SSB Single-conversion superhet transmitter

 

Juggled some wires from the receiver I made yesterday to get a transmitter.  The video shows me picking up the signal on my G90 (I'm transmitting into a dummy load).

My signal path is going microphone to the input of the first Si5351 (where the antenna was coming in on the receiver).  I switched the wires coming out of the Si5351 (so I didn't have to reprogram anything for now), so the ~9MHz is going into the first mixer and the ~5MHz is going into the second (opposite of the receiver).

I'm using this Adafruit board that has an electret microphone and amplifier together.  I made a little board that has a blocking cap (22uF - what I had handy) to block the DC bias the board has.  In the configuration, it is doing 60db gain and is enough to get 80mv P-P to the dummy load (~ -18dbm).  Will probably need to use another amplifier to drive the QRP Labs 10W linear amp.

SSB Single-conversion superhet receiver

 

I strung together a bunch of parts to make an SSB receiver.

Details (mostly for my later reference)

• Currently set up for 20m
• Bandpass filter is the QRP Labs design, just with parts I had laying around.  Don't have as nice of capacitors as his kit (they are no-name trimmer caps) and the bandwidth is more like 2MHz vs his 1.4MHz, but the insertion loss was about the same (-1.75dB) and does the job.
• I have one RF amplifier in - an AliExpress "low noise 30db" thing.  (I need to remeasure what gain I'm actually getting at 9V.)  Will probably replace for a MostlyDIYRF IF amp that has AGC.
• Both mixers are AD831 boards off of Aliexpress.  Want to see about swapping them out for some of the DBMs I built when playing with the SolderSmoke DCR.
• The audio amplifier is one from SparkFun, a class D amp based on the TPA2005D1 chip.
• The speaker is a Jameco Valuepro "4" 8 Ohm 5W 800Hz-10kHz Paper 97 dB"
• The IF bandpass filter is the Mostlydiyrf one that I posted about before.  I made little stand-alone matching transformers for it.  I'll probably swap it out for a similar crystal filter I made years ago.
• The VFO/BFO is from a MostlyDIYRF "UDVBM-1 Universal Digital VFO/BFO".  Will replace with my own arduino/si5351/display setup if I actually box this up, as this board is really nice for mocking things up and would like to keep it out for that.  Also, the code he has for this has a few things I'd change (the VFO shows the actual frequency, not the frequency+BFO, and the "click to change what digit you are changing" doesn't go high enough).  There are a lot of libraries out there or might roll my own.

No matter how many times I think about it, figuring out what side of the IF filter to put the VFO/BFO always does my head in, and of course I had it on the wrong side to start.  I like using FT8 to test, as you always know there are going to be strong signals there, and if my iPhone decodes it, we're probably good to go.

I think next step is that I want to make it a transmitter.  Really just need to make an amplifier for the microphone, use the QRP Labs 10W amp (must remember to take out the 40M bpf it has in it).  Use one of my various RF amps to get the power up enough to drive that.  I have various LPFs I've made over time sitting around.

Then see about making it a transceiver.  I have some relay modules that are ready-to-use with microcontrollers (have the diodes in for the current spike, etc.)  I think will be mostly figuring out how I want to attach different parts, updating the code to handle switching, etc.  The whole "boxing and socketry" thing is the part I'm least good at, so might take quite a while to get to it.

Anyway was well pleased 

Playing with a new crystal filter

 I recently put together this board - I wanted a crystal filter that looked a bit neater than the one I built previously.  Here is the board and it's frequency response directly.

That is a lot of ripple, as the filter does not want to "see" 50 ohms.  The email from the vendor said that this has an approximately "native" impedance of 150 ohms.  So I made input/output transformers to get to approximately that.  Was better, but still a lot more ripple than I wanted.

I found this post talking about matching the impedance of the filter to fix the filter's response.  It didn't click at first exactly the procedure, but eventually got it.  Inserting a resistor in series with the filter increases the impedance it "sees", and the filter response adjusts accordingly.  So I made to little fixtures of sma connectors with an in-line potentiometer.  The only ones I had were 10k, so it was touchier than I'd have liked.  But found the impedance looked more like the 300-400 ohm range.  Note that the impedance the filter "sees" is the amount of inline resistance + 50 ohms (from the nanovna).

I made a new transformer that had a 7:20 turn ratio, and got a much better result.  (So far have only made one on the input, still using the lower-impedance 5:9 transformer on the output.)

The bandwidth is 3.8KHz at the moment.  I need to have a play with the other capacitors provided to get the value down to something better.  (This is why the caps are on there the way they are at the moment - to easily remove them to try others.)