Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

AM demodulator help

  1. Dec 9, 2011 #1
    Hey all,

    I'm building an AM demodulator (like the ones in radios) and it seems that I have some problems with the circuitry.

    I plan to modulate the AM signal with 200kHz sine wave, but some has suggested that this frequency was too high and I should rather use a lower frequency carrier wave. Also, currently I am using a highpass filter with
    R= 10 ohms / C= 1 uF ~ cutoff freq: 150kHz
    and a lowpass filter with
    R = 6ohms / C=1 uF ~ cutoff freq: 250kHz.
    For some reason, however, the wave after the filters seem to be dampend by a great deal and does not recover the original wave very efficiently.

    All inputs will be appreciated.

    Thanks.
     
  2. jcsd
  3. Dec 9, 2011 #2
    Modulating with a 200KHz is way way too high. You want to start with a carrier frequency of a few hundred kHz and modulate with a signal of say 5KHz to play with. For a carrier that can accept 200KHz modulation frequency, you need carrier to be way into 50MHz. And at that the side band will cause too much interference to other radios.

    You use too low a resistor for your filters. You cannot use 50Ω or so resistor and a big cap. It takes a lot of power to drive that low impedance. You want to start with say 1KΩ or higher, then most of the transistors or op-amp can drive easily. You do the calculation like what you do by scaling down the capacitance. Only when you get to much higher frequency then you need to use 50Ω type of impedance.
     
  4. Dec 9, 2011 #3
    Are you radiating your signal? If you aren't radiating the signal it really doesn't matter what frequency you use. What is your carrier frequency? Are your filters used before or after the demodulator? Can you post your circuit?
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: AM demodulator help
Loading...