EI0CL as Heard on a uBITX in the Dominican Republic

 27 December 2022: My old friend Mike EI0CL:

https://www.youtube.com/watch?v=LYlI603BOXY

Who was Jean Shepherd?

Find out in this 16 minute preview video: 

https://www.youtube.com/watch?v=83rIeZVqdcs 

EXCELSIOR!

Report from the Dominican Republic -- HI7/N2CQR

https://www.youtube.com/watch?v=KK2ISLCxCh8&t=8s

WATCH Jean Shepherd in 1982 (with Letterman)

https://www.youtube.com/watch?v=3naRDmT3QGA 

A Blast from the Past: TR on Homebrewing (sort of)

 

"It is not the critic who counts; not the ham who points out how the homebrewer stumbles, or where the builder of rigs could have built them better. The credit belongs to the ham who is actually at the workbench, whose hands are scarred by solder and metal and glue; who strives valiantly; who errs, whose amp oscillates again and again, because there is no effort without error and shortcoming; but who does actually strive to build his rigs; who knows great enthusiasms, the great devotions; who spends himself in a worthy cause; who at the best knows in the end the triumph of homebrew  achievement, and who at the worst, if he fails, at least fails while daring greatly, so that his place shall never be with those cold and timid operators who neither know victory nor defeat.”

Some Direct Conversion Receiver History

Here is the article by Wes Hayward and Dick Bingham that started it all:  

https://worldradiohistory.com/Archive-DX/QST/60s/QST-1968-11.pdf

page 15 

Here's a discussion by Wes of the original project: 

https://www.n5dux.com/ham/files/pdf/Direct%20Conversion%20Receivers%20History%20-%20W7ZOI.pdf

Here is an article about DC receiver in phasing rigs by Gary Breed K9AY: 

https://worldradiohistory.com/Archive-DX/QST/80s/QST-1988-01.pdf

page 16

Roy Lewallen W7EL's Optimized transceiver (with a direct conversion receiver): 

https://worldradiohistory.com/Archive-DX/QST/80s/QST-1980-08.pdf

page 14

Jerry KI4IO on Building a DC Receiver

https://groups.io/g/qrptech/message/17

Michael Black wrote on March 5, 2014 at 3:54 PM

Isn't it a bit dated?

When "direct conversion" receivers came along in 1968 (the concept existed before, just not the name), it was to build simple receivers. They took over from regens (which of course for the purpose of CW and SSB, were "direct conversion"), and kind of bumped simple superheterodyne receivers out of the magazines.

And they were easy to build, so long as the meaning of the dots were standard, but good performance was elusive. Endless articles about better mixers or more front end selectivity, and still the same basic results The Heathkit HW-7 comes along, and endless mods to that, but still no perfection.

Slowly the move was back to simple superhets, especially with some of the early seventies ICs intended for radio, and then ladder filters came along (actually they came early at least by 1974 from the UK and/or France, but while they got mention in North America early-ish, it took some years before the KVG filters were pushed aside and ladder filters got the spotlight).

And then wham, in the mid-eighties someone caught on. The problem with direct conversion receivers wasn't the mixer (well not once it was a balanced mixer) or lack of front-end selectivity, it was the matter of properly terminating the mixer. The problems that had been there all along were gone. And direct conversion receivers started their climb to being complicated receivers.

I guess it was that receiver by Gary Breed in QST circa 1986 with diode balanced mixers and termination that changed things. A new concept, but not really, I remember an article in QST in 1974 where a DBM diode mixer for VHF was properly terminated, and yet the concept went no further until a decade later.

Actually, I think there is a tiny bit about mixer termination in "Solid State Design for the Radio Amateur" but it never went so far as to say "this is what we need".

Or perhaps that tiny transceiver by Roy Llewellyn in QST was the first, I cant' remember. It certainly used a diode mixer with termination for the receiver.

And that set the stage for Rick Campbell's various receivers, all counting on termination of the mixer.

The ideas can often be there, but not applied because technology doesn't allow it yet, or just not looking that far beyond this month's construction article.

Michael

Kludge. Rhymes with Fudge. On PBS!

At about 12:26 in this video, David Brooks uses the word "kludge" on the Public Broadcasting System. He pronounces it CORRECTLY!   (But then he and the host question whether it is a real word.) 

https://www.youtube.com/watch?v=G1qSEbr8W7I 

Did Marconi Cross the Atlantic with a Coherer? No.

