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
