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Project Notes

#515 FM Converter

Playing with a simple FM frequency conversion circuit, and demonstrating how it can be used for external tuning.


Here’s a quick demo..



Frequency converter circuits are used to shift a signal band. A common application in the past was to shift VHF/UHF TV signals (say ~35.3 MHz) to the FM broadcast band so they could be received with a normal FM radio.

Although I’ve no practical use for this, it is interesting to investigate how it is achieved with so few components.

The idea and design for my build comes from Lechoslowianin’s video on Youtube:



I’ve made a few modifications and component substitutions.

  • BF245 n-channel depletion mode JFET - replaced by J201
  • AAP155 germanium diode - replaced with 1N60P
  • C1, originally 47pF, replaced with 5pF
  • C2, originally 51pF, replaced with variable capacitor (actual range 9pF - 52pF)
  • L1: total ~0.48µH. 0.9mm copper wire, 9 turn air coil ~30mm long, ø8mm, with tap on the 4th turn from ground
  • L2: 8.5µH. About 30 turns of 0.1mm enamelled wire on ø5.5mm adjustable inductor shell with ferrite core

These may be far from ideal, but are what I arrived at for a workable circuit that transposes the FM broadcast band (rather than shift another band into the FM broadcast band). As built, this circuit is more like an external FM tuner rather than a band converter.

Performance? Not that great - very sensitive and prone to hum - but it basically works. Here’s my version of the circuit in EasyEDA:


How does it work? To be honest, I only have a sketchy appreciation so far. The tank circuit is pretty self-explanatory (FM tuner), but I think the magic occurs between the inductor tap and “peak detector” diode, producing the frequency-shift. I’d like to get a handle on the theory for how the band conversion calculations could be done, but a bit more research is required first.

Credits and References

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This page is a web-friendly rendering of my project notes shared in the LEAP GitHub repository.

LEAP is just my personal collection of projects. Two main themes have emerged in recent years, sometimes combined:

  • electronics - usually involving an Arduino or other microprocessor in one way or another. Some are full-blown projects, while many are trivial breadboard experiments, intended to learn and explore something interesting
  • scale modelling - I caught the bug after deciding to build a Harrier during covid to demonstrate an electronic jet engine simulation. Let the fun begin..
To be honest, I haven't quite figured out if these two interests belong in the same GitHub repo or not. But for now - they are all here!

Projects are often inspired by things found wild on the net, or ideas from the many great electronics and scale modelling podcasts and YouTube channels. Feel free to borrow liberally, and if you spot any issues do let me know (or send a PR!). See the individual projects for credits where due.