Jagadish Chandra Bose

A while back I posted the re-mastered version of the excellent "Secret Live of Machines" episode  on radio.  Among other amazing things, Tim and Rex build a spark radio transmitter and a receiver that uses a coherer and a tapper.  They even set up a demonstration and sent signals from the pier to the shore.  Very cool.  

I shared this with George WB5OYP of the Vienna Wireless Society because he had been looking carefully at the gear that Marconi allegedly used to make that first transatlantic contact.  George wondered if Marconi could have really done this with a coherer as his detector; he was -- for good reason -- skeptical.  Could a glass tube filled with metal filings really detect radio waves sent from across the mighty Atlantic?  

Marconi claimed that he did it with a coherer as the detector: 

On December 12, 1901, Marconi attempted to send the first radio signals across the Atlantic Ocean, in spite of predictions that the radio waves would be lost as the earth curved over that long distance. He set up a specially designed wireless receiver in Newfoundland, Canada, using a coherer (a glass tube filled with iron filings) to conduct radio waves, and balloons to lift the antenna as high as possible. The signals were sent in Morse code from Poldhu, Cornwall, in England. Marconi later wrote about the experience:

"Shortly before midday I placed the single earphone to my ear and started listening. The receiver on the table before me was very crude -- a few coils and condensers and a coherer -- no valves, no amplifiers, not even a crystal. But I was at last on the point of putting the correctness of all my beliefs to test. The answer came at 12: 30 when I heard, faintly but distinctly, pip-pip-pip. I handed the phone to Kemp: "Can you hear anything?" I asked. "Yes," he said. "The letter S." He could hear it. I knew then that all my anticipations had been justified. The electric waves sent out into space from Poldhu had traversed the Atlantic -- the distance, enormous as it seemed then, of 1,700 miles -- unimpeded by the curvature of the earth. The result meant much more to me than the mere successful realization of an experiment. As Sir Oliver Lodge has stated, it was an epoch in history. I now felt for the first time absolutely certain that the day would come when mankind would be able to send messages without wires not only across the Atlantic but between the farthermost ends of the earth."

https://www.pbs.org/wgbh/aso/databank/entries/dt01ma.html

-------------------------------------

I mentioned this in SolderSmoke Podcast #242.  This resulted in a very interesting message from Steve AB4I: 

The reason that I am writing is to comment on the coherer and Marconi's transatlantic test. One of my research interests in my doctoral studies was the development and evolution of early radio detectors.  Marconi did not use a coherer for the successful transatlantic tests, but secretly used a detector and telephone receiver that had been invented by the Indian polymath Jagadish Chandra Bose of Calcutta.  Bose's iron-mercury-iron detector was sensitive to a wide range of wavelengths and he used the detector in his 60-GHz millimeter wave and experiments. Bose presented his results to the Royal Society in London in 1899 and his paper was published in the Proceedings of the Royal Society the same year.  Marconi came by the mysterious mercury coherer detector through a friend in the Italian Navy who constructed the device from Bose's paper in the Proceedings in an effort to improve the performance of the Marconi equipment aboard . The Bose detector was superior to anything that Marconi had and was key to the success of the transatlantic tests and for Marconi's subsequent successes. Marconi then filed a patent for the detector in his own name in 1902, even though it was not his invention.  

A lot of nasty business went on in the early days of wireless. The scandal around the "Italian Navy coherer" raged for years, but eventually the role of Bose was revealed. The popular view of Marconi as radio inventor extraordinaire is idealistic, because he did not actually invent anything, but he was very good at dragging laboratory hardware into the real world to serve practical ends.  In every case, crucial parts of Marconi's patents were stolen or copied from other sources and successfully defended through aggressive litigation, deep financial backing, and extensive public relations through advertising and newspaper interviews. Marconi absolutely deserves recognition for his successes in the development of practical wireless communications although he is not noted for his ethics. Marconi's reputation is a bit tarnished nowadays, but that of Jagadish Chandra Bose has blossomed and he is now acknowledged for his epochal work that was fully a half-century before his time.

