More Info on Tom's Amazing Junkbox 20 meter Receiver (using FPGAs)

 

Back in June we had a post about an amazing junkbox receiver project carried out by Tom in SW England: 

https://soldersmoke.blogspot.com/2021/06/toms-receiver-20-meter-superhet-built.html   

This was a receiver built around SBL-1 mixers and 10.7 MHz filters salvaged from an old satellite receiver.   It was also the first radio receiver project undertaken by Tom.  His use of FPGA technology is especially interesting. 

We asked Tom for more info and he kindly provided it: 

http://soldersmoke.com/tom20receiver.pdf

http://soldersmoke.com/tomRadioschematics.pdf

http://soldersmoke.com/tomFPGAdesign.pdf

Tom also sent me Firmware sourcecode  that may be reusable for STM8 users, and the FPGA design file (for Quartus users). If anyone has a GitHub or similar site that can host these files, please let me know and we will send them to you. 

Mythbuster Video #16 -- GLOWING NUMERALS! In Juliano Blue!

I added two San Jian frequency counters to the front panel. In addition to making the rig a lot easier to operate, they add a classy touch of Juliano Blue to the project.

I got my counters here: https://www.ebay.com/itm/224223678132

There is a limitation of some of the the San Jian counters: https://soldersmoke.blogspot.com/2021/02/a-problem-with-san-jian-plj6-led-counter.html

But this limitation didn't cause any problem with this rig:  In this case I just plugged in the IF frequency of 5.2397 MHz.  I connected the input to my VFO running around 9 MHz.  For 20 meter signals, I select the "up" option;  the San JIan counter just adds the IF frequency to the VFO Frequency.  For example 9 + 5.2397 = 14.2397 MHz .   For 75, I select the "down" option.  Here the San Jian just subtracts the IF frequency from the VFO frequency -- for example 9 - 5.2397 = 3.7603 MHz.  

The band select switch operates relays that select the proper band-pass and low-pass filters.  This switch also alternatively turns on either the 20 meter San Jian or the 75 meter San Jian. 

Mythbuster Video #15 The Mythbuster Signal As Seen in the NA5B WebSDR

Mehmet NA5B has an excellent WedSDR receiver in Washington D.C., about 9 miles east of me. I often use it to check my signal quality. I think this video shows that the 10 pole crystal filter is working and is producing a signal with very sharp drop-off outside the 2.7 kHz passband. You should focus your attention to the passband (yellow vertical lines) near 3895 kHz. That's me.

Once, when I was describing my 40 meter DIGI-TIA to an SDR guy, he seemed surprised that I was using a -- gasp -- crystal filter. "Your skirts must be atrocious!" he said. My HDR sensibilities were deeply offended.

I had hoped that the 10 pole crystal filter would produce skirts so nearly vertical as to make my signal indistinguishable (in the waterfalls) from the SDR signals. At least at this low signal level, it appears to be working.

Mythbuster Video #14 -- ON THE AIR!

On August 5 I put the Mythbuster on the air, making QSOs on both 75 and 20 meters. This video is from the following day -- I was on 75 meters. My first QSO with this rig was on 20 with S57DX. That was a good omen. TRGHS

I had no feedback or spur problems with the BITX40 module amplifier circuit. And I didn't release any IRF510 smoke. There is, of course, a lot more output on 75 than on 20. That's just the nature of the IRF510. I get about 4 watts out on 20 and about 9 watts out on 75 (with a 12 volt supply). At Pete's suggestion I might replace the IRF510 with a Mitsubishi RD06HHF1.

I had a couple of minor problems getting the transmitter going. I will describe these in a future video.

Mythbuster Video #13 -- RF Power Amplifier, and Relay Switching Plan

In this episode we enter into the most fraught part of the construction project: the production of RF power. This is where amplifiers stubbornly turn into oscillators, and where components release magic smoke, or at least burn the fingers of hopeful builders.

I kind of ran out of room when I built the low-pass filters. But, thinking ahead, I wanted to have them on a separate board. And it is good that they ended up in the far corner of the rig.

Just going from one band to two bands adds to the complexity of the rig. I had to add two relays, one to switch the low pass filters, the other to switch the bandpass filters. I ended up with 5 DPDT relays in this transceiver. It was very helpful to have a plan and a diagram for the relays and all the switching.

It looks like each of the three RF amplifier stages provides about 15 db of gain -- about what I need to get to the 5 watt level.

