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

Magnus Danielson magnus at rubidium.dyndns.org
Sun Jan 26 18:10:02 CET 2014

On 26/01/14 16:55, cheater00 . wrote:
> 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?

It works, if you have the phase quadrant of both signals.
This is why you have an elaborate filter to achieve this for the audio 

> 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.

Well, this is why we have frequency shifter modules, as it combines the 
filter, mixer and oscillators into one system.


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