Re: How can I subtract one frequency from another ???

If you're going to be using pure tones and can amplify them enough so
they're clipped to resemble square waves, by far your best and simplest
solution is to use a D flip flop as the mixer. Don't use TTL logic for this
and shunt excess signal voltage to ground and the positive supply with
diodes. The advantage of a D flip flop is that it produces only the
difference frequency so there's no need to filter the output. Input one
signal on the D input and the other on the C input. You can take the output
at either Q or Q bar. If you send music to it however, you're going to get
a strange output waveform.

- jgreimer

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@xxxxxxxxxxx> wrote in message
> Frank wrote:
>> Hi all,
>> A pretty basic question, but I seem to be having a brain block about what
>> approach to take for this application.
>> I would like to take 2 different frequencies, between lets say DC to
>> 1000Hz,
>> and subtract them from one another to get the output signal frequency as
>> the
>> difference between the 2 signals.
>> For example, 400Hz in one input, 410Hz in the 2nd input, the output will
>> be
>> 10Hz.
>> My first instinct was to use an op amp as a comparator, then I thought of
>> a
>> differential amplifier, then, I stumbled across some mixer schematics,
>> and
>> PLL schematics, and then some really complex filter IC's, by that time I
>> was
>> well confused.
>> I just want this to be as simple as possible, one IC if at all possible
>> and
>> some periferal passives.
>> Could one use an LM324 configured as a differential amp? What confuses me
>> is
>> the CMMR part of it, and the fact that the differential amp seems to only
>> "differentiate" voltage differences, and not "frequency" differences,
>> however the common mode rejection will reject like frequencies from both
>> inputs.
>> I'm pretty rusty on my op amps, so I was hoping someone might be able to
>> at
>> least point me in the right direction with this.
>> Thanks!
>> ;)
> LM1496 chip maybe?
> --
> Real Programmers Do things like this.