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Monday, March 6, 2023
Pictures from Farhan's Hyderabad High-School Direct Conversion Workshops
Sunday, March 5, 2023
Farhan's High-School Direct Conversion Receiver Workshops in India
We are starting to see similar efforts in different parts of the world -- Andreas with university students in Germany, Daniel with high school kids in Canada. We hope there will be others.
Sunday, February 26, 2023
First QSO with the High-School Receiver -- 100 mW to Dipole. (with videos) -- Homebrew to Homebrew!
This little contact is a reminder of the fun that can come from using simple, homebrew, QRP gear. It is really amazing that the very first contact with this receiver was with another homebrew station. This all reminds Dean and me of something we have been telling the students: the little DC receiver they are building is not a toy -- it is capable of being used in real, long-distance contacts.
Saturday, February 25, 2023
Video by KK4DAS on Progress in High-School Direct Conversion Project
Progress Report: High-School Students Build Diode Ring Mixers (Board #2 of 4). Hyderabad Soul Added to the New Machines
A team from the Vienna Wireless Society was back in the local high school Thursday and Friday of this week, helping the students finish their variable frequency oscillators and build their diode ring mixers. Club President Dean KK4DAS was in the lead, and did an amazing job working with the school and procuring all the needed parts. Mike KD4MM and Don KM4UDX provided patient and understanding help to the students.
On the oscillators, the students had to add about six parts to install a buffer circuit built around a J310 FET. They also had to replace some of the 3D printed coil forms for the main-tuning variable inductor. (Dean KK4DAS made some really nice forms -- see below.) Several teams of students experiences were very pleased to get their oscillators running.
Then it was on to the diode ring mixer. We had planned on having the students wind their own trifilar toroids, but we realized that this might be too much -- it would add a lot of time to the build, and would introduce a lot opportunity for error.
I remembered that Farhan had given me a big supply of FT-37-43 trifilar toroids that had been assembled in Hyderabad. We decided to use these transformers. We reasoned that this was not a big deviation from our DIY ethos -- after all, we didn't ask the student to wind their audio transformers, nor did they wind the RF choke in the VFO buffer. But we faced a problem: the Hyderabad transformers were all wound with the same color wire on all three turns. This would make it hard for the students to figure out which wire went where (there were 12 such wires on each mixer board!). I figured out how to do this: The night before, I soldered together the center tap wires, and I twisted together the input coil wires. We told the students to first solder the center taps in place, then solder the two free wires to the diode ring, and finally untwist the input coil wires, soldering in these connections. This worked.
Before we started, I gave the students a quick class on the essentials of mixers. And I pointed out that we were using transformers made in Hyderabad India and donated by our friend Farhan. I told the students that whenever we include parts given to us by a ham radio friend we are adding "soul to the new machine." Indeed, Farhan's toroids added a lot of soul.
We have been insisting that the students have each stage tested before moving on to the next. This week we used signal generators to put RF and VFO energy into the mixers, and oscilloscopes to make sure that audio was coming out.
The students are making good progress. After today's session we did an estimate of where each of the projects stand at this point:
Oscillation without the buffer: 11
Oscillation with the buffer: 5
Mixer built and tested (but no diplexer yet): 5
Mixer working, diplexer built 2
Tuesday, February 21, 2023
Farhan's Direct Conversion Receiver
Thank God for the Wayback machine. For a moment I feared that this article about Farhan's DC-40 receiver had been lost. (Phonestack is now some Vietnamese vendor. ) But the WayBack Machine archive came through for us.
https://web.archive.org/web/20171109081542/http://www.phonestack.com/farhan/dc40.html
Farhan's receiver has been covered on this blog before, but it is especially relevant for us now that we are immersed in our own direct conversion receiver project. Farhan was working with his niece, who was a student. We are working with high school students.