As for the coherer, we still do not have a full understanding of how the thing actually works.  The cohesion effect of small particles clumping together in the presence of a static charge has been known from antiquity as evidenced by dust bunnies under beds through the ages. There were coherer-like lightning arrestors used on telegraph lines just after the American Civil War and in 1879 David Hughes found that a carbon microphone with loose contacts could detect arcing in nearby equipment and from considerable distances too.  He was told that the phenomenon was nothing new and he just missed the discovery of radio waves.  Thanks to some monumentally bad advice we now speak of Hertzian Waves instead of Hughian Waves.  Branly made a detailed study of resistance changes in metal particles and is generally acknowledged as the inventor of the coherer detector. Oliver Lodge coined the name 'coherer' and demonstrated the detection of Hertzian waves in 1894 a few months after Hertz's death.  Lodge wrote a tribute to Hertz, which was to inspire the young Marconi to begin his own experiments with Hertzian waves.

--------------------------------

Hack-A-Day looked at all this back in 2016:  

https://hackaday.com/2016/01/19/j-c-bose-and-the-invention-of-radio/

Here are the key passages:  One improvement invented by Bose in 1899 was the iron-mercury-iron coherer, with a pool of mercury in a small metal cup. A film of insulating oil covered the mercury, and an iron disc penetrated the oil but did not make contact with the liquid mercury. RF energy would break down the insulating oil and conduct, with the advantage of not needing a decoherer to reset the system.

Bose’s improved coherer design would miraculously appear in Marconi’s transatlantic wireless receiver two years later. The circumstances are somewhat shady – Marconi’s story about how he came up with the design varied over time, and there were reports that Bose’s circuit designs were stolen from a London hotel room while he was presenting his work. In any case, Bose was not interested in commercializing his invention, which Marconi would go on to patent himself.

---------------------------------------

Here is a lot more background on Dr. Bose: 

http://www.cse.iitm.ac.in/~murthy/sirjcbose.pdf

----------------------------

I think the more we learn about Marconi, the less admirable he seems. 

Faust Gonsett and the SB-33 in 1963

 

Click on the images for better views

When this ad appeared in 73 Magazine in February 1963 I was 4 years old, living on Manhattan Island.  Pete N6QW was in the Navy, heading to Midway Island. 

Pete writes: 

-------------------

This ad has a tremendous impact on the foundations of our hobby. The SBE-33 was pure genius in its design and implementation.

1. It is a hybrid rig using Germanium transistors –the transistor was only 15 years old
2. The Mechanical band switching showed the strong use of mechanical assemblies
3. The small size was simply amazing
4. The Bi-lateral circuitry predates any Bitx circuits.
5. The urban legend was that a team of illuminati were involved in its design (Don Stoner is one name that pops up)
6. The Japanese were a quick study and the FTdx100 in 1967 is a result, only better.
7. Many are still around in shacks. I have three

 

Gonset was well known for innovative designs – the Gooney Box is another example. Look at all of his compact mobile equipment.

 

The next point – the final owner of SBE was Raytheon thusly the next generation of SDR Radio Equipment for the US Air Force can trace its pedigree to the SBE-33.

 

This was the appliance box of 1963. I saw my 1^st SBE-33 (August 1963) when likely you were in the 2^nd Grade and I was headed off to Midway island.

-----------------------

 

I have an SBE-33 that N6QW sent me. Thanks again Pete!

Also, I'd like to note that W6VR had a very cool name.  Faust Gonsett.  I just sounds like the name of a real radio guy.  Google says this of the given name Faust: 

"Faust as a boy's name is of Latin origin, and the meaning of Faust is 'fortunate, enjoying good luck.'   Indeed. 

Save the Antenna!

SolderSmoke fans have an interest in saving this antenna because it is the site of one of the most amazing RF troubleshooting stories of all time:  Wilson and Penzias were trying to track down some noise.  At one point they thought it might be the result of bird droppings.  Uh, no, it was really the result of the Big Bang!  Please sign the petition: 

https://tworivertimes.com/holmdels-historic-big-bang-antenna-at-risk-residents-say/

 

https://actionnetwork.org/petitions/save-big-bang-antenna

R-390s, KWM-2s, Airplanes, and Magnetic Loops -- A Really Interesting Interview with Ted Robinson K1QAR

Eric Guth 4Z1UG has a really interesting interview with Ted Robinson K1QAR.  

