Low frequency signal generator

You can easily enough (with appropriate software) generate a very low frequency in a .wav file, but the earlier reason for doing it wbut virtually all playback devices roll off below 20 Hz.

Here's a webpage on modifying a soundcard's input to respond down to DC, perhaps its output could be similarly modified:

But I think that's a dodgy kludge either way.

Here's a 'real' interface with two analog outputs:

-----

Summing signal one and signal 2 in an operational amplifier cannot create other frequencies, assuming the opamp is working in the linear region. Like i said, you need a non-linear device.

f1 and f2.

You are not going to get a amplitude modulate signal by putting two signals through a linear device. You need a non-linear element to "multiply" the signals.

So what *does* come out if I put f1, f2 into a summing (or difference) op amp?

So two sine waves go in, and from a single o/p comes...? If I add two sine waves, f1 f1 in Audacity I get a signal amplitude modulated

Why would a summing amp be different?

Dirk, I would advise you to make a frequency analysis in Audacity and then tell us how high the level is at the beat frequency.

SinA + SinB = 2 * Sin(A+B)/2 * Cos(A-B)/2.

Which I think on the scope should look like a sinewave at a frequency of (A+B)/2, amplitude-modulated at (A-B)2.

OTOH.

SinA * SinB = 0.5 * [ Cos(A-B) - Cos(A+B) ].

Which produces separate sum and difference frequencies.

Out the opamp comes V1 = cos(f1*t) + cos(f2*t) .

Periodically with f2-f1 (or f1-f2), the two cosines will interfere constructively or destructively. So the ENVELOPE of the signal does contain f2-f1.

STILL if you would look on a spectrum analyser you see only TWO different frequencies.

So the envelope can be detected by rectifying and low-pass filtering Vout, to get the V2 = a*cos((f2-f1)*t) signal. (AM-detection). A very simple circuit that does this would be:

V1 --|>|---+----+---- V2 | | | | --- R | | --- C | | GND --------+----+---- GND

I guess that RC must be chosen somewhere around 1/(2*pi*fh), where fh is the highest of f1 and f2.

In short: The nonlinear element (diode in this case) is essential, and using only an opamp will not bring you the difference frequency.

Bull!

That is not what you said or even implied in previous posts.

Each time someone fed you info about what you need to do to obtain a sum and difference signal, you argued. What we all told you was simple facts about amplitude modulation. Law? Yes. The only post processing you talked about was "simple filtering to get f1-f2." That won't work no matter what is the assumed "spirit" of what you want to do.

Well, yes. That's what I was talking about originally (ie adding post processing) but kept being told it wasn't possible. Seems everyone looks at the letter of the law and not the spirit.

Thanks Dirk

The Consensus:- The political party for the new millenium

The point being that in order to get what I want a summing op amp is essential as part of the solution. I had that, and so I needed the rest of it. Which others have kindly supplied.

You're welcome Dirk.