80 Meter SSB - Part 2 - Superhet Receiver

Some time ago, I purchased an RF board off of ebay for a few bucks so I could repurpose the crystal filters. I figured that I’d use them someday and I guess today is that day.

Below is a picture of the board as I received it.


There were three German KVG filters, an XF-9 S 42, and XF-9 S 43, and an XF-9 S 44. The S 42 filter is an AM/DSB filter, the S 43 is LSB, and the S 44 is USB. They are all centered on 9MHz.


After removing the filters, I modified my 80 meter direct conversion receiver. I added an additional ADE-1 mixer and put together two IF amplifiers on each side of the S 43 crystal to convert the direct conversion receiver into a superhet.

Below is the schematic for the identical IF amps. The only change is the input/output transformers.


From the little I could find about these crystals, the input impedance seems to be 1k ohms, so for the output transformer of the first IF amp, I wound a 7:14 turn transformer on an FT37-43 to transform the 200 ohm output of the amp to the 1000 ohm input of the filter. Between the filter and the second IF amp is a 6:26 turn transformer taking the 1k filter down to 50 ohms for the input of the second IF amp. From the second amp to the mixer is a 12:6 turn transformer taking the 200 ohm output to the 50 ohm input of the mixer.

I also read that the crystal filter should have 24pF in parallel to the input and output. The radio worked fine without it, but I added a 22pF cap on the input and output and it did seem to help.

Below is the current configuration. There is still a little noise present, but it is receiving quite well.


Next step is to make it transmit.