A project log for K8TRC 6m QRP transceiver

A 6m SSB/CW amateur radio transceiver.

trevor-rh-clarkeTrevor R.H. Clarke 06/21/2020 at 20:460 Comments

I designed and build the preselector this weekend. It's a 3rd order Bessel response. The schematic and Qucs simulation results are below.

Preselector Schematic
Simulation Results

The plots show a response from about 51 MHz to 55 MHz, a bit high for a weak signal 6m radio. I've found that stray capacitance and such usually pull the response lower when I actually implement a filter on an FR4 based prototype board so I went ahead and used the values as above.

I used SMD parts with C0G capacitors and standard value inductors. I happened to have the caps around otherwise I would have used something more temperature stable, we'll see how these do.

Initially, the center frequency was around 50 MHz with an upper range (-3 dB) of 52 MHz. This was a little low but usable. I removed 100 pF shunt capacitor to try and pull the frequency up a bit. The new center is at 50.26 MHz with a -3 dB range or 47.5 MHz to 53.1 MHz which is a bit better. Since this will be a radio for weak signal work in the lower part of the band I only need reasonably passage through about 51.6 MHz or so. While I could have dialed it in a bit more I didn't feel like looking for other capacitor values in my junk bin so this will do. The blue traces are the original plots and the yellow-green traces are with the 100 pF cap removed.