IMD in Transmitters -- Splatter? Or Signal Strength?

https://www.newsvhf.com/conf2024/PresPapers/WA1MBA-IMD_in_Transmitters.pdf

Here is a good (and very recent) article on IMD ("splatter") produced in transmitters. The focus is on VHF, but much of this is relevant to HF operators.  I found the footnotes on the ARRL "Clean Signal Initiative" to be worrisome.  They seem to just be assuming that all ham operators will be using commercial gear, and the "OEM" needs to be made to meet certain standards. This seems to leave the homebrewer out in the cold.  I can see where someday soon, the "standards" will exceed the capability of analog homebrewers.  That would be bad.  

The role that signal strength plays in the perception of "splatter" is often misunderstood by the "waterfall police."  We often we hear some irate waterfall policeman screaming that,  "You are 40 over and far too wide."   Here is a good quote from the article on this point: 

"If you have a calibrated spectrum display (as many SDR’s are these days), you can directly measure the level difference in dB. If it is 30 dB or more, then it could be an acceptably “clean signal”, even if it is bothersome. Most ham voice communication is conducted with less than 30 dB signal/noise, and in that case the unwanted IMD is buried in the noise."

And even in a low noise environment,  if the signal is 40 db over S9. that would mean the signal PEP is at -33dbm.  If the IMD products are 46 db down from the signal peak, that means your IMD products are -79 dbm.  That is S-8!   That signal will look quite wide in the waterfall, but it would be within FCC specs, right? The problem here is not so much distortion, as signal strength.  And let's remember that "legal limit" is usually a misnomer:  FCC regs require hams to use the minimum power necessary, not 1.5 kW on every single QSO. 

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

A Surprisingly Good Movie from the Late 1960s: "The Ham's Wide World" (Video)

I found this movie to be surprisingly good.  Narrated by Arthur Godfrey, it features Barry Goldwater, and a lot of other hams.  There is a homebrewer too!  Lots of  old rigs we know and love:  a Drake 2-B, a couple of Galaxy Vs, a Benton Harbor lunchbox, Heathkit SB-series rigs, many Swans, and was that an HQ-170 that I saw in there?  There are also many cool antennas, including a 15 meter quad set up by a bunch of Southern California teenagers. 

Near the end, when they visit ARRL Headquarters, we briefly see none-other-than Doug DeMaw, W1FB!  FB!  

Please take a look at this video and post comments about the rigs, antennas, and radio amateurs that you see in the film. 

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

 

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

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

QST Repeatedly Got Sideband Inversion Wrong

It kind of pains me to do this. These articles are from a long time ago, and the author is an esteemed  Silent Key,  but the myth about the origins of the USB/LSB convention is still out there, and as a homebrewer of SSB gear I feel obligated to point out these examples of the error that that myth is based on.   

Last Friday, Pete WB9FLW and I were talking about homebrewing SSB rigs.  I recommended a series of QST articles by Doug DeMaw.  "Beginner's Bench:  The Principles and Building of SSB Gear" started in QST in September 1985. There were at least five parts -- it continued until January 1986. (Links to the series appear below.) I hadn't looked at these articles in years, but when I did, a big mistake jumped right out at me:  In the first installment, on page 19, Doug  makes the same mistake that he made in his Design Notebook:

"Now comes the conversion section of our SSB generator.  We must move (heterodyne) the 9-MHz SSB signal to 3.75-4.0 MHz. Our balanced mixer works just as it does in a receiver. That is, we inject the mixer with two frequencies (9 MHz and 5 MHz) to produce a sum or a difference output frequency (9 - 5 = 4 MHz, or 9 +5 = 14 MHz) If we are to generate 75 meter SSB energy, we must chose the difference frequency. We could build an 20-meter SSB transmitter by selecting the sum of the mixer frequencies. The RF amplifiers and filter (FL2) that follow would then have to be designed for 14-MHz operation.  In fact, many early two-band homemade SSB transmitters were built for for 75 and 20 meters in order to use this convenient frequency arrangement.  The use of upper sideband on 20 meters and lower sideband on 75 meters may be the result of this frequency arrangement (the sidebands become inverted when switching from the difference to the sum frequency.) " 

Those last two sentences are incorrect.  They repeat the "Myth," or the "Urban Legend" about the origins of the LSB/USB convention.  Contrary to what many hams now believe, with 9 MHz filter and a 5.2 MHz BFO it takes more than just switching from sum frequency to difference frequency to invert one of the sidebands. 

There are two conditions needed for sideband inversion to take place:  

1) You have to be taking the difference product (DeMaw got that right) 

2) The unmodulated (VFO or LO) signal must be larger than the modulated signal. (DeMaw and the ARRL obviously missed that part.  Repeatedly.) 

This is another way of stating the simple, accurate and useful Hallas Rule:  Sideband inversion only occurs when you are subtracting the signal with modulation FROM the signal without modulation. 

For DeMaw's claim to be correct, one of the SSB signals going into the balanced mixer would have to invert, and the other would have to not invert.  Let's see if that happens: He has the sideband signal being generated at 9 MHz and the VFO running around 5 MHz. 

9 - 5 = 4  But we are not subtracting the modulated signal FROM the unmodulated signal.  SO NO INVERSION

9 + 5 = 14   We are not subtracting at all.  SO NO INVERSION.  

