# How can I subtract one frequency from another??

F

#### Frank

Jan 1, 1970
0
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!

K

#### Ken Smith

Jan 1, 1970
0
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.

400Hz -----------
XOR ------/\/---+--------- Output
410Hz ----------- !
---
---
!
GND

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.

C

Jan 1, 1970
0
B

#### [email protected]

Jan 1, 1970
0
Charles said:

This is fine as far as it goes, but multiplying two sine waves produces
both the sum and product frequencies - 400Hz and 410Hz produce both
10Hz and 810Hz.

The higher frequency is easy enough to filter out in this particular
example, but for the general case you would want to search on "single
sideband modulation" or look it up in Horotwitz and (Win) Hill's "The
Art of Electronics".

Using logic gates or comparators to do the multiplication produces even
more components, as you get the higher harmonics in addition to the
basic sum and difference.

R

#### Robert Baer

Jan 1, 1970
0
Frank said:
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!

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

K

#### Ken Smith

Jan 1, 1970
0
Rich Grise said:
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.

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

[...]
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

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

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