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Frequency Synth Chip for 40Mhz range

Discussion in 'Electronic Design' started by Bob, Mar 22, 2007.

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  1. Bob

    Bob Guest

    There are dozens of PLL synthersiser IC's that could be used
    along with a bunch of extra bits.

    You need to give more detail to get sensible answers.

    How many frequencys within that band do you need?
    FM? Some other modulation scheme?
    Do you want to send analog or digital signals?

    For a single fixed frequency with a typical varactor diode
    FM modulator and superhetrodyne FM receiver it may make
    sense to use a couple of crystal oscillators instead
    of a synthersiser.

    RF takes experiance to get things to work well.

    Being a bodger at heart for 40Mhz I'd be considering
    buying a load of basic 27Mhz CB radio's, adding
    a little board with a couple of crystal oscillators
    for TX and RX LO and changing about a dozen
    capacitors. Guessing from the retail prices
    CB radios must be available wholesale for about 5$
    each. Not ideal if you have portable use in mind.

  2. Hawker

    Hawker Guest

    Anyone know of a Freq Synthesizer chip that can be used for the ISM band
    (40.660-40.70Mhz). We may have an application where we will need to roll
    our own TX/RX.

    Since this is a low volume product (a few hundred a year) I would prefer
    to find a low power off the shelf TX/RX set in this range if anyone
    knows of any.

  3. For these frequencies the easiest solution would probably be to use a DDS
    chip (Direct Digital Synthesis), that will produce any waveform you want
    with tremendous flexibility. Downsides : quite high power consumption when
    active (may be 50mA), not very cheap (5$ ?). See

    Alternatives would be discrete PLLs, or even fixed frequency oscillator
    (quartz based ?), as there are very very few integrated transmitters working
    in these frequencies. To go further a close study of your requirements would
    be needed (in particular regarding spectral requirements : noise, FCC/CE
    approvals, stability, etc).

    PS : If you need an external expert to design your product, just ring us !

  4. Hawker

    Hawker Guest

    Well to answer your questions.
    I may need about 200 of these so modifying a CB is not practical.
    The ISM band is 40.660-40.70MHz so it would be nice if I could select
    anything in that range with say, 5Khz steps. It may be possible we want
    to jitter the frequency for reasons I can't go into here. Modulation is
    not really important. The device just needs to know a carrier is there.
    We may want to modulate something (FM probably) just so we can lock onto
    that and know we are not getting other interference. What we are
    interested in is variance in signal strength in the receiver based on
    carrier level, not any actual information. So I need both a transmitter
    and receiver solution.

    I realize we will probably hire an RF person to do this, but I want to
    get an idea what our options are in parts first.

    am I any clearer?
  5. colin

    colin Guest

    Theres quite a lot of chips with PLL and some with VCO as well,
    depends if you want one that has both integrated or not,
    there are some PLL that have much higher frequency,
    ie 1ghz but these things are also able to work with a reference frequency of
    up to 100mhz,
    therefore you could provide it with a reference frequency of 1ghz from a
    frequency multiplied XCO into the vco port
    and put a LC vco into thhe ref port. would give you a good resolution.

    Colin =^.^=
  6. Hawker

    Hawker Guest

    thanx, That is very helpful. I don't really have a grasp on the number
    of bits I need though. It seems like if it is two few I would get to
    many harmonics on the output, but perhaps not.
    I also need to figure out the receiver.

    I also appreciate your offer for help. Unfortunately my client insists
    that all contracts and supplier and US so he can say "designed and build
    in the US". I do digital design for this client and he wanted me to do
    this project. I told him I was not an RF engineer but he seems to want
    us to do the work anyway. I'm trying to act more as a project manager
    than engineer and doing some preliminary some leg work.

  7. Here's something I had in the archives. I might even have a PCB

    Frank Raffaeli
  8. Barry Lennox

    Barry Lennox Guest

    Motorola had a whole lot of them in the p/n range MC145162 to 70. The
    max freq varied, but was between 60 and 185 MHz.

    AFAIK, they dropped the range several years ago, but they are still on
    the market, and it should be possible to get the small number you

  9. In a DDS-based design the number of bits is probably not a bit issue for
    this application, as it has links with the frequency resolution (linked to
    the DDS phase registers bit length, but you will have at least a 1Hz
    resolution with the worse DDS) and with harmonic content on the output
    (linked to the DDS DAC resolution). Harmonics are quite easy to filter, but
    the most difficult aspect of DDS are spurious signals in the output, which
    are linked to all of the parameters... You will find simulation tools on the
    AD web site, but you will have to carefully check it with the exact
    frequency and modulation parameters of your application, as well as with FCC
    regulations. Often you will find that you have both to select a particular
    main crystal frequency in order to "move" the spurious far enough from your
    carrier, and to add good pass band filters to remove them.
    US guys often want to do things alone, like in I... (no, I will not say it,
    as I don't want this thread to go be jeopardized :)

  10. I wasn't aware this band was used in the U.S.

    You can make a cheap (<$14, in low volume) high performance TX-RX up
    to about 70 MHz with an amp, ADC, FPGA, a micro-controller, and a
    switch. No pesky analog mixers, modulators or demodulators necessary.
    You don't need an RF guy ... 40 MHz is D.C. ;-)

    Frank Raffaeli
  11. Hawker

    Hawker Guest

    Thanx, looks like a good start,
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