I really like Farhan's blow-by-blow description of the build. There are raw emotions here: He speaks of his hatred of LM-386s, and of how he thought of using the copper clad board as a projectile. His niece wonders about the possibility of evil spirits in the receiver. The battle against AM breakthrough is very familiar. (I like the RF choke idea.) You won't find candor like this in QST or QEX.
Farhan's DC-40 project was one of the inspirations for our high school effort. In fact, when we first went to the school, I left behind a direct conversion receiver that I had built. Taped onto the bottom of the receiver was a quote from the DC-40 article and a picture of the Wizard of Hyderabad. (See above, and click on the picture for a better look).
This week we will inject some more Farhan-ismo into our receiver. The time has come to build the mixer. Like Farhan, we will go with the diode ring. Winding the transformers would be very time consuming. I remembered that on his visit, Farhan had left me a box of trifilar toroids wound by the seamstresses of Hyderabad using FT37-43 cores. We will uses these in our build. They will add a lot of soul to the new machine.
Saturday, February 18, 2023
Progress Report: High-School Students Melt Solder and Successfully Build Oscillators
Dean KK4DAS and I were at the high school on Thursday and Friday of this week for the construction (by the students) of the variable frequency oscillator stage of their analog, discrete, direct-conversion receivers for 40 meters. Most of the students have already obtained their Technician Class licenses, so they are already radio amateurs. Both the licensing classes and the receiver build are being done with the assistance of the Vienna Wireless Society.
A week earlier Dean and I had demonstrated how to build the oscillator stage using the Manhattan technique (isolation pads super-glued to copper clad boards), but this week was the first time these students were actually building anything like this themselves.
We deliberately did not "spoon feed" the students. We told them that while we would be on-hand to help, THEY would have to do the building. They would have to layout the pads on the PC board, select the parts (from a table set up by Dean), and do the gluing and soldering. We did not hand the students bags of parts, or prepared PC boards. This was not going to be a kit building session. We wanted this to be real homebrewing.
We had parts for 15 receivers. But on the first day there were more than 60 students. So four students per project. On the first day we actually ran out of soldering stations.
We cautioned the students against dawdling. We told them to get on with it, and to "make haste slowly." We also injected an element of competition into the build by announcing that the first team to achieve oscillation would win. (Prize still TBD).
By the end of the Thursday session, many boards had been built but there were not yet any oscillations. We reconvened on Friday afternoon -- Dean and I set up support/troubleshooting stations.
Right off the bat, one of the students came up with a board that he wanted to test. After one quick correction (enamel still on the oscillator coil leads), my frequency counter showed that it was oscillating. I fired up my DX-390 receiver and we heard the loud tone. We had a winner!
In the following hour or so, Dean and I did troubleshooting on about 10 more boards. We found some of the problems that we would all expect (because we have all made these mistakes ourselves!):
-- There were cold soldering joints. We showed the students how to properly solder -- usually they just had to re-heat some cold-looking connections.
-- A few of the Zener diodes and transistors were wired in backwards (been there, done that).
-- A few of the feedback capacitors were of the wrong values. Dean and I had brought some good caps, so the students were able to quickly swap out the parts. This was another good lesson.
-- There were a few wiring errors -- these were quickly corrected.
It was exciting. One-by-one we would hear the whoop-whoop as the DX-390 confirmed that another oscillator was OSCILLATING! The students really liked to HEAR the oscillations that they had created. We reminded them at the beginning that they would be taking DC from a little square 9 volt battery and turning it into RF that could (if connected to an antenna) be heard around the world, or in our case be used to receive signals from around the world.
We got eight of the oscillators going. We think the students will be able -- without much help from us -- to get the remaining seven oscillators going.
They learned a lot. They learned about the ease, flexibility, and usefulness of the Manhattan technique, and we think they could see how this represents a basic kind of PC board design. Their soldering skills improved a lot. And they learned how to troubleshoot: Is the layout correct? Are any parts wired in backwards. Is the soldering OK? Are any of the parts bad (or of incorrect values)? Most importantly, they learned that they CAN build circuits themselves, and actually get them working.
