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Digital FM transmitter

Discussion in 'Embedded' started by Richard Sloan, Aug 23, 2005.

  1. What minimum processor/DSP would it take to mimic a FM transmitter, that is
    to say there would be no RF components, just an output voltage driving a
    small power amp, now I am thinking of this for low power applications that
    would fall under FCC Part 15 250uv/m @3m type applications.....

    I have search the web but find it difficult to find any info like this.

    Thanks for your time,
    Richard.
     
    Richard Sloan, Aug 23, 2005
    #1
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  2. Richard Sloan

    Bob Guest

    I recall a design in the 70s that used a quad NAND gate. Even that's not
    minimum, but you seem to want to use digital and "no RF components". What do
    hope to gain with a DSP?

    Bob
     
    Bob, Aug 24, 2005
    #2
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  3. Richard Sloan

    Tauno Voipio Guest


    In principle, you can generate any modulation with
    an I/Q (quadrature) modulator, but even here some
    RF parts are needed.

    You're pushing for a very fast DSP to generate the
    broadcast band FM (about 100 MHz). The minimum sample
    (and DSP cycle) rate will be a little over 200 Msamples/s.

    Even with a quadrature RF generator and suitable
    modulators (balanced modulators / analog multipliers),
    generating the suitable modulating signals for
    wide-band FM is a PITA (pain in the lower back).

    For a low-power signal, a simple analog oscillator
    and modulator is far easier.

    For good examples and basic knowledge for this kind
    of things, get the ARRL <http://www.arrl.org/>
    handbook and read the chapters of transmitters and
    modulation methods.
     
    Tauno Voipio, Aug 24, 2005
    #3
  4. maybe a hybrid version. Do the FM etc at 10.7MHz with a DSP, a common
    IF frequency then use a bit of analogue to bump it up VHF


    martin
     
    martin griffith, Aug 24, 2005
    #4
  5. Richard Sloan

    Tauno Voipio Guest

    Wide-band FM is still a PITA to create with a DSP, but
    very easy with analog means. There has to be some hidden
    advantage to use a DSP (or a bet).
     
    Tauno Voipio, Aug 24, 2005
    #5
  6. Richard Sloan

    Jan Wagner Guest

    One AD9952 (a DDS), followed by a small output filter and power amp
    or buffer (a single transistor should do). You can program it at
    25mbit/sec serial interface, and this give you (as a rough guess)
    1MHz of FM bandwidth.

    - Jan
     
    Jan Wagner, Aug 24, 2005
    #6
  7. Richard Sloan

    Thomas Magma Guest

    Thomas Magma, Aug 24, 2005
    #7
  8. And then? It can not be possible to do all thats needed in this part?

    It has at least 16 ADC's that can sample at good freq to do good FM?

    FM as in 106.7Mhz?
     
    Richard Sloan, Aug 26, 2005
    #8
  9. Sorry, 88-108Mhz FM :)

    I thought it would give you better THD, SNR, stereo seperation

    Thanks.
     
    Richard Sloan, Aug 26, 2005
    #9
  10. Richard Sloan

    Thomas Magma Guest

    My point was that the original poster did not specify what kind of FM
    transmitter was being mimicked. Frequency modulation, in it's simplest
    form, only requires an interface to the control voltage of a VCXO at the
    appropriate frequency.

    P.S. FM doesn't only exist at 88-108 MHz.

    Thomas
     
    Thomas Magma, Aug 26, 2005
    #10
  11. Richard Sloan

    Wim Ton Guest

    Analog Devices has an app. note with a basic DSP dev. kit and one of their
    DDS chips. They use a mirror/harmonic of the DDS signal to get to the 3m
    band. May need some filtering to comply with FCC rules.

    Wim
     
    Wim Ton, Aug 26, 2005
    #11
  12. Richard Sloan

    Tauno Voipio Guest

    As the frequency setpoint is stepped at the audio
    sampling rate (48 kHz or 44.1 kHz in the AN-543),
    there will be extra sidebands at the sample rate
    and its multiples. This is because there is no
    audio signal reconstruction filter before the
    modulation (and in this application there's no
    way to include it). The only way out from this
    problem is to change the frequency setpoint at
    each RF sampling cycle which would need a
    200 MHz+ audio sample rate.

    The signal is crisp and clean on a FM receiver
    on channel, but there is plenty of crud outside
    the allowed bandwidth.

    The trick used by the AD app note is to use
    the aliased frequency (125 MHz clock - set
    frequency) as the payload output.

    The app not device is OK for a directly connected
    FM receiver, but not suited to be connected to
    an antenna.
     
    Tauno Voipio, Aug 26, 2005
    #12
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