Category: TJ DC RX

First QSO with the High-School Receiver — 100 mW to Dipole. (with videos) — Homebrew to Homebrew!

Alan W4AMV in Raleigh NC

Dean KK4DAS and I have been working with a local high school. The students are building a direct conversion receiver for 40 meters.

We’ve been giving out prizes for the first team to complete each stage. I wanted to give one of the teams a little oscillator that could b heard with their receiver. So this morning, using a 7040 crystal from the AF4K (SK) company, I threw together a one transistor oscillator. It has just 8 parts, including the key:
I had a low pass filter in the antenna tuner. The antenna was a low-to-the-ground 40 meter dipole. The transmitter was putting out around 100 milliwatts.
N2CQR’s Ten Minute Transmitter
The Reverse Beacon Network showed that I was getting out quite well:

Then I thought, wait a second, let’s make a contact with the prototype high-school direct conversion receiver.

With the receiver hooked up, I again called CQ on 40 CW. BOOM! Very quickly Alan W4AMV in Raleigh NC came back to my call. Wow! That’s 222 miles. And a quick check of QRZ.com revealed that Alan is a homebrewer. Then Google reminded me that his work has been featured on the SolderSmoke blog. TRGHS.

I was so excited during this contact that I almost forgot to film it. But I did manage to get some short clips of the QSO in progress. You have to listen carefully, but you can hear our calls in there while Alan is transmitting (listen for the lower tone):

And in this clip you can watch me transmit using the 10 Minute Transmitter:

UPDATE (Feb 27 2023): I asked Alan about the rig he was using: “A PLL EXCITER DRIVING A PAIR OF FETS PUSH PULL ABT 50 w to an inverted L at 55 feet. The Rx a single conversion 9 MHz IF and it is connected to an active antenna in the trees out in the woods abt 25 feet up. Uses an automotive whip antenna about 3 feet long. “

Alan’s Rig

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.


Thanks Alan!

Progress Report: High-School Students Build Diode Ring Mixers (Board #2 of 4). Hyderabad Soul Added to the New Machines

Dean KK4DAS works with students

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.

Students at work on the receiver

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.

Manhattan Mixer Pads

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.

One of Farhan’s transformers

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.

How the transformers were prepped


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.

One of the test set-ups for the mixers


The students are making good progress. After today’s session we did an estimate of where each of the projects stand at this point:

We are building 15 receivers.
Oscillation without the buffer: 11
Oscillation with the buffer: 5
Mixer built and tested (but no diplexer yet): 5
Mixer working, diplexer built 2

During the next month or so the students will build the bandpass filter and the audio amplifier, and put all the boards together to complete the receiver.

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.

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.

We’ve often reminded the students that what they are doing is NOT easy. This is hard. As new radio amateurs, they are taking on a project that most older hams never dare to take on. They like the challenge. They are homebrewing a real receiver.

SolderSmoke Podcast #243 — HI7/N2CQR, uBITX mods for 10 meters, High-School Direct Conversion Receiver Project Launched (Success!) Mailbag

DC RX and one of the PTO boards we built as demos last night.

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)

VK2BLQ Builds the High-School Direct Conversion Receiver

It is truly a thing of beauty. Stephen VK2BLQ built it just as we designed it, so his build (like those of several others) serves as confirmation that this design is ready for the high school students.

Stephen writes:

Bill,


Don’t worry, you are not alone out there.

Here is my build; sorry that the front panel is overexposed and hard to see, but it is plywood.

I did follow the schematic but due to the contents of the junk box there have been some component changes.

The only thing that I had to buy was the 3/16 x 50 mm (2 inch) brass screw.

My calculations for the coils for the PTO and BPF were a little bit off necessitating padding down the PTO with a further 100 pF (easier than remaking the coil and mounting) and removing a few turns from the T50-6 toroids.

Like other people have found: the audio takes off at full volume; I am thinking but not yet tried adding decoupling between R5 (15K) and C2(47 uF). It isn’t the actual values of the electros as I had to use 100 uF so might be the audio output getting back into the earlier stage.

The tuning range I get is our 7000 to 7200 KHz and some shortwave stations above and below, Turning the screw is a little bit fiddly, but once tuned the vfo is quite stable and the audio sounds good.

Best wishes,

Stephen

VK2BLQ
I really like the ruler near the tuning control. Is that (I hope!) the frequency readout?

Rick N3FJZ Completes Build of High-School Direct Conversion Receiver Project — LISTEN!

Rick N3FJZ has completed his the direct conversion receiver that we will soon be building with students at a local high school. See video above.

We are hoping that a number of people will build the receiver as we designed it. Some folks have sent us versions of the receiver that they have built, but these versions often include significant deviations from our design, rendering them less-than-useful in checking our work. Rick built it just as we prescribed. His build is very useful in confirming the validity of our design. So if you are working on one of these receivers, I would encourage you to — for the moment — dispense with innovations and build it the way Rick did: as per the design we have been using.