The real payoff came each time oscillation was achieved. The students were really amazed and pleased. I could tell that some of them weren't really sure their little device was actually creating the signal they were hearing. So while we listened to the DX-390, I asked them to disconnect and reconnect the battery. Confirmed. Oscillation! Smiles. It was really great.
Soon, after finishing up some PTO odds and ends, we will move on to the other stages. We'll probably do the bandpass filter or the mixer next. Then the AF amp. Then put it all together into a full receiver. We think each stage will get easier and easier to build as the students learn and improve their homebrewing skills and their self-confidence.
Friday, February 10, 2023
SolderSmoke Podcast #243 -- HI7/N2CQR, uBITX mods for 10 meters, High-School Direct Conversion Receiver Project Launched (Success!) Mailbag
February 10, 2023
SolderSmoke Podcast #224 is available.
http://soldersmoke.com/soldersmoke243.mp3
Video here: (32) SolderSmoke Podcast 243 (video) Hi7/N2CQR, uBITX, Success with High School Receiver Project - YouTube
Pete N6QW had technical difficulties this morning. He insisted that the show must go on. Pete will be back for the next episode.
Travelogue:
Bill in the Dominican Republic for all of January.
HI7/N2CQR Eastern
tip of the island. uBITX and dipoles.
20, 17, 10. CW and
SSB. SSB was tough and I had reports of
RF getting into the signal.
Went to CW.
Worked VWS Mike KA4CDN, and Walter KA4KXX on 20CW.
Finally moved up to 10 CW. Lots of contacts. Even though uBITX very QRP
on ten.
I am modifying the uBITX now.
Copper tape shielding to keep RF out.
Low power out not
the fault of the IRF-510s. The problem
is the 2N3904s.
Will replace with 2N2222 in To-18 cans.
Dean KK4DAS putting KD8CEC software into Arduino. I gave up.
Who sent me this orphan uBITX?
SolderSmoke Shack South in final phase of construction.
SHAMELESS COMMERCE DIVISION:
Patreon!
Bezos Shopping!
DC Receiver Project
Local High School radio club.
Simple: Like
Herring Aid 5 and Wes’s original.
Farhan’s four
stages:
BP Filter, Diode Ring, PTO, AF amp.
Simple Colpitts PTO SURPRISINGLY STABLE.
Simple and easy.
No chips. No complicated circuits.
Guys have helped test out the design: Rick N3FJZ, Walter KA4KXX,
Daniel VE5DLD, Stephen
VK2BLQ and others.
First session last night: We demonstrated build of the PTOs.
They worked (thank God).
Open Circuits book.
Envelope Detection Controversy
Save the Antenna – Book “Losing the Nobel Prize” K1JT
MAILBAG
--Dean KK4DAS 10 meter DSB!
Tiny SA ULTRA! FB
--John AC2RL on Elmer W3PHL DSB guy
--AC3K reports inventor of Fender Stratocaster guitar was a
ham: W6DOE
--AF8E was doing POTA.
I worked him. He said my rig had presence. FB
--Alain F4IET FB DSB rig with mic in Cigar can!
--Daryl N0DP worked him on SSB. He is homebrewing
--Steve N8NM was in for repairs but is on the mend.
--Rick G6AKG working with sub-harmonic mixers and logic chips
--Paul HS0ZLQ Built DC receiver but looking for something
else to build. No DSB!
--Steve AB4I – Coherer, Jagadish Chandra Bose, and Marconi
--Eldon KC5U
Worked VK5QD right after me and mentioned SolderSmoke FB
--Todd K7TFC is building the DC RX.
--Tony G4WIF and Ian G3ROO using automotive relays for
antenna switching. FB.