Doug's convenient frequency scheme WOULD work if he'd just switch the frequencies of the filter and the VFO.  With a sideband generator on 5.2 MHz and a VFO around 9 MHz you do get the happy 75  LSB, 20 USB arrangement without the need to switch the carrier oscillator/BFO frequency.   That is what happened in the Swan 240, and that is what I have in my Mythbuster rig. I am listening to both 75 LSB and 20 USB without changing the carrier oscillator/BFO frequency.  My filter/BFO/product detector is set up for USB.   With this arrangement the 75 meter LSB signals DO invert, and the 20 meter USB meter signals do not, so both are able to make use of my USB BFO/product detector without shifting the BFO frequency. 

This error shows up again in DeMaw's the May 1989 QST article "A Four Stage 75-meter SSB Superhet" (reprinted in the ARRL's QRP Classics book).  Here he writes: 

"Should you want to cover both the 75- and 20-meter bands you can build a 20-meter version of FL-1 and band switch the two filters. As with the 75 meter only version, an IF of 9.0 MHz (Y1) is required. With this arrangement the 20 meter band will tune backwards from the 75 meter band, but upper- and lower-sideband reception will occur, as required, without changing the BFO frequency (Y2). This two band scheme with a 5-MHz VFO is an old one!"   NOTE: FL1 is the bandpass filter, not the IF filter.  

Doug's mistakes in this area may simply be due to the fact that he was more of a CW guy.  And this is something that is quite easy to confuse:  9 and 5 will get you to 75 and 20, but you have to make sure the VFO is at 9 if you want to make use of sideband inversion and avoid having to shift the BFO/ carrier oscillator.   I've made this mistake myself: 

https://soldersmoke.blogspot.com/2012/05/usblsb-urban-legend-debunked.html

In October 1993 I wrote to DeMaw about his Four Stage 75 meter SSB Superhet.  I think I was looking for details on how to put it on 20 meters.  As I recall, Doug wrote back telling me to just pick 20 meter values for the input bandpass filter.  Had I done so, I would have discovered that  -- for the reasons cited above -- this just wouldn't have worked on 20.  His BFO and filter were set up to receive LSB signals. That's fine for the incoming 75 meter LSB signals.  But on 20 -- contrary to DeMaw's thinking -- there would be NO sideband inversion. I'd be trying to listen to 20 meter USB signals with a receiver set up for 20 meter LSB.  

Did anyone else notice these errors.  Were there ever errata notices in QST on this?  

This is a reminder that you should take all technical articles and schematics with a grain of salt.  Many contain errors. We are all human, and this is a complicated subject with lots of details. 

https://pe2bz.philpem.me.uk/Comm/-%20ARRL/Build-Service-Design/Receivers/PrincipleOfBuildingSSBgear/8509017.pdf

https://pe2bz.philpem.me.uk/Comm/-%20ARRL/Build-Service-Design/Receivers/PrincipleOfBuildingSSBgear/8510027.pdf

https://pe2bz.philpem.me.uk/Comm/-%20ARRL/Build-Service-Design/Receivers/PrincipleOfBuildingSSBgear/8511016.pdf

https://pe2bz.philpem.me.uk/Comm/-%20ARRL/Build-Service-Design/Receivers/PrincipleOfBuildingSSBgear/8512037.pdf

https://pe2bz.philpem.me.uk/Comm/-%20ARRL/Build-Service-Design/Receivers/PrincipleOfBuildingSSBgear/8601029.pdf

W7ZOI: Direct Conversion Receivers -- Some Amateur Radio History

http://w7zoi.net/dcrx68.pdf

Farhan and Pete WB9FLW alerted me to this wonderful article by Wes Hayward, W7ZOI.  I guess my interest in DC receivers must have been noticed by the Google algorithm because I am bombarded by ads extolling the virtues of "Zero IF."   Hey Google -- I'm already a believer!  I was converted by W7ZOI's 1968 article in QST. And my belief in the technique has been greatly reinforced by his November 2018 50th anniversary article. 

There is so much good stuff in Wes's look-back piece.  The travails of trying to write for QST are presented very well.   And we learn that none-other-than Doug DeMaw himself is responsible for the use of the word "presence" in describing amateur radio audio.  

This article has inspired me to take a new look at the DC receiver I built last winter.  Mine needs some work. I think it is kind of deaf.   It could probably benefit from a diode ring  detector.   But it already has presence.   

http://w7zoi.net/dcrx68.pdf

Thanks Wes.  And thanks to Farhan and Pete for the heads up. 

FDIM: Bob Crane Interviews Ed Hare W1RFI

I always associate "Ed Hare" with that picture of him holding an HW-8 in the ARRL lab.

I was supposed to edit this one a bit at the beginning, but I forgot.  Sorry about that Ed. 

Thanks to Ed and to Bob Crane for the interview. 

http://soldersmoke.com/FDIM17W1RFI.mp3

Meeting up with ARRL CEO Tom Gallagher, NY2RF

On April 12, Tom Gallagher NY2RF was in DC and was kind enough to make some time in his schedule for us to get together and talk about radio.  As we mentioned on a recent podcast, Tom, who has recently taken over as Chief Executive Officer of the American Radio Relay League, is a true FB ham.  He has a restored Drake station that he keeps on the air, and was recently talking up the Michigan Mighty Mite during his interview with Eric 4Z1UG in the "QSO Today" podcast.  And he is a SolderSmoke listener.  It was great to finally meet Tom.  We are all lucky to have him at the ARRL.