I really enjoyed hearing Ted's inspiring story: 

https://www.youtube.com/watch?v=SUUzlKMMANg

https://www.qsotoday.com/podcasts/K1QAR

Listeners will like the discussion of the R-390 and the KWM-2.  And his talk about airplanes.  And the joy of repair.  

Here is Ted's QRZ.com page:  https://www.qrz.com/db/K1QAR

Thanks Eric!  Thanks Ted! 

Is Envelope Detection a Fable? Or is it Real? Diodes, Square Laws and all that

HOW ENVELOPE DETECTION (SUPPOSEDLY?) WORKS 

Most of us grew up with the above diagram of how a receiver detects (demodulates) an AM signal. Here is how they say it works: 

--  Because of the way the sidebands and the carrier in the transmitted signal interact, we end up with a signal whose "envelope" matches the frequency of modulation. And we just need one side of the envelope. 

-- We used a simple diode to rectify the incoming signal. 

-- A simple filter gets rid of the RF. 

-- We pass the resulting signal through a capacitor and we get  audio,  which we listen to. 

REASONS FOR SCEPTICISM

But recently, a member of my local radio club has questioned this explanation of AM detection.  He maintained that "envelope detection" is not real, and that was actually happening was "square law" mixing. I guess there are reasons for skepticism about the envelope detection explanation:  The envelope detection explanation does seem very (perhaps overly) simple. This does sound a bit like the kind of "dumbed down" explanation that is sometimes used to explain complex topics (like mixing). Envelope detection does seem consistent with the incorrect insistence from early AMers that "sidebands don't exist." (Of course, they do exist.)  All the other detectors we use are really just mixers.  We mix a local oscillator the incoming signal to produce audio.  Envelope detection (as described in the diagram above) seems oddly different.  

Denial of envelope detection can even be found in the ARRL handbook:  On page 15.9 of the 2002 edition we find this:  "That a diode demodulates an AM signal by allowing its carrier to multiply with its sidebands may jar those long accustomed to seeing diode detection ascribed merely to 'rectification.'  But a diode is certainly non-linear. It passes current only in one direction and its output is (within limits) proportional to the square of its input voltage. These non-linearities allow it to multiply."  

ISN'T THIS REALLY JUST MIXING, WITH THE CARRIER AS THE LO? 

It is, I think, tempting to say -- as the ARRL and my fellow club member do -- that what really happens is that the AM signal's carrier becomes the substitute for the VFO signal in other mixers.   Using the non-linearity of the square law portion of the diode's characteristic curve, the sidebands mix with the carrier and -- voila!  -- get audio. In this view there is no need for the rectification-based explanation provided above. 

But I don't think this "diode as a mixer, not a rectifier" explanation works: 

In all of the mixers we work with, the LO (or VFO or PTO) does one of two things: 

-- In non-switching mixers it moves the amplifier up and down along the non-linear characteristic curve of the device.  This means the operating point of the device is changing as the LO moves through its cycle.   A much weaker RF signal then moves through the device, facing a shifting operating point whose shift is set by the LO.  This produces the complex repeating periodic wave that contains the sum and difference frequencies. 

-- In a switching mixer,  the device that passes the RF is turned on and off.  This is extreme non-linearity.  But here is the key:  The device is being turned on and off AT THE FREQUENCY OF THE LO.  The LO is turning it on and off.  The RF is being chopped up at the rate of the LO. This is what produces the complex repeating wave that contains the sum and difference frequencies.  

Neither of these things happen in the diode we are discussing.  If you try to look at the diode as a non-switching mixer, well, the operating point would be set not by the carrier serving as the LO but by the envelope consisting of the carrier and the sidebands.   And if you try to look at is as a switching mixer you see that the switching is being controlled not by the LO but by the envelope formed by the carrier and the sidebands.  

Also, this "diode as a mixer" explanation would require the diode to be non-linear.  That is the key requirement for mixing.   I suppose you could make a good case for the non-linearity of solid state diodes, but the old vacuum tube diodes were quite linear.  The rectifying diode mixer  model goes back to vacuum tube days.  The "diode as rectifier" model worked then.  With tubes operating on the linear portion of the curve, the diodes were not  -- could not -- have been working as mixers.  We have just substituted solid state diodes for the tubes.   The increased non-linearity of the solid state diodes does introduce more distortion, but the "detection by rectification" explanation remains valid. 

