AM demodulation--> get envelope as well as carrier

Discussion in 'Math' started by ashwini1, Aug 3, 2009.

  1. ashwini1

    Thread Starter New Member

    Aug 1, 2009
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    Hi,

    I am working on AM modulation. At receiver end i want to separate my modulating signal with carrier signal. (i want to have both the signals available after demodulation.)

    Is there any way, where i can get back both the signals.

    Thanks !
    Ashwini
     
  2. AlexR

    Well-Known Member

    Jan 16, 2008
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    What you are looking for is called synchronous demodulation. A google on the subject should bring up plenty of useful links.
     
  3. ashwini1

    Thread Starter New Member

    Aug 1, 2009
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    Thanks for hint ... it helped me a lot ..
     
  4. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Dear Ashwini:

    I have a lot of experience with practical synchronous A.M. receivers; we used them almost exclusively in ionospheric research. Let me know if you need any "inside" secrets. :)

    eric
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
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    I have made some synchronous demodulators for skin conductance measurement. This method is the heart in all lock-in amplifiers. In a digital version the modulated signal and the reference/carrier are sampled. Then they are multiplied (sample by sample). By applying a low-pass filter you get the signal part. A high-pass filter should give you the carrier.
     
  6. ashwini1

    Thread Starter New Member

    Aug 1, 2009
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    Thanks for all the replies ..

    The basic intention is to calculate the modulation index or percentage modulation at receiver end. When i go with normal am demodulation i.e., multiplying with carrier signal and apply LPF with cut-off freq of carrier frequency, i am facing problem that due to dc component present in demodulated signal i am unable to calculate the correct modulation index. I am unable to filter this low freq/dc component completely.

    So i thought that, if i get carrier signal back at my receiver i can calculate the dc shift due to it and subtract the same from demodulated signal.

    If there is any other better way to get back the original signal in more faithful way plz let me know and that is appreciated .

    Thanks,
    Ashwini
     
  7. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Greetings:

    The modulation monitor used in A.M. broadcast stations has to contend with this problem. The traditional way of doing this is as follows;

    1) Rectify the carrier with a LONG TERM integration.
    2) Apply this voltage to the inverting terminal of a differential amplifier
    3) Rectify the envelope with a SHORT time constant. (Value determined by bandwidth and "diagonal clipping" restrictions
    4) Apply this voltage to positive terminal of differential amplifier.

    Output of diff amp will give you the modulation level, positive or negative.


    Catch 22: You can determine DC carrier shift ONLY at the transmitter. If DC carrier shift is measured at a distant receiver, there is really no way of knowing where it originates! This is why the FCC had/has very stringent requirements on DC carrier shift for broadcast stations!

    If you assume that there IS no carrier shift, the method above works for any remote receiver. In practice, DC carrier shift is pretty minimal...even in the ancient of days. :)

    What is your precise application? It may be a "non-issue"

    Eric
     
  8. t06afre

    AAC Fanatic!

    May 11, 2009
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    Sorry can not help you. I work with skin conductance, and I am very happy I do not need to remove the DC from the envolope. Since my signal is in range 0-5Hz. It is hard to filter DC from a signal very close to DC.
    Here is something you can try. After the mixing, calcualte the mean. Then subtract this value from the result after the low pass filter.
     
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