"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.”
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
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.)
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:
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.
http://www.cse.iitm.ac.in/~murthy/sirjcbose.pdf
----------------------------
I think the more we learn about Marconi, the less admirable he seems.
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 1st SBE-33 (August 1963) when likely you were in the 2nd Grade and I was headed off to Midway island.
-----------------------
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.
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.
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
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