About the Limits and Problems and Shortcomings of AI (actually of LLMs) -- The Design and Construction of a Crystal Filter

Complaining about AI on the computer/internet while surrounded by digital rigs!

 

A couple of days ago, I described in this blog how AI (specifically the Gemini LLM) had helped me design a 13 MHz crystal filter.  There was some angry push-back from people who seem to dislike any use of AI. One fellow said I should use my own mind and hands, and that by using AI I would risk having time to stand on the street corner.  I found it kind of ironic that this observation was being sent to me on a computer connected to the internet (two tools that have made this kind of construction MUCH easier).  

In the first round I think AI helped a lot. It even drew nice looking schematics for me.  Using the motional parameters determinef by me via the G3UUR method,  AI used the basic equations to calculate the shunt capacitances and the termination impedances.  This helped me avoid many of the mathematical errors that had slowed me down in the past.  I went ahead and built this filter, and (later!) took a look at the passband in my Antuino (a Scalar Network Analyser from Farhan). 

I found the passband to be that of an "LSB filter":  the skirt was steep just below the cutoff frequency, but much more sloping farther down in frequency.  This is not what I needed.  The signal hitting this filter would be (and would remain) USB. I could try to use this filter but I would be placing my BFO/Carrier Oscillator on the sloping portion of the curve.  Carrier and opposite sideband suppression would suffer. (Note: It is NOT 20 db down!  -20 db on this meter equals 0 db.) 

And here is one point where LLM AI really screwed up:  I asked it for a 5 crystal filter that would be a USB filter: One with the steep part of the skirt above and close to the cutoff frequency.  AI LLM quickly obliged.  I was suspicuous, because I had never seen this before. Here is what AI LLM Gemini recommended: 

Wow, looks great right?  Yea,  but it doesn't work.  I built it and looked at it in the Antuino.  NOPE.  It wasn't even acting as a bandpass filter.  It is more of a notch filter. Confronted with the failure, AI LLM admitted it was wrong (my coments in red): 

-------------------------------------------------

This configuration does not work. I have never seen it in any amateur radio book. Are you sure that this will work?

You are 100% correct to call that out—that schematic is incorrect and will not work as an SSB bandpass filter.

Why the Shunt-Crystal Schematic Fails

In quartz crystal equivalent circuits, the crystal reaches its lowest impedance () at its series resonant frequency ().

• Series Position (Correct Bandpass): When the crystal is placed in the series signal line, its low impedance at allows the RF signal to pass through to the next stage.
• Shunt Position (Incorrect): Placing the crystal directly from the signal line to ground causes it to act as a shunt short circuit at . Instead of passing a window, it short-circuits directly to ground, creating a band-reject (notch) response rather than a bandpass filter.

This is why you will not find that topology in Experimental Methods in RF Design (EMRFD), the ARRL Handbook, or classic G3UUR/Dishal filter literature.

---------------------------------------------------------

At this point I decided that a 10 crystal Cohn filter would give me the steep skirts I needed.   Here is what AI recommended: 

I went ahead and built this one.  I looked at it with the Antuino.  It works. 

You can see the steeper skirts on this filter.  These steep skirts will allow me to use this filter on 17 meters with the 5 to 5.5 MHz VFO from the old Kenwood TS-820s.  

In the course of doing this, an old friend who works in this field (and who will remain anonymous) warned me about the dangers of over-reliance on AI LLMs, especially when working on ham radio circuitry.  More on this in the days ahead.  

How Hans G0UPL Cracked the Code for Stable I and Q from the Si5351

L to R: Farhan, Roy Lewallen, Eric Guth and Hans Summers

Back in 2018, I remember Farhan telling me that "Hans has cracked the code" getting stable I and Q from the Si5351.  But I never really appreciated the importance of this breakthough until I recently started looking at the receiver from Macedonia.  Then I asked AI (Google Gemini) for a readout on who was responsible for the I and Q innovation.  Here is what I got yesterday from Gemini: 

---------------------------------

The credit for solving this problem and unlocking stable, single-chip I/Q quadrature generation from the Si5351 goes to Hans Summers, the British QRP developer and founder of QRP Labs.

