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

[Donald Tillman]

On Jan 26, 2014, at 7:55 AM, cheater00 . <cheater00 at gmail.com> wrote:

> 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.

Again, your description is confusing, but I think you're describing a tunable heterodyne filter.  Basically, given an audio input and a carrier frequency:

    multiply audio input by carrier frequency ->
    through a fixed filter ->
    multiply again by carrier frequency 

So it modulates the input up in frequency, filters, and then demodulates back, and the tuning of the filter relative to the input is determined by the carrier frequency.

There's also a quadrature version.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com