# Modulate difference freq.

Discussion in 'Electronic Design' started by Harry Lang, Jun 10, 2007.

1. ### Harry LangGuest

I am producing a 100Hz beat signal by mixing 3KHz and 3.1KHz
sinewaves.

As an experiment, I was wondering if it is possible to frequency
modulate the two KHz sinewaves while keeping the 100Hz difference
frequency constant.

If so, what type of circuit would be required.

Harry Lang

2. ### Guest

Best to do this as a simulation on a PC unless you use DDS that is.

3. ### JamieGuest

2 very stable and maybe MILL spec VCO's that get biased
from the same source using all the same component values that
are closely selected for matches..
You could at least then discover the linearity of the components
you're using ..something on the same lines as CMR (common mode
rejection) I guess.

4. ### Phil AllisonGuest

"Harry Lang"

** A linear frequency modulator - silly.

........ Phil

5. ### MooseFETGuest

Do you have a control input to the 3KHz signals? Are you using VCXOs
or DDSes to make the signals?

If you have no control over the signal source, you can use phase
modulation to do the frequency modulation. Phase and frequency
modulation differ by one integration. If you are working over a
modest frequency range, this may be the way to go.

How accurately do you need to keep the 100Hz difference?

Do you have to start with the 3KHz and 3.1KHz or can you change the
circuit however you wish so as to make the desireed output?

6. ### John O'FlahertyGuest

Suppose there were a modulation ratio that could do that.
Then,
(3100 * Am * c) - (3000 * Am) = 100
where Am is instantaneous modulation amplitude, and c is some
constant.
Then (3100 * c) - (3000) = 100 / Am
and 3100 *c = 100/Am + 3000
Suppose Am = 1; then c = 1
Suppose Am = 2; then c= 0.983...

You would either have to supply a non-linear modulation function to
one of the modulators, or use something other than simple FM on the
two signals. Maybe a PLL on one of the signals.

7. ### Tom BruhnsGuest

An FM modulated signal offset a constant frequency is still an FM
modulated signal. Imagine one signal is the FM-modulated 3kHz. Mix
that signal with a constant 6.1kHz and take the difference; be sure
the mixer is very well balanced with respect to the input
frequencies. Now you have an FM-modulated 3.1kHz signal. Mix the
3.1kHz and 3.0kHz signals and take the difference; it will always be
0.1kHz.

8. ### John O'FlahertyGuest

Right, that's something other than simple FM applied to two starting
signals; and it sounds like a good way to do it.

9. ### Tom BruhnsGuest

Well, actually, that was a brain fart on my part! That SHOULD have
been a circuit to simply add 100Hz to one, not to generate one that
goes up while the other goes down. Sigh. Replace the 6.1kHz with
0.1kHz, but then be careful that you filter out the difference
frequency, and do the filtering in a way that preserves the phase
relationships: in other words, make sure that you delay both the
3.0kHz and 3.1kHz paths by the same amount, including the filtering.
It would likely be easier to generate, say, 100kHz FM and mix that
down to 3.0kHz and 3.1kHz, passing each through a low-order filter
that has essentially the same delay for both of the outputs.

The problem if the delay is unequal: imagine a modulation frequency
of, say, 200Hz, and a delay in the two paths that differs by 1/400 of
a second. The the modulations will be out of phase: one will go up
while the other goes down in frequency, and the difference is
modulated. That's extreme, but to have the difference exactly
constant, you need to have equal delays.

Cheers,
Tom

10. ### John O'FlahertyGuest

That's interesting. It seems to be a pretty high precision
requirement. Would it be possible to detect the 100 Hz difference, and
trim the delay accordingly?

11. ### Tom BruhnsGuest

Well, yes, I suppose so, though _why_ anyone would actually want to do
something like this I don't know!

Actually, things aren't all that bad if you start with a high
frequency and mix it down with two mixers fed by local oscillators
100Hz apart to 3.0kHz and 3.1kHz. If you start with a high enough
frequency, say several hundred kHz, a simple filter will kill the
local oscillator and FM source feedthrough and the sum frequencies,
and it can have high enough cutoff frequency that the delay difference
between two copies of the filter can be well under a microsecond with
no trimming of 5% tolerance parts (for the one I just simulated,
anyway). But as you say, it's possible to get things like this to
auto-calibrate if it's important.

Cheers,
Tom

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