Farhan and the SBITX on Hack-A-Day

 https://hackaday.com/2021/08/06/sbitx-hackable-hf-sdr-for-the-raspberry-pi/

AFTIA! The Audio Frequency Termination Insensitive Amplifier from W7ZOI

 

It really pays to check Wes Hayward's web site.  I looked yesterday and found something that we really need:  A termination insensitive amplifier for audio frequencies -- an AFTIA.  Just last week I was looking at the AF amplifier of my Mythbuster rig, wishing that it had TIA properties.  Now, thanks to Wes, we have a circuit available: http://w7zoi.net/audio-fba.pdf

And let's not forget that it was Wes Hayward W7ZOI and Bob Kopski K3NHI whose 2009 article alerted us to the RF variety of termination insensitive feedback amplifiers. 

Mythbuster Video #12 -- Bandpass Filters

The really cool part comes at the end when I put the scope probe on the output, then on the input of the bandpass filter.  Exciting stuff my friends! 

Mythbuster Video #11 -- Block Diagram and Overview of the Transmitter Circuits

Much of the transmitter is now done. This video looks at the circuitry and how it all fits together.

Mythbuster Video #10 -- A Front Panel for an Al Fresco Rig

Video: Rob Sherwood NC0B on Transceiver (and Especially Transmitter) Performance

 Rob Sherwood NC0B is one of the real authorities on receiver performance.  Many of us have relied on his ratings of commercial receivers for many years.  His recent presentation to the Madison DX Club has a lot of really interesting information. There is also, I think, some stuff that homebrewers will find distressing. 

Just some things that I noticed: 

-- Rob mentioned a move back to 9 MHz IF filters and a move away from dual-conversion rigs with a high IF.  He also mentioned the combination of a 9 MHz IF and a 5 MHz VFO as a way of easily getting on both 75 and 20 meters.  

-- Rob discussed phase noise from synthesizers, a topic we discussed at length (some would say ad nauseum!) a year or so ago. 

-- Rob really praised the "Pure Signal" system of one of the SDR manufacturers.  He showed the completely rectangular waterfall display of a Pure Signal transmitter.  I'm afraid that simple crystal rigs might never live up to this standard.  An embrace of this high standard could discourage the construction of simpler, HDR rigs.  We should not let the perfect be the enemy of the good!  

-- We often hear SSB ops complaining that some other SSB op is "splattering all over the band."  It often turns out that what is really happening is that a clean SSB signal is just overloading the receiver of an operator who does not know how to turn off his pre-amp or turn on an attenuator.  Rob shows us how to really know if the problem is in fact at the other end:  He looks at key clicks from two different CW signals on 160 meters.  Both are at roughly the same level in his receiver  But one is clicking all over the place while the other is not.  With this kind of comparative info, we can be sure that the problem is the transmitting station's fault. 

-- In discussing when to turn on the pre-amp (or the attenuator) Rob revives the old practice of just listening to the band noise. If you can hear the band noise when you switch from dummy load to receive antenna, you have enough RF gain.  Adding more will only make things worse. 

-- There was an interesting question about how to evaluate the performance of receivers when there are many signals inside the receiver's passband.  This is the case with FT-8.  Rob said this situation needs more research. 

I don't mean to be critical here -- Rob is the guy who evaluated commercial rigs.  And he is a contester.  So his presentation is, of necessity, going to have a very "appliance operator" orientation.  There seems to be an assumption that the only "rigs" that modern hams can use are commercial products. At one point Rob admits that most hams just can't repair these rigs. There is much for homebrewers to learn from experts like Rob, but presentations like this also remind us of what a tiny minority we really are, and how most hams have moved completely away from the old ham tradition of building our own rigs.  

Thanks to Rob Sherwood and the Madison DX Club.  And thanks to EI7GL for alerting us to this important presentation. 

Video: Introduction to the TinySA Spectrum Analyzer

The TinySA has some very cool capabilities, and this short intro video provides a good sense of what it can do.  

I am learning how to use the TinySA so that I can check the output of my Mythbuster transceiver (I now have the first portion of the transmitter working.)  I tried to use the TinySA to check the carrier and opposite sideband suppression on my new Mythbuster transceiver, but I think the max Resolution Bandwidth (3 kHz) is too high for me to do this.  Please let me know if I am missing something.  That would have been a very useful capability.  

The rest of the videos are here: 

https://www.youtube.com/channel/UCh140tYlt5ZvDMVy0B1eTSg

It also functions as a signal generator that also provides AM and FM modulated signals.  You can also have a waterfall on the spectrum display.  Very nice. 

I have not yet figured out how to listen to the signals.  This is one of Erik's videos -- it looks like you have to solder in a connection for audio out. 

Thank you Erik Kaashoek.