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

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!
>
> ;)

You were on the right lines with mixers. Apply the two input signals to a
balanced mixer, and you get output components at the sum and difference
frequencies. A mixer is a multiplier, and can be understood by thinking
about the following trig identity:

2 . cos At . cos Bt = cos (A+B)t + cos (A-B)t

Given your example of 400 and 410 Hz inputs, the sum and difference outputs
would be 810 Hz and 10 Hz, and you would need a low pass filter to remove
the unwanted sum product.



.

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