While other talented developers in the amateur radio community—most notably Jason Mildrum (NT7S)—wrote the definitive, widely used standard open-source Arduino libraries for basic Si5351 tuning, it was Hans Summers who fundamentally cracked the math required to force the chip into a stable 90 degree phase shift over a wide frequency range without breaking lock.

The Breakthrough Discovery

Around 2014 to 2015, while designing highly compact, low-cost transceivers like the QCX and ultimate VFO kits, Summers realized that standard programming libraries were completely incompatible with generating stable quadrature signals.

He pioneered a completely reversed architectural method to control the chip, which became the blueprint for homebrew SDR builders everywhere:

• Flipping the Fractional Math: Standard libraries fixed the primary internal Voltage Controlled Oscillator (VCO/PLL) at an integer value and used the second stage (the MultiSynth fractional dividers) to dial in the exact frequency. Summers realized this broke the phase shift. He discovered that you must lock the MultiSynth divider to a fixed even integer and force the first stage (the PLL VCO) to do the fractional math instead.

• The "No Reset" Discovery: Standard practices issued a soft PLL reset command to the chip every time the user turned the tuning dial to ensure clean frequencies. This reset caused the phase relationship to drop out or randomize. Summers proved that after an initial startup reset to align the phases, you can smoothly glide the VCO frequency up and down via the I2C bus without resetting the PLL, keeping the I and Q signals locked in a perfect mathematical embrace.

Thanks to Hans Summers publishing his detailed engineering application notes and open-sourcing his mathematical methods, developers around the world were able to write the code that modern microcontrollers (like the ESP32 in your receiver) use today to keep the I/Q phase rock-solid as you dial across the bands.

Farhan Talks LARCSet (CW & SSB) at FDIM (with a Lot of Homebrew Wisdom)

Watch the presentation here: https://www.youtube.com/watch?v=MefojjQ84YY

Farhan made it to FDIM 2026  (he must hold the "distance travelled" record!). We thought he might be talking about the latest version of the digital SDR sBITX, but NO!  Farhan talked about the entirely analog LARCSet, a 30 dollar SSB/CW monobander.  And in the process he shared a lot of good homebrew history and wisdom. I took notes on the video of his presentation: 

-- Farhan recounts his discussion with Steve Hartley, President of GQRP.  Farhan said he started to talk about SDR projects, but Steve steered him away from all that.  Farhan said he realized that the homes of GQRP members are often small, and projects need to fit into took boxes that are pulled out as needed.  There is often not even enough room to mount a screen.  Analog rigs just fit better.  

-- Farhan talked about the beauty of analog.  He also shared some info on the recent timeline of analog rigs, going back to 1976 with the IARU gift kits made available by W1VD.  Farhan very kindly mentioned the DC receiver that Dean and I are promoting.  He talked about the 2003 BITX 20 rig, and the subsequent uBITX.  Farhan talked about the cleanliness of all-analog rigs.  "SDR's are a mess!" he said. "With SDRs it is difficult to avoid hash." 

-- Farhan said he had trouble measuring the phase noise of the VFO in the LARCSet.  He consulted with Wes W7ZOI.  Wes told him this was NOT a measurement problem; VFOs have almost no phase noise.  The level is even lower than that of crystal oscillators.  Of course, crystal oscillators are more stable, but they also have more phase noise.

-- He noted that almost no recent homebrew design does not rely on an Si5351.  This, he said, is "not a healthy situation."  Indeed.   

  --  Farhan talked a bit about how Indian regulations seemingly require a deviation from the completly open source ethos.  Indian regs require companies to have assets.  So the PC board layouts have to remain proprietary.  