We know that our design is not perfect. But we have decided to stick with it because it is very simple and very easy to explain. Examples: We know there is an impedance mismatch between the mixer and the AF amp. But fixing this would introduce complexity that we want to avoid. And the receiver works fine with the imperfection. We know that a push-pull AF amp would probably work better than the one we have. But we do not want to have to explain push-pull amps, biasing schemes, and PNP transistors in this short introductory course. So we stuck with three common-emitter AF amp circuits and an 1K-8ohm transformer.

Rick did a really excellent job not only in building this receiver, but also in documenting it. His diagrams and drawings are really superb. We will probably use these in our presentations to the students:

We will keep all of you informed on the progress of this project. We will begin this week. But if you are still working on the receiver, please send us your work, even if it comes in after we begin the course.

Thanks Rick!

Direct Conversion Receiver Bandscan — 40 Meters early on a Thursday Morning — With W1AW/4

I superglued a San Jian frequency counter to the front panel of my High School Direct Conversion receiver. Then I tuned it through the 40 meter band. You can hear the W1AW/4 station. And several SSB stations.

Rick N3FJZ Builds the Mixer for the High School Direct Conversion Receiver

Wonderful progress from Rick N3FJZ. Having completed the PTO, Rick went on to build the diode ring mixer and diplexer. (See video above.) He then connected these to boards to a band pass filter and an AF amp from previous project. And wow, the resulting receiver sounds really great! I think it sounds better than the ones Dean and I have built.

— It will be interesting to see how Rick’s receiver sounds with the very simple AF amplifier that we are using.

— We need to get Rick a coil form for the PTO variable inductor! And getting him the PTO form will allow him to dispense with the varactor circuit (which, I must say, is pretty cool).

— In the PTO, for the coupling capacitor (C15) , I now have a 260 pF NP0 cap. But your .1 uF ceramic disc seems to be doing just fin. So maybe we don’t need the NP0.

— I’m glad that Rick grounded the brass screw. Without that, the hand capacitance effect is bad.

— I think results will be much the same with the dual tuned circuit bandpass filter that we use. But it will be interesting to see if Rick’s triple tuned circuit helps with AM breakthrough (I think I heard Radio Marti when Rick tuned to the top of the band).

— I haven’t had a hum problem and I don’t think Dean has either. Dean is running his on two 9V batteries. Maybe that would take care of the hum. Our PTOs are completely Al Fresco!

— The Traders Net sounds like fun!

Thanks Rick! 73

N3FJZ Builds PTO for High School DC Receiver

Rick N3FJZ has stepped forward and is building the direct conversion receiver that Dean and I have been working on. This will be big help in our testing of the receiver. See above for the video report on his build of the PTO stage.

When I saw this, I found myself thinking two things:

1) Those really are “rigs.”

and

2) I am not alone!

Years ago, I was having a tough day on 40 meters. The Waterfall Policemen were gleefully attacking my admitedly less-than-perfect SSB signal. Unbeknownst to me, someone out there was listening, and listening with a homebrew receiver. A receiver with a PTO. It was Rick N3FJZ. He made my day when he sent this video:


Here are the other blog posts on the amazing rigs of N3FJZ here:

More to follow, both on Rick’s receiver and on the work of other intrepid builders.

Thanks Rick.

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:

http://bit.ly/3WAFfLI

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!

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.

——————-

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:

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:

Direct Conversion Receiver — Simple 2-Diode Mixer Defeats Radio Marti, but Diode Ring is the Best

Here is another update on Direct Conversion receiver construction. In Northern Virginia we get very strong signals from the Radio Marti transmitter in Greenville NC. During the morning hours it is just above the 40 meter band at 7335 kHz. In the evening it is a bit higher in frequency at 7435 KHz. (in the video above I mistakenly give the morning frequency, when in fact they were on the higher evening frequency). In either case, Radio Marti has been a big source of unwanted AM breakthrough in our simple DC receivers. It now serves as something of a test of our bandpass filters and mixers.

In this video I try out the simple mixer described in detail here: https://soldersmoke.blogspot.com/2022…

The following morning, I tested the mixer with Radio Marti (in fact) on 7335 kHz. By adjusting the VFO signal input to the minimum value needed to turn on the diodes, I was able to bring Radio Marti AM breakthrough to minimal levels. But I could still hear it (weakly) in the background. Putting a very simple diplexer at the audio output of the mixer (just a .1uF capacitor in series with a 47 ohm resistor to ground) helped a lot.