Even in the "square law" region (see diagram below) an AM signal would not really be mixed in the same way as signals are mixed in a product detector.  Even in the square law region, the diode would be responding to the envelope. Indeed, the Amateur Radio Encyclopedia defines "Square Law Detector" as "a form of envelope detector."   And even in the square law region, the incoming signal would be rectified. It would be moving above and below zero, and only one side of this waveform would be making it through the diode.   Indeed the crystal radio experts discuss "rectification in the square law region" (http://www.crystal-radio.eu/endiodes.htm )  So even in the square law region, this diode is a rectifying envelope detector. 

http://www.rfcafe.com/references/electrical/ew-radar-handbook/detectors.htm

THE BEST PROOF

Here is what I think is the best proof that the "envelope detection" explanation is real:  In this video, we see someone build an envelope detector in a simulator.  Watch as he then traces the signals as they move through the diode, the RC filter, and the coupling capacitor.  He goes through it cycle-by-cycle.  You can clearly see how the rectification of the AM leads to envelope detection.

 

The rectifying envelope detection model goes way back in radio history, back to when authors did not shy away from complex technical explanations.  Terman knew how mixers worked, and his 1943 "Radio Engineers Handbook" went to 1019 pages. Terman presented it as a rectification-based detection of the envelope.  I think envelope detection is real, and that Dr. Terman was right. 

--------------------------------------

Some links that might help: 

Analog Devices  has a very good, rigorous site showing how envelope detectors work: 

https://wiki.analog.com/university/courses/alm1k/circuits1/alm-cir-envelope-detector

Here is great article by W4ENE on Envelope Detectors and how they have been used in ham receivers over the years: 

http://www.tonnesoftware.com/appnotes/demodulator/EnvelopeDemodulators.pdf

This QEX article begins with some useful vector diagrams showing how the sidebands and carrier combine to form an envelope that is equal to the frequency of mondulation: https://swling.com/blog/wp-content/uploads/2009/08/syncdetectionarrl.pdf

RF Cafe has some good graphs showing the linear and "square law" portions of the crystal diode's curve (see above): http://www.rfcafe.com/references/electrical/ew-radar-handbook/detectors.htm

The crystal radio guys have a good take on square law detection (note, they just see it as rectification, but on a lower, more parabolic portion of the curve): http://www.crystal-radio.eu/endiodes.htm

Here is a good booklet from 1955 on AM Detectors: https://worldradiohistory.com/BOOKSHELF-ARH/Rider-Books/A-M%20Detectors%20-%20Alexander%20Schure.pdf

Cargo Culting, Appliances, and Homebrewing

Hack-A-Day had an interesting short piece on cargo culting and computer coding:

https://hackaday.com/2022/12/03/cargo-culting-and-buried-treasure/  

Obviously there is a lot here that it applicable to ham radio.  Feynman admonished us to try to deeply understand what we are doing.   Do we risk cargo culting when we make use of gear that we really don't know anything about?  Or when we use a chip that we don't really understand?  (I'm looking at you, Si5351.)

I guess we can't really understand some of this stuff as deeply as Feynman would like -- can anyone describe the signal flow in a CPU chip?  I don't think so.  And Feynman would be the first to admit that no one really understands quantum mechanics. Still, as the author notes, we should be cognizant of the gaps in our understanding.  For there dragons lie.  Or opportunities to learn. The comments on the Hack-A-Day post are mostly pretty good. 

SolderSmoke Podcast #242 Mars, New Hams, Direct Conversion, SDR Console, Proficio, PSSST, 8 meters, A BIG MAILBAG

SolderSmoke Podcast #242 is available

Audio podcast:  http://soldersmoke.com/soldersmoke242.mp3 

Video:  (362) SolderSmoke Podcast #242 December 3, 2022 - YouTube

Travelogue: 

Mars at opposition.  Not as good as 2020 (see charts below)

SSSS Prep

Hearing aids and high frequency loss.

AirPods as hearing aids.

Out in the Shenandoah with a Baofeng.

Success among TJ High School students.

Satellite in space?

 

PARTSCANDY

 

Bill's Bench:

Direct Conversion Receiver:  IT IS ALIVE!  EVEN IN LTSPICE

PTO works very well.

