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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.

Electrical Schematic

My circuits

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  • \$\begingroup\$ I might try 10 Mhz on boards like that. Maybe. Maybe not. At 1 MHz 30 pF is already near to 5k! At 10 MHz, that's now near to 500 Ohms. At 100 MHz?!?!? My wiring technique would have completely changed. I don't see everything you did. But none of what I see makes me feel comfortable. But I'm a hobbyist. I pretty much completely shift mental gears when I move past about 12 MHz and through a transition from there to say 30-40 MHz by which point I'm in a different mental space. I don't do much up past 50 MHz, though. I have to work too hard. :) \$\endgroup\$
    – jonk
    Commented Aug 12, 2021 at 7:04
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    \$\begingroup\$ Agreed, Ive had my fair share of challenges leading up to this point hahah \$\endgroup\$
    – Yousif Alniemi
    Commented Aug 12, 2021 at 16:19
  • \$\begingroup\$ Also see here and here, for examples. \$\endgroup\$
    – jonk
    Commented Aug 12, 2021 at 16:25

2 Answers 2

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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.

enter image description here

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: -

enter image description here

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.

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  • \$\begingroup\$ Good to know, I will try 10nf caps to decouple! Thank you! \$\endgroup\$
    – Yousif Alniemi
    Commented Aug 12, 2021 at 16:18
  • \$\begingroup\$ @YousifAlniemi Don't ignore Andy's comment about probing techniques! This isn't a throw-away comment. Clip-on ground leads have significant inductance to them. It's not rocket-science, but it also takes care and thought to make a useful connection -- especially to some nodes. \$\endgroup\$
    – jonk
    Commented Aug 12, 2021 at 16:23
  • \$\begingroup\$ Noted; thank you for the insight! \$\endgroup\$
    – Yousif Alniemi
    Commented Aug 12, 2021 at 18:47
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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.

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  • \$\begingroup\$ Wonderful idea, Thank you! \$\endgroup\$
    – Yousif Alniemi
    Commented Aug 12, 2021 at 16:17

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