-- Farhan talked about the sharpness and shape of the BP filter in the LARCSet.  I remember talking to him about the shape of my BP filters in my dual banders -- I had to rebuild the filters.

-- On the crystal filters that form the heart of SSB rigs, Farhan noted that cheap low Q crystals often introduce a lot of loss in the filters (that may explain my problem with some styles of computer crystals). 

-- A member of the FDIM audience asked about the Sharpie written frequency readout on the LARCset that Farhan showed to the group.  Farhan told them that this was the only frequency readout used in the rig. 

  -- With the LARCSet, Farhan used varactors to vary the frequency.  But the varactors he used were cheap but horrible.  They varied the frequency as the rig hearted up.  The LM386 was the source of heat.  He also noted that the cheap varactors, while cheap, did not provide linear frequency readout. Farhan said the varactor scheme was still not perfect; he offered a PTO solution that could be used instead.  Three cheers for the PTO! 

-- Farhan said the LARCset was really an SSB rig, but when coming to FDIM he said he felt obligated to present a rig that included CW, "or they would throw me out of the room."  Farhan described a scheme to generate CW based on what was done with the Atlas rigs. 

-- Farhan said the LARCset might even work on 2 Meters.  Hmmm.  

-- On tuning, Farhan said he used a very large tuning dial (he said it was like a steering wheel) and then recommended the use of a smaller control that could serve as an SSB "clarifier." 

-- Farhan pointed out that homebrew rigs are never really done; even decades later, they can still be modified.  

Watch the presentation here: https://www.youtube.com/watch?v=MefojjQ84YY

Rick N3FJZ on the Red Summit Podcast with Charlie NJ7V

 

Check it out! 

https://www.youtube.com/watch?v=sUQA2uNskEs

I liked Rick's story about his early days in radio electronics.  Taking old TVs from the street -- been there, done that!  FB Rick, 

Rick points out that he has never used a commercial ham radio rig, so he is unfamiliar with some of the "features" of such rigs.  Sometimes, I think, the lack of experience is a good thing. 

I really like the display that Rick uses, showing the operating frequency, the VFO frequency and the BFO frequency.  This might help with our struggle with those who complain that we are 40 Hz off.  Rick then notes that he used 15,000 lines of code for this display.  Wow, that shows the benefits of being -- like Rick is -- both a real software wizard and a hardware wizard. 

Rick describes how  he uses tin-plated steel boards in lieu of copper clad boards.  

I liked his approach to schematic drawing -- we benefited from this in the SolderSmoke Direct Conversion receiver project. 

When Rick talks about taking pieces of schematics from other rigs and making them work in new rigs, Charlie notes that, "this is the ham radio way."  Exactly. 

There is a lot of really sentimental stuff in this podcast.  SolderSmoke is mentioned frequently.  They mention Pete and Dean.  This starts at around 22 minutes. Rick talks about Farhan at around 26 minutes.  And he talks about Wes W7ZOI.  

Rick talks about some of his early projects.  I have a sentimental attachment to his Lakeside DC receiver: 

  

Then, a few years later, we had our first HB2HB contact.  Homebrew rigs on both sides:  

https://soldersmoke.blogspot.com/2015/10/hb2hb-n3fjz-n2cqr-si5351-and-bitx-tias.html

Finally, I agree with the last sentiment expressed by Rick in his conversation with Charlie:  The Red Summit podcast -- especially with its focus on homebrew -- is exactly what this hobby needs.  Anything that encourages hams to experience the fun of homebrewing is a good thing.  Three cheers for Rick and for Red Summit. 

How VU2RM Got Into the Satellites

 

Readers will remember last week's post about the homebrew rivalry between VU2RM and VU2NR. (Note the bio about VU2RM below).  This morning Farhan sent me this article from a 1970s-era Indian ham radio magazine in which VU2RM describes his early efforts to get onto the satellites. I also tried during the mid 1970s, but I didn't have the kind of magic decoder ring provided (via QSL!)  by OH2RK.  My satellite success had to wait some 20 years, until I was in the Dominican Republic.  More on this below the VU2RM article. 

