[sdiy] [synth-diy] Frequency shifter which rejects negative frequencies

[cheater00 .]

Hi guys,

In a parallel thread, I have explained what lock-in amplifiers are and
how they work.
I have asked myself: what if rejecting negative and positive
frequencies, leaving only frequencies near DC, you could reject only
negative frequencies?

I'm not sure, but I believe in the digital world you can do the
four-quadrant multiplication and then take a Fourier transform of
that. But I'm very hazy on the details. Can anyone confirm or deny?

Is there an analog process that allows you to do this?

If you had a mixer (four-quadrant multiplier) that can reject negative
frequencies, you would get a low-pass or high-pass filter using
lock-in amplifiers. It would be pretty steep and fairly nice. The
frequency would also be voltage controlled.

Cheers,
Damian