abandoned Wehrmacht tank. It seems radio components were extraordinarily hard to come by in those days. Amazing... Have fun, Bill! Mike, AA1TJ ---------- Forwarded message ---------- From: Fabio Ventrone Date: Sat, Mar 27, 2010 at 9:16 AM Subject: Re: IZ0PEC de AA1TJ/QRPp To: Michael Rainey Dear Michael, Many many compliments for your qrpp station! Really exciting to have qso in this conditions... It's actually the first time I can connect qrpp oversea... Unbelievable, something we can tell our friends forever!!! I was transmitting with 4 el antenna and something more than 100w. I will have to take back my 817 and try to call dx as you bravely did!!! Best 73 qsl Fabio de IZ0PEC 2010/3/26, Michael Rainey > Dear Fabio, > > Thank you for your patience in copying my signal on 20m CW this evening. I > had been calling DX stations for several hours but you were the only one to > answer. My homebrew rig has an output power of only 10mW (0.010 watts). The > distance between us is 8793km; nearly one million km per Watt!. The antenna > here is simply an end-fed wire. > > I am amazed that you heard my 10mW signal in Rome. It's fantastic! > > Again, thank you for patiently listening for my weak signal, OM. It would > not have been possible without your very kind efforts. > > Ciao, > Mike, AA1TJ