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Amplitude Modulation with Function Generators

  1. Aug 2, 2005 #1

    Tom Mattson

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    Hello everybody,

    I am trying to use a pair of Tektronix FG 503 Function Generators to modulate a carrier wave and produce an AM modulated signal. I tried running the output of one generator into the VCF input of the other, but it seems to give me FM modulation. I did this because I am working from a lab handout that calls for function generators that have a modulation input feature.

    What, if anything, can I do to get AM?
    Last edited: Aug 2, 2005
  2. jcsd
  3. Aug 2, 2005 #2
    If the signal generator won't take amplitude control,
    you need to get a voltage-controlled amplifier.
  4. Aug 5, 2005 #3


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    You have a couple of options, depending on how comfortable you are with building support circuits. You can use a double-balanced mixer component:


    You can buy these from Mini-Circuits, and probably even through Digikey. Another option is to use a double-balanced mixer IC based on the Gilbert Cell:


    And a 3rd way would be to brew your own AM transmitter based on opamps and transistors -- similar to the Gilbert Cell architecture, but a little more precise.

    Hope that helps. -Mike-
  5. Jan 6, 2006 #4

    George Jones

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  6. Jan 6, 2006 #5


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    Just a couple of caveats about cheating the ground connection on equipment that uses a 3-prong AC mains plug....

    First, remember that the 3rd prong is there for a safety reason. The metal chassis of the instrument is connected to this safety ground so that if the AC mains Hot wire comes loose at the input and ends up touching the metal chassis (which is user-accessible and could shock you), the fuse will blow because of the short cicuit. When you cheat the ground on an instrument, you lose this safety feature. Granted, it's a pretty unlikely failure mode for most commercial instruments. But it's a more likely failure mode for a hand-built prototype like you might see in a lab....

    Second, cheating the ground only really works for low-frequency applications. The power supply in most instruments has enough leakage capacitance from its outputs to its AC mains Hot/Neutral inputs, that for high frequencies you cannot break the connection. So cheating the ground on an instrument will likely help you at audio frequencies, but not in the 10+ MHz type range.
  7. Jan 6, 2006 #6
    There was a discussion about this sort of thing a year or more back. There was a member (who no longer is a member) who insisted that AM is nothing more than a varying amplitude of a single frequency. I finally lost interest in arguing with him. A composite AM signal consists of 3 frequencies. They are the carrier and upper and lower sidebands. I told him he could achieve the same composite signal without actually changing the amplitude of anything and just summing 3 individual signals together. I posted quotes from text books and everything. He was a slippery individual who seemed to be able to make himself sound right and quite proficient at twisting around things that I and he had already posted into sounding like something else.
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