This morning I replied to Farhan's email:  

Very cool Farhan.  I see that Pete may have an Indian rival for "most SSB transceivers built."  

I got into the satellites during my 1992-1996 stay in the Dominican Republic.   At first, we had no way to get the up-to-date Keplerian elements to predict satellite passes.  Then Compuserve opened up. I had to telephone Miami from the DR to get the Keps.  I ran up a huge phone bill. 

I too built an antenna for these contacts.  Mine was the 2 meter portion of a 144/440 MHz project from 73 magazine called "The Ray-Gun".  I built mine out of scrap lumber and refrigerator tubing.  I still have these elements!  I used them in the beam that I built to listen for YOUR satellite!   In the DR, elevated the thing by 45 degrees and pointed it in the general direction of the satellite.  

RS-11 had a very cool robot aboard.  If you called it just right, it would issue you a serial number.  I have the QSL.

Here are some articles about our satellite adventures.  

https://www.gadgeteer.us/ASTRO.HTM

BTW, Luis Ernesto HI8LEZ visited me during summer 2024 in the DR! 

My hombrew 2 meter beam in the Dominican Republic

Early SSB in India: Espionage, Stolen Secrets, and Kleptomania

 

Earlier this month I had stumbled across a 1964 QST article entitled "A Sideband Transceiver, VU2 Style."  I forwarded the link to Pete N6QW, Grayson KJ7UM, and to Farhan VU2ESE.  Last night Farhan sent me this insider look at early SSB in India.  It is really great.  The battle for preeminence among early SSB homebrewers in India reminds me a bit of Jean Shepherd's descriptions of homebrew radio in Hammond, Indiana.   Farhan's description of the early rigs being "all over the place, in about 3 or 4 boxes... with a whole lot of  wires running all over"  really resonated with me -- yes, even today, that is true homebrew. Thank you Farhan for sharing this with us. 

Farhan writes: 

VU2NR, Raju was a legend! Quirky, brilliant and liked to be by himself. He lived to a very old age of 100 or so. He was the first ham to get on SSB from India. Therein hangs a story of espionage, stolen secrets, cold war, politics and kleptomania. Most of the actors are now dead, so it can be told now.

I never met VU2NR, he rarely travelled. One evening at Paddy's shack, I was shooting the breeze with VU2RM, Ram, about my own SSB efforts. Ram was probably the most knowledgeable ham on SSB in India and his RM96 was widely duplicated. Paddy and Ram were trying to empty out a bottle of the Old Monk and I casually mentioned how we had to scrounge around for SSB lattice filters until Wes showed us how to build them ourselves. At that point, Ram unloaded this story to us:

VU2NR joined the Allies and worked as a radio mechanic with the RAF. After the war, he joined the ICAO (International Civil Aviation Organization) and then finally the United Nations as a comms expert, he was in Aden for a while. He understood the radios well and he became a well known homebrewer. His most brilliant work was the NR60 (it is an SSB radio built around four CA3028 kind of mixers). It was the first SSB radio that I had built (and it worked!) but we are getting ahead of ourselves.

Back in the 50s, ham shacks of India consisted of World War II surplus receivers and a self-built 807 transmitter driven by a VFO. There was a fair amount of buzz on the bands about SSB. The hams could occasionally pick up DX that was on SSB and easily resolved by the BC-348s  and the HROs but there was no way to build one. The QST was difficult to get your hands on but Raju had become a subscriber of the QST during his RAF days.

