How to isolate leaky LO and antenna for FM radio?

Question

I am building an FM receiver. My problem is that when powering my antenna circuit or LO circuit one at a time, I get the output of those circuits appearing very strongly on my power supply lines.

When I connect both LO and antenna circuits to power, I get an extremely distorted waveform at the output of either. An FFT shows my amplified antenna frequencies, my LO frequency and a ton of large unintended peaks.

How can I electrically isolate these two circuits from my power supply and each other? Pictures of my setup are included.

Answer 1

My problem is that when powering my antenna circuit or LO circuit one at a time, I get the output of those circuits appearing very strongly on my power supply lines.

Relying on electrolytic capacitors to decouple the power lines at circa 100 MHz is naïve. They will become inductors waaaay lower than 100 MHz. Use proper HF decoupling capacitors such as 10 nF. Even SMD ceramic capacitors are not that good if you choose a value that is too high: -

Image from here.

Of course, the problem might be made to appear worse by poor oscilloscope probing techniques such as loops in the earth clip. Breadboard doesn't help either.

Answer 2

It is common practice that you shield the LO to stop it radiating and/or coupling where you don’t want it to. This is done by putting the LO into a box usually made out of tin plate. I’ve been known to use an empty soup can - use tinsnips to cut the can and you should be able to bend and solder it together using a moderately powerful soldering iron. Some newer can may have a plastic coating on the inside which makes the task a bit harder.