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How do I select a transformer for superhet? (self.askelectronics)

submitted 3 months ago* (last edited 3 months ago) by acannan to c/[email protected]

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I've recently been learning about superhet and frequency mixing and wanted to start tinkering. Specifically I'd like to try using two heterodynes in series to first frequency shift then uninvert the original audio, sort of like an analog frequency shifter.

To do this, I'd need a frequency mixer. I've been looking at a ring modulator (like https://upload.wikimedia.org/wikipedia/commons/1/1b/Diode_DBM.png) which should require 4 shottky diodes and 2 center-tapped transformers. I've had difficulty locating an affordable transformer, with good enough fidelity for audio, that also includes a center tap.

I have a few questions:

Where can I locate affordable, good-enough-for-audio transformers?
Is the ring mod approach good enough? I see there's also a gilbert cell.
Any general advice for someone just starting a project like this?

Thanks!

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[–] [email protected] 3 points 3 months ago (2 children)

I'm not sure I understand what you are trying to do... do you want to build a radio? Or are all your signals in the audio range?

Anyway:

w2aew has some great videos on diode ring mixers, like this one about winding the transformers: https://www.youtube.com/watch?v=a8ViWS61hsU
You can also get ready-made diode ring mixers (including transformer and everything) like this one: https://www.minicircuits.com/pdfs/SRA-1W+.pdf

Regarding Gilbert cells, the two popular chips are MC1496 and SA631. The 631 comes with a built-in oscillator, so it's quite handy. Unfortunately both are hard to come by these days.

[–] acannan 1 points 3 months ago* (last edited 3 months ago) (1 children)

Sorry for the ambiguity, I intend to use this for audio applications (specifically I want to shift 20 kHz to 100 kHz down to human hearing range, tunable by a pot.)

Given the price and low supply I think I'll go ahead and try to wind my own transformers--thanks for the video, seems perfect!

[–] [email protected] 1 points 3 months ago

Great, good luck with that :)

Another thing that comes to mind: for audio purposes another technique used in ring modulators for audio effects is to use a mosfet switch to mix the signal with a square wave. This has more byproducts than mixing with a pure sine, but is a lot easier to do. Since you are downconcerting, it should not matter at all if you use a square wave, since the byproducts will all be (higher-order) harmonics of the local oscillator, which you'll filter out anyway.