SM0XVM

Here is my new mixer module. It is a superheterodyne receiver with a 4 pole 10MHz crystal filter. It uses a 10MHz crystal as BFO with the option to solder in a connector for an external MCU controlled BFO.

I have decided to use surface mounted components to avoid drilling since it is boring and creates lots of dust and noise. Sadly enough I could not find SMD versions of the SA612 at my local electronics supplier so I had to use the through hole mounted version. The connectors are also through hole mounted in order to get some mechanical stress relief.

Component side of the mixer module. The RF mixer is on the left with RF in and LO connector. The right side is the IF mixer.

On the solder side we can see the crystal ladder filter and the BFO with a trim capacitor to tune the BFO. The other trim capacitor is used to tune the LC circuit at the RF input.

To fix my problems with bad selectivity in my direct conversion mixer I am planning on building a superheterodyne mixer to replace the current mixer.

My current plans look something like this, use a SA612 as RF stage mixer to convert the incoming 7MHz signals to 10MHz. I do not think that mirror frequencies will be an issue since I have a low pass filter with cutoff frequency of about 9MHz before the mixer module.

The IF filter is a 4 pole crystal ladder filter. I have not yet calculated proper values fo the capacitors but somewhere around 20 to 150pF should work.

The AF mixer has both a crystal oscillator and a connector for an external beat frequency oscillator. I plan to start by using the crystal oscillator and then switch to using a DDS based oscillator which will give me the possibility to add IF-shift functionality.

You can find the schematics below