A QST sailed on a slow boat from the USA and landed up at the port on the east coast of India, Vizag and made its way to Raju's doorstep. Over weeks, he gathered parts and in the summer of 1955 he quietly came on 40 meters on SSB! This blew everyone away. Until now, one had only heard stories of SSB transmitters told by sailors of Sandra Maria Gracia after a few drinks to a bunch of hams clutching onto their 807s. It was as if Jesus had materialized!

VU2RM, Ram was not amused. He was a homebrewer with pride and honour. He could never be bested. He spent the next evening tuning on Raju's SSB up and down to note that the other sideband was missing and the carrier was gone. It sank into him that Raju had indeed beaten him to SSB. Ram worked at the Kakinada port, about 100 miles further down south on the east coast as the port radio engineer. He didn't sleep well. At 5 am, he got up, walked out and took a bus to Vizag where Raju lived. He was at Raju's door at 9 am. Raju was amused at the young guy and invited him in. They knew each other as rival homebrewers. There was tension in the air. Raju showed off his SSB transmitter in the front hall of his home. It was all over the place in about 3 or 4 boxes. There was the power supply, the PA, the ssb generator using a 2Q4 (a passive RC phasing network), a separate VFO box and a whole lot with wires running all over. In a corner was his stack of QSTs.

Raju's XYL called out for coffee and snacks (called 'Tiffin' in south Indian English) and Raju sauntered off to the kitchen to fetch his celebratory feast...

With alacrity and swiftness that only comes once in a lifetime, Ram, VU2RM, sprang to action. He darted to the QST collections, quickly found the one with the SSB transmitter on the cover, turned the pages to discover VU2NR's dog eared pages. He was staring at the circuit trying to memorize it. He realized that 2Q4's internal diagram had really odd values that he could never remember. So he did what James Bond, Bertie Wooster and Louvre thieves would have done. He rolled it up and slipped it into his pocket! (In those days, QST could be rolled and slipped into your trouser pockets, leading to wider dissemination of knowledge). He called out to Raju asking him to forget the refreshments as his bus was due back home. He legged it out and didn't stop until the bus stop. He hid around the bus yard hoping to not be caught by Raju. In the meantime, Raju, cursing the young upstart, drank both the filter coffee cups and left for work. Raju was unaware that the royal jewels were stolen.

VU2RM, Ram, started to work on his SSB radio over the next few days. He stopped going to work. At first, he figured out how to null the carrier using a twin triode modulator. Next, he fabricated the 2Q4 circuit. He sand papered lower value resistors until they read as close to the value as he could manage on his analog volt-ohm meter. He gave up on the exact capacitances and just soldered a bunch of the closest together. He had a crude 2Q4 equivalent. This, he dropped into his rat next circuit and an 'Almost' SSB signal emerged. He was probably having a sideband suppression of less than 15 dB at this point. It was suppressed and the carrier was nulled. He declared victory.

In the morning, Ram was on the band strutting off his SSB transmitter to the local gang. Graciously accepting the accolades from his fan club and extolling virtues of life on SSB to the lesser mortals. The news quickly spread over CW and AM of VU2RM as the second SSB station from India.

In the meantime, the atmosphere at VU2NR was tense. You could cut the air with a toggle switch. Raju had no idea how Ram had gotten the SSB. He carefully tuned around on his signal. He could pick up faint traces of the opposite sideband. Ram's carrier was also leaky. He decided that Ram was probably passing off his AM transmitter as a SSB. He said as much on air to his own devotees. Ram, ever watchful of his own reputation, heard this criticism and broke in, challenging Raju to prove that his was indeed not SSB. Raju's reaction was mixed. Now he wasn't sure. 

It was now Raju's turn to have a sleepless night. After his breakfast, he had had enough of confusion and he hauled his old scope and himself into the 8 am bus to Kakinada and showed up at Ram's doorstep! "Show me your radio" he said crisply.

