Tuesday 12 June 2018

Input filter for SA612 mixer

We are planning to build a simple Direct Conversion RX, but with a Si5351 digital sythesiser VFO. The Si5351 has a square wave output with lots of harmonics. In order to limit these and avoid unwanted mixer products a simple filter is to be put infront of the VFO input.

IMG 0233
This is simply a 22pF capacitor and a 22uH inductor (commercial resistor-style) loaded by a 51R resistor. The Si5351 happily drives a 51R load with about the right input level for the SA612. The response of this arrangement is,

Screen Shot 2018 06 12 at 12 50 35

Which looks good. Now to try it in practice...

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