AM Modulation

Discussion in 'The Projects Forum' started by mattturner81, Jun 17, 2009.

  1. mattturner81

    Thread Starter New Member

    Jun 17, 2009
    I am working on creating an AM sine wave utilizing an Agilent 33250A Function Generator. I would like to modulate the sine wave using an external digital stimulus that I am generating with a Xilinx Spartan 3 development kit to produce and ASK (amplitude shift keyed = modulation depth of 100%) signal. My problem is this:

    The spartan3 dev kit has a voltage level of 3.3V on output ports when high and 0V on output ports when low (digital CMOS levels). The external modulation input of the 33250A requires a -5V to 5v full-scale signal. Therefore I must level shift the output of the spartan board to -5volts when low, and +5 volts when high to achieve a modulation depth of 100% for ASK. As my modulation signal is digital, I don't really care about the inbetweens.

    More simply, I have a digital signal that is either 0V or 3.3V. When the signal is 0V, I want to run it through a converter that outputs -5V. When the signal is 3.3V, I want it rasied to 5V.

    Suggestions? I have access to both a split voltage (-20 to +20) power supply and am interested in any transister/op-amp solutions and IC solutions that may exists.

  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008

    A dbm or a PIN diode switch will do this admirably
  3. beenthere

    Retired Moderator

    Apr 20, 2004
  4. SgtWookie


    Jul 17, 2007
    Optocouplers might be a good solution for matching your 0v-3.3v to -5v/5v signal levels. They're available in 6-pin DIP single and 8-pin DIP dual configurations, as well as many others. You'd need a current limiting resistor for the input diode, and a resistor to +5V from the collector for the output.

    See the attached for the idea I'm talking about. You may need to click on the image once it comes up to get rid of distortion. Note that the 4N25 optoisolator I used is of the "old 'n' slow" variety; faster IC's are available.
    Last edited: Jun 17, 2009