Simple Amplitude Modulation Circuit

Discussion in 'The Projects Forum' started by bruzzac, Jul 10, 2008.

  1. bruzzac

    Thread Starter New Member

    Jul 10, 2008
    Hi guys,

    I'm not sure if you have heard of DCF77, but I need to create a simple DCF77 generator which is basically a 77.5kHz carrier that has 1 bit a second encoded into the carrier. It is lowered to 20% of the amplitude for either 0.1 or 0.2 seconds at the start of each second depending on whether the bit for that second is a 0 or a 1.

    I have tried using a simple voltage divider network such as the one below, but with a transistor and resister in parallel with the second resistor which would control the amplitude. However I am getting a weird waveform.. the negative side of the sine wave does not have the same amplitude as the positive side.

    Code ( (Unknown Language)):
    2. ---/\/\/\/\-------/\/\/\/\-------
    Code ( (Unknown Language)):
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    I would like to know if anyone has had any similar experiences with this kind of modulation scheme and see if anyone has any suggestions.
    Thanks in advance

  2. Wendy


    Mar 24, 2008
    Sounds like clipping, but without the whole schematic it is impossible to be sure. Need input.

    Are you switching between the two points with a transistor or what?
  3. bruzzac

    Thread Starter New Member

    Jul 10, 2008

    this is basically what i have come up with.. the resistors have been setup accordingly so that it should bring the 77.5kHz carrier down to be about 0.2%-0.25% however, with this setup I am getting a waveform like the one attached. So I was wondering if there was another way of implementing such an switching attenuation network?

    Also, I am not really able to use an amplifier, as I will only have a positive 5V available. Although, I maybe able to use an amp with biasing the signal 2.5V.
  4. thingmaker3

    Retired Moderator

    May 16, 2005
    Since the transistor conducts in only one direction, the divider attenuates only the negative going peaks. If you had used a PNP instead of an NPN, only the positive going peaks would have been attenuated.

    Perhaps a voltage-controlled amplifier would better suit your needs.

    I suppose a clever person might be able to use a triac instead - don't know for sure.