Diode ring -- really needs a diplexer, Radio Marti.

AF amplifier simplicity.

But WHY can't you listen to DSB on a DC receiver? Now I know.

How does a diode detector work?  Is the envelope real?  Is it square law?

The benefits of writing... 1967 and 1966 articles on PTOs and 2Qs.

 

Shameless Commerce Division:

-- MOSTLY DIY RF!  NEWS FROM PORTLAND! Get your free Michigan Mighty Mite. 

-- YouTube Goal Reached.  Thanks! Keep watching.  Subscribe!

-- Keep buying from Bezos using the link on the right-hand column of the blog page.

-- Become a Patreon sponsor!  Left hand column of blog page.

-- I have ads on the blog page, but I have configured to avoid troublesome ads -- dating sites, etc.

 

 Pete's Bench

-- Stepper motor, LCD and Arduino for my little DC RX?

-- SDR Console

-- PSSST, BOMS, Schematics and spoon feeding...

-- Proficio SDR by Multus

-- 8 meters?

  

Mailbag

-- Alan Yates VK2ZAY now also W7ZAY PTOs, trivial motors, a broken ankle...

-- Dhaka Jack AI4SV formerly of Cyprus, Madagascar and Northern Virginia. has moved to France!

-- John WB5OAU/K5MO An old friend.  FMLA as "Glowbugs Noir"

-- Dale Parfitt W4OP on the Homebrew 2Q from 1967

-- Nick M0NTV Glue Sticks, PTOs, DC receivers and AM breakthrough testing.

-- Todd K7TFC suggests “cool” names for DC RX  PT Cruiser? PT109? PT73?

-- Levi replacing Selenium diodes in a Globe VFO.  I am not alone!

-- Juanjo EC5ACA wants to build DC RX .  FB.

-- Dave designed a discrete LM386.  Picked up by Jenny at HackaDay.  Can you build this?

-- George Zaff.  HamRadio Workbench Spiritual Brother of SolderSmoke.

-- Alain F4IET -- Still building DC receiver.  FB.  Sorry I got the call wrong.

-- Drew N7DA Building Pixies with 3D forms.

-- Toni G6XMO in Sheffield getting a 3D printer business going: https://www.whizz3dparts.co.uk/

-- Chuck KE5HPW restoring an old SW-54.  Pete is skeptical.

-- Lex and Jesse like Colin's placement of WYKSYCDS sticker on his Homebrew rig.

-- Jim KI4THZ joined the Vienna Wireless Society -- FB  on the faculty at GMU

-- Tony G4WIF suggested mechanical counter for DC RX PTO freq readout.  I have some in the junk box.

-- Our old friend Jonathan-san in W0XO now a Patreon sponsor.  Origato!

-- Thomas K4SWL sent him video of Tiny SA watching Vatican Radio sign off for the day.

-- Farhan and Chuck Penson liked blog post about Heathkit Digital Rig SS-8000 1978!

-- Ed KC8SBV working on DC receivers -- I recently used the Peppermint Bark box he sent.

-- Old friend Bob KD4EBM on the linearization of the R-390s.  Hard to homebrew one of those!

-- George from VWS trying to figure out how (if?) Marconi got his coherer to work DX...

-- Steve EI5DD sends Connaught Radio news: https://www.docdroid.net/Q1lBoyi/crnews1222-pdf#page=36

 

Won’t have another Podcast until the new year so Happy Holidays to all!  Merry Christmas, Happy New Year! 

 

But why? Why Can't I Listen to DSB (or AM) on my Direct Conversion Receiver?

I've said this before:  I just seems so unfair.  We just should be able to listen to DSB signals with our beautifully simple homebrew Direct Conversion receivers. I mean, building a DSB transmitter is a natural follow-on to DC receiver construction.  And we are using AM shortwave broadcast stations (Radio Marti --I'm looking at you)  to test our DC receivers for AM breakthrough.  But when we tune these stations in, they sound, well, awful.  So unfair!  Why?   Unfortunately it has to do with laws.  Laws of physics and mathematics.  Blame Fourier, not me.  

