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RF Front end for a ADC

Discussion in 'Electronic Basics' started by ma, Nov 29, 2005.

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  1. ma

    ma Guest

    Hello,

    I like to do build or buy an RF front end for an ADC to be able to
    capture RF signal and process with them.



    Perfectly I like to cover the frequency range of FM and AM radios (150MHz).

    The RF would have an antenna interface and a heterodyne stage. The ADC that
    I have can sample up to around 100 MSpS but I like that my sampling rate be
    around 20 to 30 MSpS so the heterodyne should down convert to 10MHz.

    For AM radio receiving I think I don't need the heterodyne stage and I can
    sample at the rate of 1o MSpS.



    And my question: Where should I start? Can I buy anything such as this? My
    budget isn't very high so I can't buy anything more than 100pound or so on.



    Any help appreciated.



    Best regards

    Mansour
     
  2. Dave Platt

    Dave Platt Guest

    I like to do build or buy an RF front end for an ADC to be able to
    A lot of off-the-shelf FM radios use a double-conversion system... an
    RF stage, oscillator, and mixer, with the output of the first mixer
    being centered at 10.7 MHz. There's usually a tuned or crystal filter
    stage of some sort (bandwidth of perhaps 100 kHz) with some
    amplification, and then a second oscillator and mixer which creates a
    lower-frequency (often 455 kHz) IF.

    You could probably start with an off-the-shelf battery-powered AM/FM
    radio (a several-decades-old one would probably be easier to work
    with), tap off the signal at the output of the first mixer, and
    amplify it enough to give you a useful input for your ADC (a
    Minicircuits or similar monolithic amplifier would be an easy way to
    get gain). You might be able to use the existing IF strip and its
    amplifiers, if you jumper around the crystal or tuned-transformer
    filters and convert it to a broadband amplifier.

    Google for "software radio" - there are a number of projects working
    on RF front ends and ADCs like this, and there might be some which
    have a bandwidth as wide as you are looking for.
     
  3. Joerg

    Joerg Guest

    Hello Dave,
    Most of them are single conversion. 10.7MHz into a filter (or whatever
    they nowadays call 'filter'), then into the FM demodulator. The 455kHz
    section that runs in parallel is for the AM band(s).

    The only radios with any decent performance I have seen are the ones
    over $1000 or the old tube sets.

    Regards, Joerg
     
  4. ma

    ma Guest

    Hello,
    I aonly need the antena amplifier and hytrodyne section. No
    demodulation. I want to do demodulation in software.
    so my questos are:
    1- How can I design a simple antenna amplifer that connect to an antena from
    one side and amplify the signal to a suitable level for a adc (1Vpp).
    2- How can I design a hetrodyne section that can convert from 100MHZ to
    10MHz?

    best regards
     
  5. Joerg

    Joerg Guest

    Hello ma,

    You could still use an old radio. Just tap off at the mixer output.

    I suggest to get a copy of the ARRL Handbook. That shows in great detail
    how it's done.

    Regards, Joerg
     
  6. Dave Platt

    Dave Platt Guest

    so my questos are:
    "Experimental Methods in RF Design", also available through the ARRL,
    would also be a good reference source. Lots of detail there about the
    design of local oscillators, various sorts of mixers, IF amplifiers,
    and so forth.
     
  7. Joerg

    Joerg Guest

    Hello Dave,
    And then there is always the easy way out: Whip out the credit card, buy
    a Mini Circuits mixer and a nice synthesizer chip from AD. But that kind
    of spoils the sportsmanship.

    Regards, Joerg
     
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