How can I subtract one frequency from another??

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

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 need a device that multiplies to cause frequencies to add and subtract. A logic gate, flip-flop, diode, analog multiplier and camera shutter are all example of things that multiply.

Try making columns in a spreadsheet program to see what various operations will do, or get out a pencil and paper and do a little bit of trig. identity review.

You can also do it with certain ceramic capacitors, but this only works when you don't want it to.

Use a count-up / count down counter IC; the difference is seen at the output.

The multiplier already acts like a "balanced modulator" - what the OP needs to do is distinguish the difference frequency - if the reference freq. is unpredictable, then he might need a tunable low-pass filter.

Yeah, don't try to do it in logic - but, if you're only going up to 1 KHz, you could do it in a micro with a couple of a/d channels, if you don't need your answer instantaneously. :-)

Cheers! Rich

In article , Rich Grise wrote: [...]

He could use a D-flip-flop. They give you the difference as a square wave (with the odd glitch now and then)

If you use a good micro, 50KHz can be done.