Over the years there has been a lot of handwaving about this problem.  From Doug DeMaw, for example: 

In his "W1FB's Design Notebook," Doug wrote (p 171):  "It is important to be aware that two DSSC (DSB) transmitters and two DC receivers in a single communication channel are unsatisfactory.  Either one is suitable, however, when used with a station that is equipped for SSB transmissions or reception. The lack of compatibility between two DSSC (DSB) transmitters and two DC receivers results from the transmitter producing both USB and LSB energy while the DC receiver responds to or copies both sidebands at the same time."

That's correct, but for me, that explanation didn't really explain the situation.  I mean we listen to AM signals all the time.  They produce two sidebands, and our receivers respond to both sidebands, and the results are entirely satisfactory, right?  Why can't we do this with our Direct Conversion receivers?  I struggled with this question before: https://soldersmoke.blogspot.com/2015/07/peter-parker-reviews-dsb-kit-and.html  You can see in that post that I was not quite sure I had the answer completely correct. 

It took some discussion with a fellow Vienna Wireless Society member, and some Googling and Noodling for me to figure it out.  But I think I've got it: 

Imagine a station transmitting a DSB signal at 7100 kHz with a 1 kHz tone at the AF input.  There will be signals at 7101 kHz and at 7099 kHz.  Assume the carrier is completely suppressed. 

We come along with our DC RX and try to tune in the signal. 

Remember that they heart of the DC RX is a product detector, a mixer with the VFO (or PTO) running as close as we can get it to the suppressed carrier frequency (which we can't hear).  

Lets assume that we can somehow get our VFO or PTO exactly on 7100 kHz.  The incoming signals will mix with the VFO/PTO signal.  We are looking for audio, so we will focus on the difference results and ignore the sum results of the mixing.  

The difference between 7101 and 7000 is 1 kHz.  Great! And the difference between 7099 and 7000 is 1 kHz also.  Great again, right?  We are getting the desired 1 kHz signal out of our product detector, right?  So what's the problem?  

Here it is: SIDEBAND INVERSION.  Factoring in this part of the problem helps us see the cause of the distortion that plagues DSB-DC communication more clearly. 

Remember the Hallas Rule:   Whenever you subtract the modulated signal FROM the unmodulated signal, the sidebands invert.  So, in this case, we are subtracting that 7099 "lower sideband" signal FROM the 7100 VFO/PTO signal.  So it will invert.  It will become an upper sideband signal at 1 kHz.  We will have two identical 1 kHz signals at the output.  Perfect right?   Not so fast. Not so PERFECT really.  

The perfect outcome described above assumes that our VFO/PTO signal is EXACTLY on 7100 kHz.  And exactly in phase with the suppressed carrier of the transmitter.  But if it is even SLIGHTLY off, you will end up with two different output frequencies, signals that will move in and out of alignment, causing a wobbling kind of rapid fade-in, fade-out distortion.  You can HEAR this happening in this video by Peter Parker VK3YE, starting at 6:28:

And you can see it in this LTSpice simulation.  

This LTSpice model just shows two diode ring mixers.  The transmitter is on the top, the receiver is on the bottom. The transmitter has RF at 7100 kHz at L1 and audio at 1 kHz at R1.   The receiver has the VFO at 7100.001 L7,  DSB from the transmitter at L12 with audio appearing at R4.  It is instructive to watch the output as you move the VFO frequency.  If you move the VFO freq away from the transmit carrier osc frequency you will see the distortion.  Here is the netlist for the LTSpice simulation: 

http://soldersmoke.com/DSBDC.net

On paper, using simple mixer arithmetic, you can tell that it will be there. With the VFO/PTO just 1 Hz (that's ONE cycle per second) off, you will end up with outputs at 1.001 kHz and at .999 kHz.  Yuck.  That won't sound good. These two different frequencies will be moving in and out of alignment -- you will hear them kind of thumping against each other.    And that is with a mere deviation of 1 Hz in the VFO/PTO frequency!  We are scornful when the SDR guys claim to be able to detect us being "40 Hz off."  And before you start wondering if it would be possible to get EXACTLY on frequency and in phase, take a look at the frequency readout on my PTO.  

Now consider what would happen if the incoming signal were SSB, lets say just a tone at 7101 kHz.  We'd put our VFO at around 7100 kHz and we'd hear the signal just fine.  If we were off a bit we'd hear it a bit higher or lower in tone but there would be no second audio frequency coming in to cause distortion.  You can hear this in the VK3YE video:  When Peter switches to SINGLE Sideband receiver, the DSB signals sound fine. Because he is receiving only one of the sidebands. 

