Low frequency signal generator

[Rich Grise]

Summing them won't work?

No. You have to modulate one with the other, or multiply them with
an analog multiplier. In fact, I've heard of an analog multiplier
that's optimized for balanced modulator use.

I wonder if that's really the case, if your analog multiplier
is "linear". It should be simple for someone who paid attention
in that class to answer this one with arithmetic.

The f1+f2 can be filtered out. It should be easy if f1,f2 >> f1-f2

True.

Cheers!
Rich

 

[Ben Bradley]

Some audio editing software allows you to generate a modulated signal.

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:

http://www.qsl.net/om3cph/sb/d cwithsb.htm

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

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

http://sine.ni.com/nips/cds/view/p/lang/en/nid/14604

 

[Robert Baer]

Summing them won't work?



The f1+f2 can be filtered out. It should be easy if f1,f2 >> f1-f2

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.

 

[Dirk Bruere at Neopax]

No. You have to modulate one with the other, or multiply them with
an analog multiplier. In fact, I've heard of an analog multiplier
that's optimized for balanced modulator use.

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

--
Dirk

The Consensus:-
The political party for the new millenium
http://www.theconsensus.org

 

[Don Bowey]

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

f1 and f2.

 

[Dirk Bruere at Neopax]

f1 and f2.

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
100% at f1-f2

Why would a summing amp be different?

--
Dirk

The Consensus:-
The political party for the new millenium
http://www.theconsensus.org

 

[Ban]

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 100% at f1-f2

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.

 

[Don Bowey]

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
100% at f1-f2

Why would a summing amp be different?

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.

 

[Tony Williams]

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

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.

 

[KoKlust]

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.

 

[Dirk Bruere at Neopax]

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.

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
http://www.theconsensus.org

 

[Don Bowey]

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.

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.

 

[KoKlust]

In short: The nonlinear element (diode in this case) is essential, and

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

You're welcome Dirk.

 

[Dirk Bruere at Neopax]

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.

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.

--
Dirk

The Consensus:-
The political party for the new millenium
http://www.theconsensus.org