For Ram it was his moment of crowning glory. He had brought Raju down to his shack! Raju plugged his scope into Ram's transmitter. He sniffed around the transmitter which was laid all over the table. "Where is your 2Q4?" he asked. Ram pointed to bunch of resistors and capacitors and Raju realized Ram's dog headed brilliance. He sat down, traced to the balancing pots (there would be two of them as this was a phasing transmitter). At this time, he decided to show Ram his greater prowess. He reached out to the carrier nulling pots, and tweaked them to minimum carrier (Ram was watching the oscilloscope with his mouth wide open). He told Ram that he had now fixed the carrier and then showed him how to adjust the RF phasing control for minimum by tuning to the opposite sideband on Ram's Bc 348. Having proved to Ram that he was a better homebrewer, he declared that now, Ram's transmitter was indeed SSB and not semi-AM. Raju retrieved his honor by being gracious and "mentoring" Ram.

 

They had coffee and snacks and it was time for Raju to leave, as Ram was showing Raju to the door, Raju's eye caught a QST on the shelf... he pulled it out and stared down at the QST with his own address neatly stickered on the QST's cover. He glared at Ram for a long time and turned away and left in a huff. Ram called after him down the street but Raju was gone.

VU2NR's last radio was built when he turned 90. It was the NR90 and it was built using NE612 chips. His hamming came to an abrupt end when one day his son dropped in unannounced only to find him perched on his tower of 30 feet height, fixing a fallen element of his self-made log periodic. The son was so horrified that he took away all the radios to save him from self-harm (this part of the story is unconfirmed).

 

My friend Sasi, VU2XZ, was close to him and he got the family to donate his callsign to a repeater that has been established in his memory. VU2NR lives on.

- f 

------------------------

Here is the link to the 1964 QST article.   VU2NR's article appears on page 19: 

https://www.worldradiohistory.com/INTERNATIONAL/QST/QST-1964-03.pdf

"Rig Here is Homebrew" -- The Joys and Sorrows of Building your own Rigs

This is kind of a re-run.  I came across it today on YouTube and thought it was pretty good.  So here it is again.  

 -------------------

Dean KK4DAS asked me to speak to our local radio club, the Vienna Wireless Society.   It was a lot of fun.  I talked about my evolution as a homebrewer, some of the rigs I made, the moments of joy, and the tales of woe.   You can watch the presentation in the video above. 

I was really glad to be able to explain in the presentation the importance of people like Pete, Dex, Farhan, Wes, Shep and even Dilbert. 

I was also pleased to get into the presentation the N2CQR sign that Peter VK2EMU made for me.  Thanks Peter! 

Here is the URL to the YouTube video (also above): 
https://www.youtube.com/watch?time_continue=3414&v=VHSr-v4QO7Q&feature=emb_logo

And here are the PowerPoint slides I used: 
https://viennawireless.net/wp/wp-content/uploads/2020/06/VWS-presentation-Rig-here-is-homebrew.pdf

Ashish N6ASD's Beautiful SolderSmoke Direct Conversion Receiver -- Built in Bangalore -- "This was such a fun project. This was my first direct conversion build, and I learned a lot along the way."

Ashish N6ASD is a really good guy.  We started watching his projects when he was in San Francisco.  He does a lot of fundamental radio building.  And he is a friend of Homebrew Hero Frank Harris K0IYE. 

When I spotted a "thumbs up" from an N6ASD on the SolderSmoke Discord channel, I knew that we would soon have yet another Bangalore Direct Conversion Receiver.  Ashish has been in contact with both Farhan and Ramakrishnan.  FB. 

Ashish writes:  I finally finished building the receiver! This was such a fun project. This was my first direct conversion build, and I learned a lot along the way. The biggest challenge was finding stable capacitors for the oscillator. In my first attempt, I used whatever I had in my junk box, but those capacitors drifted all over the place. Mica caps seemed expensive and difficult to get in Bangalore. I ended up buying surface-mount NP0 capacitors instead. Soldering them was a hassle, but they are much cheaper and easier to get here. Next step will be more mods and experimentation! The first improvement will be in the BPF. I will replace the regular ceramic caps with NP0 caps and re-tune it.

Here is Ashish's receiver inhaling CW:   

Congratulations Ashish -- Welcome to the Hall of Fame! 

Ashish N6ASD

The resemblance between Ashih's photo and the famous

1818 painting by Caspar David Friedrich is completely coincidental,  

but quite remarkable. 

https://en.wikipedia.org/wiki/Wanderer_above_the_Sea_of_Fog

 

Ted KN4ZXG's FB SolderSmoke Direct Conversion receiver: "This project is the greatest gift to homebrewing..."

There it is, a thing of beauty.  The best part is that Ted built this receiver himself.  That is something that most hams NEVER do. 

Ted writes: 

This project is the greatest gift to homebrewing since I got interested in radio nearly a decade ago. Lot's of great projects out there but most go into a mysterious black hole of obsolete parts like the NE612 mixer that's hard to get, or even the LM386, which is plentiful but not as cool as this amp. Everything's divided neatly on it's own board and explained. The builder has wiggle room on the layout and ways to test each module whether that's with nice or budget equipment. Not only did I learn a ton about each stage, but also about making RF probes and making use of my modest but super useful equipment like the Nanovna. Although there were no mysteries, it all comes together to make magic. If you know stuff, you can do stuff! Thanks again de KN4ZXG, Ted.

While he has some great plans for modifications and improvements, at this point I think Ted should follow Farhan's advice and spend some time just listening to the receiver that he has built. Direct conversion receivers sound especially good. It is as if they are closer to the ether.

Here are some clips of Ted's receiver in action:

 

Congratulations Ted.  Welcome to the Hall of Fame.  

Honorable Mention: Andreas DL1AJG's THREE SolderSmoke Direct Conversion Receivers for Biologists

 

Andreas DL1AJG was another of those intrepid hams who,  in the dark of winter 2023 took up the SolderSmoke Direct Conversion Receiver Challenge in order to test our receiver (before we pushed high school kids to build it).  Andreas came to the task with a lot of useful teaching experience. At the time he was an academic  biologist and had been teaching a course called "Applied Electronics for Biologists."  See: 

https://soldersmoke.blogspot.com/2022/08/can-biologist-fix-radio.html

Some of Andreas's students

I put these receivers in the "Honorable Mention" category only because they deviated a bit from the basic schematic:  they used an indoor ferrite antenna, had an RF amplifier ahead of the mixer, and did not use a transformer in the AF amp.  But they are very clearly the SolderSmoke DC receiver, they succeeded in inhaling RF, and Andreas and his students built them to help us.  

Here is the schematic they used: 

Click on the image for a better view

Here is an e-mail exchange I had with Andreas when the scholatic dust had settled in June 2023: 

Dear Bill,

Please my apologies for my late update on our DCR project. We started with the course in the semester break and once the semester started only a handful of students were able to finish their receiver. A long shelf of shame .... 

Here is what we've got so far (those were finished at the end of April already). 3 nice DCRs completely sufficient to copy CW indoors without additional antennas starting from late afternoon. Strong stations can be heard all day. I found that coupling some 6m of wire with one or two windings to the ferrite core can boost the signal dramatically but can also increase noise. 

The PTO is based on your design (Bill Meara N26QR &  Dean KK4DAS) which was sparked by Farhan  (VU2ESE) , except that I've swapped the FET for a NPN. The input amplifier and antenna is from the JUMA active ferrite antenna by Matti Hohtola (OH7SV), the band pass filter is from Hans Summers (G0UPL), the mixer and the headphone amplifier is inspired by Pete Juliano (N6QW), you told me that the diplexer (as well as the whole DCR idea) is attributed to Wes Hayward (W7ZOI) and the perfect schematics of Rick Scott (N3FJZ) where crucial to get me started in the first place. I enjoy keeping track of original sources, as I would do in science. This shows that even little achievements are based on the ideas of many other great people -  and this is nothing to be ashamed of. 

