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!
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Thanks to Andreas DL1AJG. Welcome to the Hall of Fame.
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German build, organized, clean and efficient ;-
ReplyDeleteTo my defense there was some rationale for all the deviations:
ReplyDelete1) the ferrite antenna was a high-Q tuned circuit which my students could investigate and adjust with a little DSO with FFT functionality as well as the miniVNA. It allowed to assess the antennas directionality and we could talk about electric and magnetic fields. Furthermore, everybody is used nowadays to whitness the wonders of the AEther without paying too much attention to a large antenna. I wanted everything on one board.
2) the push-pull topology is the minimalist case where I could talk about cross-over distortion, and where NPN and PNP play nicely together. From a didactic point of view it was very important.
3) for the IN-amp using a FET for a circuit which runs otherwise entirely on transistors seemed overkill. Moreover, I would have been forced to introduce the FET and where it differs from a BJT - it was to much work at that time :D
It was great fun!
Thanks a lot and all the best,
Andreas
73 de DL1AJG
Andreas: Thanks. I prefer real antennas, in part because younger builders have gotten the idea that electronic devices should work without external antennas. Antennas just seem so old fashioned. But for ham gear, there is no getting around them. We recommended 33 feet of wire and a 33 foot counterpoise. On the push-pull topology, we thought about that, but concluded that it would add complexity and make the circuits harder to explain. On the RF amp, we just think there is no need for one on 40 meters. Going back to the very first solid state ham DC receiver in November 1968, we see no RF amp ahead of the mixer. But Andreas, we were not emphasizing the need to "build it as we designed it" when you and your students were doing this. We were (and are) very grateful for your efforts. Thanks again. 73 Bill N2CQR
ReplyDelete