Receiver Mixer and Filter module

By | 14th August 2017

Having re-evaluated the receiver mixer and filter stages I have re-designed the front-end to accommodate the first and second mixers with both the roofing filter and SSB filter daughter PCB’s on one single module, this allowed easy assembly and modular testing of the filter stages.

Each filter PCB has the impedance matching networks along with the crystal filter, the first mixer filter is a 10.7MHz, 7.5KHz bandwidth roofing filter followed by an Inrad 455KHz, 2.8KHz bandwidth SSB filter, both of these filters are terminated with a simple L-Match network consisting of a Toko 10K variable inductor and a trimmer capacitor to allow tailoring of the matching networks be done in-circuit. This was done using a Spectrum Analyser and tracking generator, allowing me to “Tune for maximum smoke” (TFMS a phrase I borrowed from Pete Juliano N6QW) and tune out any ripple.

I tested this module following assembly and was supprised to learn it worked first time – no errors, I had to adjust the mixer diplexers slightly to get the best signal to the input of the post mixer amplifiers, I was keen to test this module so using the demodulation function of the Spectrum Analyser I tuned the analyser to 455KHz and plugged in the headphones, tuned the DDS VFO to 11.852MHz (11.852 – 10.7 = 1.152Mhz) and with a random length of wire in the mixer RF input I was able to hear Magic 1152/Metro Radio II AM broadcast, little distorted due to the narrow SSB filter but still perfectly audible.

Here are a few pictures of the finished module taken by my good lady, who is a keen amateur photographer so an opportunity for her to practice her skills.


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