The same thing happens when we try to tune in an AM station using a Direct Conversion receiver:  Radio Marti sounds awful on my DC RX, but SSB stations sound great. 

My Drake 2-B allows another opportunity to explore the problem.  I can set the bandwidth at 3.6 kHz on the 2-B, and set the passband so that I will be getting BOTH the upper and the lower sidebands of an AM signal. With the Product Detector and the BFO on,  even with the carrier at zero beat  AM sounds terrible.  It sounds distorted.  But -- with the Product Detector and BFO still on --  if I set the 2-B's  passband to only allow ONE of the sidebands through,  I can zero beat the carrier by ear, and the audio sounds fine. 

There are solutions to this problem:  If you REALLY want to listen to DSB with a DC receiver, build yourself a synchronous detector that gets the your receivers VFO EXACTLY on frequency and in phase with the transmitter's oscillator.  But the synchronizing circuitry will be far more complex than the rest of the DC receiver. 

For AM, you could just use a different kind of detector.  That will be the subject of an upcoming blog post. 

Please let me know if you think I've gotten any of this wrong.  I'm not an expert -- I'm just a ham trying to understand the circuitry. 

The 40 Meter Direct Conversion Receiver We Have Been Working On -- Comments Welcome

Above is the screenshot of the LTSpice model of the 40 meter Direct Conversion receiver that Dean KK4DAS and I have been working on.  I will post a larger scale version of the picture below.  Click on the images for a better view.  Comments welcome.  Please let us know if you find any errors or mistakes.  Realize that we wanted to keep this all simple, discrete, and entirely analog. 

Here (I hope!) is the net list for the LTSpice model: 

http://soldersmoke.com/DCRXLTSPICE.net  

First, one of the surprising things about the LTSpice model: IT IS ALIVE!  I never had a VFO or PTO actually turn on for me in LTSpice.  This one did!  So I just connected the PTO to the Mixer and the receiver works in LTSpice.   I just put an RF signal at the receiver input, and you can see the resulting AF across the 8 ohm resistor at the audio amp output.  I was even able to calculate the precise frequency of the PTO:  7078 kHz.  As in the real world, in an effort to stabilize the frequency, I changed the capacitors to NP0 in LTSpice.  Very cool.  Dean joked that all we need is a way to get RF in and audio out and we will have made an SDR receiver.  

About the receiver:  

--  Four stages that will be built by students Manhattan-style on four copper clad boards: Bandpass filter, diode ring mixer, Permeability Tuned Oscillator (PTO),  AF Amplifier.  

-- The bandpass filter is a simple dual-tuned circuit device based on the info on the QRP Labs site.  (Thanks Hans!)  We out a 10k pot as an RF gain control between the antenna and the filter. 

-- The mixer is a standard diode ring.  We included a diplexer at the output using a circuit from the famous W7EL  Optimized transceiver. (Thanks Roy!) 

-- The Permeability Tuned Oscillator is a very simple and very stable Colpitts design developed by Farhan VU2ESE.  We added a simple FET buffer using the circuit in Farhan's Daylight Again rig.  (Thanks Farhan!) 

-- The AF amp is a very simple three transistor amplifier based loosely on designs from Forrest Mims and from the Herring Aid 5 receiver. Both these designs use just two stages -- we added a third and put an AF gain pot between the first and the second stages. There is an impedance mismatch between the diode ring and the AF amp, but we found that most of the proposed solutions were more trouble than they were worth, so we left it as is.  

--Thanks to Wes W7ZOI for his November 1968 QST article on the solid-state DC receiver. Wes's article inspired our efforts.  

Dean and I have both built these receivers.  They work very well.  Dean has even decoded FT-8 with his. We used Radio Marti at 7355 kHz to test for AM breakthrough -- with the diode ring, the diplexer, and the RF gain control we were able to bring the AM breakthrough down to acceptable levels. You can see many videos of my receiver in action over on my YouTube channel:  (355) SolderSmoke - YouTube

Here is a larger image of the schematic (click for a full view): 

And here is a nicer schematic done by our friend Walter KA4KXX: