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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
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 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.
-- 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/
Won’t have another Podcast until the new year so Happy Holidays to all! Merry Christmas, Happy New Year!
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.
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.
Here is a larger image of the schematic (click for a full view):
Below is a video from 2006:
This reminds me of a cool project I have not yet done: modifying the TinySA to allow the user to listen to the station: https://soldersmoke.blogspot.com/2021/10/how-to-listen-with-your-tinysa.html I notice that Dean KK4DAS (my colleague in DC receiver design) was the only commenter on the blog post describing the TinySA mod. TRGHS. We need to to do this.
Here are the reports showing when Vatican Radio and Radio Marti were on the air on November 18, 2022:
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