2011-03-19
ARNSW Homebrew Group 2011 Challenge
The ARNSW Homebrew Group is doing another build challenge this year. Like previous years, this one is to build a practical piece of ham equipment. This year, a simple receiver for the 80 metre band. To make it even more practical and useful it was decided the receiver should be for 3.595 MHz, so one can listen to the weekly ARNSW/WIA broadcast from Dural.
I was a bit late starting the build and only just got it working before the broadcast on Sunday... It remains to be put into a case with an integrated audio stage.
Video
Circuit
The RF circuit is based around 3 J310 JFETs, one is a simple Colpitts local oscillator feeding the remaining two as a cascode mixer. There is some limited front-end pre-selection using a tuned circuit (HF IF can from the junkbox) in the gate of the lower mixer JFET. The LO signal is applied to the gate of the upper JFET and AF is extracted at the drain load resistor. Antenna input is either low-Z into a link winding on the front-end tuned circuit, or high-Z into the hot-side of the tuned/JFET gate circuit, directly or through a small capacitor for intermediate impedances.
The LO tank is a T68-7 core for stability, and selected NP0 capacitors getting me into the right frequency ball-park. An ancient bee-hive trimmer (older than I) allows tuning about 100 kHz but still has good spread for fine adjustment.
The AF stage is still on a breadboard, but is a Sallen-Key low-pass filter using a BJT emitter-follower, then a common-emitter BJT amplifier driving the input of my amplified speaker. I'll probably finalise the receiver with an integrated amplifier, or perhaps discrete...
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