# Signal Multiplication

1. Feb 9, 2008

### n0_3sc

In engineering we're always taught block diagrams in communication systems, specifically multiplying two signals...
How exactly do you multiply 2 seperate signals electronically? What does the circuit diagram look like?

2. Feb 9, 2008

### Corneo

You use a mixer. Block diagram of a mixer is a circle with an X inside. How does a inside of a mixer look like is another story.

3. Feb 9, 2008

### f95toli

The simplest type of mixer is just a non-linear element of some sort, usually a diode.
Nowadays mixer can be quite comlicated but in old-style crystal radios you can even see the diode which in their case consists of a thin wire in contact with a semiconducting crystal, the voltage-current characteristics of the metal-semiconductor interface is non-linear and can therefore be used to down-convert the radio signal back to audio frequencies.

4. Feb 9, 2008

### rbj

a PWM chopper circuit, with an LPF, can multiply two signals at low frequencies. it's just a sorta variation of looking at the "mixer" or "non-linear element" way of doing it.

also, there used to be these things we called Analog Computers that had very high quality op-amps in them in blocks for integrators, abs(), exp(), and log(). with a exp() and log() blocks, you can accomplish multiplication of positive voltages ("one quadrant multiply"). to do 2-quadrant multiply, i think there is something called a Gilbert cell that does it and i think they've been put together in a manner to accomplish 4-quadrant multiply (where you don't care about the sign of either voltage getting multiplied).

check out

http://www.analog.com/en/subCat/0,2879,770%5F847%5F0%5F%5F0%5F,00.html
http://www.analog.com/en/subCat/0,2879,770%5F848%5F0%5F%5F0%5F,00.html
http://www.analog.com/en/subCat/0,2879,773%5F862%5F0%5F%5F0%5F,00.html

http://www.analog.com/analog_root/static/raq/moreInfo/computation.pdf

you can get some pretty good app note and other information like that from Analog Devices. now they do a few different kick-ass DSPs (with warts) but when i was in college, they were purely analog.

5. Feb 10, 2008

### n0_3sc

f95toli:
How is a diode used to multiply them? I understand its a nonlinear element but is their an example circuit of how you would implement a diode and use it too multiply two seperate signals?

6. Feb 10, 2008

### n0_3sc

rbj:
That pdf file was exactly what I wanted to see - thanks.