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what adc to use for digital am receiver

Discussion in 'Electronic Design' started by Bhargav, Apr 21, 2007.

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  1. Bhargav

    Bhargav Guest

    Hi All,

    I am thinking of designing a digital am receiver, I dont know what
    typical am voltage levels would be and what a/d convertor( i mean what
    should be the resolution) to use for it. Can anybody help?

    Thanks.
     
  2. linnix

    linnix Guest

    What are you trying to digitize?
    Digitized tuner or digitized output?
    They drives different requirements.
     
  3. Bhargav

    Bhargav Guest

    HI Linnix,

    My radio would consist of a antenna and then a Low pass filter
    (analog) to select only the AM band. After this lowpass filter I want
    to use a a/d converter so that I can carry out the rest of
    demodulation( like downconversion etc) digitally.

    Thanks.
     
  4. Bob

    Bob Guest

    Peak voltage from the antenna will vary from a few tens of
    microvolts to volts depending on your antenna and
    how close you are to the nearest AM transmitter.

    My guess is that with a 10 bit ADC it would be normal to
    not have enough bits to demodulate weak stations with the
    input gain at a level where strong local stations are not
    clipping.

    Bob
     
  5. linnix

    linnix Guest

    That's why you need the tuner to block the strong stations and
    AGC (Automatic Gain Control) to normalize the tunned signal.
    10 bits should be fine for voice. Don't expect HiFi, digital or not.
     
  6. joseph2k

    joseph2k Guest

    You are not getting OP's goal, it is to digitize and simultaneously
    demodulate all channels at once. 12 bit 20 MSPS ADC is minimum for the
    task, more bits means better channel seperation and S/N for any channel.
     
  7. Tom Bruhns

    Tom Bruhns Guest


    If you are not too close to a local station, and if your local
    stations don't run a lot of power, you should be OK with about 0dBm
    full scale. If you have one station that is very strong (e.g., 5kW a
    kilometer away) I would recommend a passive notch filter to attenuate
    it. If you set full scale at about 0dBm, you should do fine if your
    ADC has a noise level of about -120dBfs/Hz. You'll want an ADC with
    low intermodulation distortion and good linearity to low levels. You
    only need a sample rate a little over twice the highest frequency of
    interest, but beware of the alias protection requirements. We
    generally use about 11th order elliptical filters. Nice thing is that
    the AM broadcast signals are generally pretty strong, so unless there
    are local hams or other users of the HF band, the potential alias
    signals shouldn't be terribly strong compared with the AM broadcast
    stuff. But because local conditions can vary drastically from place
    to place and even time to time, be prepared to either characterize
    your spectrum beforehand, or adjust things after you build the
    receiver. The actual number of bits in the ADC is less relevant than
    you might think, provided it has low enough noise and good enough
    linearity and distortion performance. Also, you can trade off sample
    rate for number of bits, to some extent: decimating a faster sample
    rate will get you more bits (slowly).

    If it were my project, I'd look to digitize at 5Ms/s; the antialias
    filter would cut off above 2MHz and would be at least 100dB down at
    3MHz and above. (11th order elliptical should get you close to 120dB
    if you're careful in its construction and use good coils.) I think
    there are a few good ADCs that will do 5Ms/s and 16 bits...but since I
    generally work at much higher sample rates, I'm not sure what the
    current state of 5Ms/s parts is.

    Cheers,
    Tom
     
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