--Dave WA1LBP Great to hear from my fellow Hambassador (Okinawa)
Older post comments:
--Scott VO1DR was also in CF Rockey’s class! (Blog comment)
--Aurora Aug 4, 1972: Twelve people shared memories. (Blog comment)
--Will WN1SLG Googled novice call and was led to
my Novice log.(Blog comment)
Tuesday, February 7, 2023
VK2BLQ Builds the High-School Direct Conversion Receiver
Sunday, February 5, 2023
Rick N3FJZ Completes Build of High-School Direct Conversion Receiver Project -- LISTEN!
Thursday, February 2, 2023
Direct Conversion Receiver Bandscan -- 40 Meters early on a Thursday Morning -- With W1AW/4
Sunday, January 29, 2023
Rick N3FJZ Builds the Mixer for the High School Direct Conversion Receiver
Friday, January 27, 2023
N3FJZ Builds PTO for High School DC Receiver
Tuesday, January 24, 2023
Update on High School Direct Conversion Receiver Project + .asc File
So far, in response to my recent request for testing, no one has stepped up to build the DC receiver Dean KK4DAS and I are testing out. We did get a couple of comments explaining why guys are opting not to help, but so far no other builders are actually melting solder in response to our recent request.
So Dean and I decided to each build second versions of the receiver. That will bring the total finished build population to 4. I finished my second version yesterday. Picture above. It works great.
One change: The emitter resistor on the final AF amp was too low in value. The transistor and the transformer were getting hot. I switched from 10 ohms to 100 ohms and the problem disappeared. I have made the change on the LTSpice Schematic. Here is the .asc file (I hope!) :
http://soldersmoke.com/DCRX.asc
Dean posted the .asc file (and some other info) here:
If you have trouble accessing that file, please let me know and I will try to e-mail it to you. In any case the schematic appears here:
https://soldersmoke.blogspot.com/2023/01/a-call-for-builders-please-help-us-test.html
You will notice that this Spice schematic actually works! The PTO turns on, and I put a simulated RF signal at the antenna port. Audio appears at the output.
Time is getting very tight. Dean and I will begin presenting this project to the high school students on February 2. So it is not too late to help. But helping is, of course, strictly voluntary -- if you are reluctant to build this thing, DON'T!
Saturday, January 21, 2023
A Call for Builders! Please help us Test this Receiver! Please Build this Receiver!
This is the Direct Conversion receiver that Dean and I have built. We plan to have students at a local high school build it, starting in early February. We would like to have some others build it, to make sure that the design is re-producible without problems.
Please build this receiver! But we ask that you build it exactly as per the schematic above and below. Innovation can come later -- for now we just want to make sure this thing works, that there are no errors in the schematic, and that it can be built by the students with minimum woe. Thanks in advance!
Dean or others with 3D printers may be able to supply the plastic form for the PTO inductor.
We know of one other builder, but he is having some trouble. We would like to confirm that this design is sound.
-------------------
Here is a larger image of the schematic (click for a full view):
Sunday, January 15, 2023
Working Walter KA4KXX from Hispaniola
Walter KA4KXX in Orlando has been a prolific builder of rigs for many years, and has been a great friend of SolderSmoke: Here are some of the SolderSmoke podcasts and blog posts in which Walter's solder melting was mentioned: https://soldersmoke.blogspot.com/search?q=KA4KXX
As we approach the end of our current stay in the Dominican Republic, I could not miss out on the chance to work Walter with his homebrew rigs. Even though the space weather was stormy, and my dipole was droopy, we arranged to meet up on the high end of the 20 meter CW band this morning. See the results in the video above. A solid QSO with Walter. He says it is HB2HB, but truth be told I was on a uBITX that was built more by Farhan than by me. But this was a great contact. Walter started with a 50W rig, then switched to his 3 watt rig with a DC receiver. FB
Here is the e-mail I received from Walter after the QSO:
Dear Bill:
Sunday, December 25, 2022
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
Saturday, December 3, 2022
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.
-- 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!
Friday, December 2, 2022
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.
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.