I could also hear break through from Spanish-language broadcasts from Vatican Radio on 7305 kHz (using the 250 kW transmitter in Greenville NC) from 11:30-11:45. Perhaps most surprisingly, I was also getting AM breakthrough from 40 meter FT8!

Here is a short video showing the simple two-diode mixer in action during the morning hours:

I also tried out the more common two diode mixer with trifilar toroid. (In this one, the VFO turns both diodes on, then turns both of them off). The results were similar to what I got with the other two diode mixer.

We are trying to develop four circuits — bandpass filter, mixer, variable frequency oscillator, and audio amplifier — that will be simple enough for construction by high school students, but not so simple as to compromise performance. We want the receiver to work well.


So far, my conclusion is that the best results come from the diode rig mixer with two trifilar toroids. Here is a short video showing the diode ring in action on the morning of November 9, 2022:

DC Receiver: 100db Gain? Diplexer? VFO in a box?

Here is a progress report on Direct Conversion Receiver developments. Dual Tuned Circuit, Diode Ring with Diplexer, PTO VFO from Farhan’s Daylight rig, two stage 49 db BJT AF amp with a transformer. It works very well. I discuss: Shielding of the VFO — necessary or not? Why brass in the PTO? Do we really need 100db in a receiver, especially with ear buds? Sourcing the AF amplifier’s transformer. Using W7EL’s diplexer. (I think it has solved my Radio Marti breakthrough problem). Developing a DC RX circuit that can be built by students. I end with a bandsweep of 40 meters that includes CW, FT-8, SSB, and AM

How the Diode Ring Multiplies by 1 and -1 — “The Secrets of the Diode Ring” — Plus another Bandsweep with the DC RX

A deeper look into how the Diode Ring detector works: “the particular go of it.” Here I rely on a wonderful diagram from the RSGB. This diagram clearly shows how in this circuit, the switching action of the diodes — controlled by the VFO — results in sum and difference frequencies at the output. This is amazingly illuminating. I then tried to build this actual circuit. It works, but I am also getting a lot of AM breakthrough from a local AM station (WFAX) and Radio Marti at 7335 kHz. I will try again. In any case, the diagram shows how the diode ring does its thing! I need to beef up the Band Pass Filter. I tuned around a bit on 40 meters — you can listen. Students at a local high school have been trying to get the DC receiver I loaned to them going — they may be confused by the intricacies of SSB tuning. I will see them next week.

Here is the RSGB diagram that reveals the secrets of the Diode Ring. (Now that could be the title of a book or movie. I claim the rights to that!) Click on the image for a better view.

Adding a Diode Ring to the Direct Conversion Receiver, And How the Diode Ring Works

I take a look back at Wes Hayward’s classic DC receiver from the November 1968 QST.
I then try to describe how diode ring mixers work (“the particular go of it”) using Alan Wolke’s excellent YouTube description as my base.

I present some drawings that I did that use the “crossed diode” diagram as the start.

I also discuss interference from Radio Marti’s 250 kW transmitter on 7335 kHz in Greenville NC.

Finally, we listen to the receiver a bit.

Alan Wolke’s Diagram
Wes Hayward’s 1968 DC Receiver

Update on the Direct Conversion Receiver — Now only 4 transistors

I took a new look at the receiver, did some measurements, and decided to take out the RF amplifier and the AF pre-amp. The receiver works fine without them, another indication that they were unnecessary. Line-up is now: Band-pass filter, Mixer, VFO, AF amp. I think there is only 49 db of gain in the entire receiver, but it is useable with an un-amplified speaker, and is a bit too loud on ear buds. Not bad for just 4 transistors. And I think we could do this with just 3 (no need for the VFO buffer).

Here is the AF amplifier I used:

Bandsweep with the New Homebrew 40 meter Direct Conversion Receiver

It is inhaling nicely but some improvements are still pending. Click on the video above to see and listen to the bandsweep done on 40 this morning.

— The front end consists of capacitive divider input impedance matching circuit, followed by one LC circuit and an FET RF amp.

— The VFO is a super-simple Colpitts design by Farhan. The two feedback capacitors do double duty in the LC tan circuit.

— I am using an old variable capacitor instead of the PTOs that we have been experimenting with.

— The mixer is singly balanced using one trifilar toroid and two diodes. We have found out that even with these three simple devices, there is significant variation in how people connect them to VFO, RF in and audio out. I think we have found the best way to do this: Be sure to put the VFO on the primary of the transformer, and let this signal turn the diodes on and off.

— For the AF amplification, I have one FET, followed by two BJTs. I have a small audio transformer between the speaker and the final AF amp. There is plenty of audio.

You may wonder why, after all the SSB superhet transceivers, I am building a simple Direct Conversion receiver. Well, we hope to help a bunch of high school kids build one, so we need to be really familiar with how it works. And I find that as simple as it is, there is still a lot to learn in a project like this.