Signal Multiplication

  1. 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. jcsd
  3. 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.
     
  4. f95toli

    f95toli 2,414
    Science Advisor
    Gold Member

    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.
     
  5. 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.
     
  6. 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?
     
  7. rbj:
    That pdf file was exactly what I wanted to see - thanks.
     
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