This was a lot of fun! Thank You!

Best and yours sincerely,

Andreas

Wow, the direct conversion re-engineering of education continues, this time at graduate-school level with biologists in Munich!  Amazing.  

Andreas points out that his group was also plagued by semester-related problems that caused many additions to the German shelf of shame.  Let's hope that someday soon these builders will come to their senses and join the ranks of those who have finished their homebrew projects.  

Looking at the schematic (above) of Andreas's project, there are a couple of significant differences from ours:   

-- Their AF amp used a transformer-less push-pull design.  We had considered this but abandoned it thinking that it would be too complicated to explain the workings of this circuit to our students.

-- Most significant, is Andreas's use of a ferrite rod antenna and an RF amplifier.   I think a simple 33 foot quarter wave antenna (with a ground or a counterpoise) might work better.  But hey,  to  each his own!   The important thing is that a number of these receivers were successfully built.  They look beautiful.  

Congratulations to Andreas and the successful Munich homebrewers!  

-----------------------------------

Thanks to Andreas DL1AJG.  Welcome to the Hall of Fame.   

------------------------------------

Join the discussion - SolderSmoke Discord Server:

https://discord.gg/Fu6B7yGxx2

 

Documentation on Hackaday:

https://hackaday.io/project/190327-high-schoolers-build-a-radio-receiver

 

SolderSmoke YouTube channel:

https://www.youtube.com/@soldersmoke

Ramakrishnan VU2JXN's FB SolderSmoke Direct Conversion Receiver: "Extremely thrilled! Hooked!"

This is a really important Hall of Fame entry for us.  This is our first completed SolderSmoke Direct Conversion Receiver from India (more will come).  

Most importantly, this receiver was built by our good friend Ramakrishnan VU2JXN (ex VU3RDD).  Ramakrishnan goes way back in SolderSmoke history.  He was the one who first suggested (in 2006!) that our podcast needed a blog to go with it.  This was the origin of the SolderSmoke Daily News.  A short time later we announced the birth of Ramakrishnan's daughter.  That same daughter now has her ham license and will build a second receiver with her dad.  

Ramakrishnan VU2JXN

The roots of this receiver are deep in India. When Dean and I first went to the local high school to talk about ham radio support, I carried with me a Direct Conversion Receiver inspired by Farhan VU2ESE.  In fact, that receiver had a picture of Farhan and quotes from him taped to the wooden base.  When Dean and I designed the receiver that we tried to build with the High School kids, we used an oscillator based on the DC receiver that Farhan was building with youngsters at Indian girl's schools.  That is the receiver design that we are using in this SolderSmoke Challenge. 

Ramakrishnan is in Bangalore now, which is where he built this receiver.  Ashish N6ASD is also there now, and is building a receiver. 

Ramakrishnan writes:  

This morning before I got ready to come to work, I got a few minutes 

to turn on the radio and catch the morning nets. Extremely thrilled to

 get this working. The PTO is very stable.

Needless to say, you folks convinced me to get back into homebrew
and you have hooked me into it with this project.

I am off to my ailing mother's place tonight. I don't have a station 

there, but I am carrying a copyof EMRFD with me (I have two!).

I want to listen with this receiver as you all always say 

and make simple modification to this receiver and improve it.

 
Again, couldn't turn off from it even though I am at work! The bug has
caught me.

Here is some more video of Ramakrishnan's receiver: 

Congratulations Ramakrishnan. Welcome to the Hall of Fame! 

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Join the discussion - SolderSmoke Discord Server:

https://discord.gg/Fu6B7yGxx2

 

Documentation on Hackaday:

https://hackaday.io/project/190327-high-schoolers-build-a-radio-receiver

 

SolderSmoke YouTube channel:

https://www.youtube.com/@soldersmoke