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super fast divide-by-N

Discussion in 'Electronic Design' started by Thomas Magma, Aug 16, 2005.

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  1. Thomas Magma

    Thomas Magma Guest


    I am looking for a fast divide-by-N chip with a high bandwidth. I'm hoping
    for an input frequency of 1 GHz but would probably settle for one around 80
    MHz. Duty cycle doesn't matter for the DSP application I have in mind but I
    do need to run at 3V.

    Thanks to anyone who knows of one and cares to share,

  2. Jim Thompson

    Jim Thompson Guest

    ...Jim Thompson
  3. Thomas Magma

    Thomas Magma Guest

    Thanks Jim,

    Nice parts but I can only see a divided by 2 and divided by 4 from that
    company. I'm kind of looking for a divide by N. Maybe up to 32 or so.

  4. PeteS

    PeteS Guest

    Once you have divided by 2 or 4, you are in the realm of fast logic
    (look at the various fast logic families).

    One search I would suggest is 'fast prescalers'.

    Jim has set you on the right path :)


  5. Jim Thomas

    Jim Thomas Guest

    Above, you imply that there are only 32 values of N (1-32). Can you
    multiply by 1/N instead?
  6. Jerry Avins

    Jerry Avins Guest

    That's a pretty broad range, Thomas. Like me asking a bank for a loan of
    $1,250 and saying I'll settle for a dollar.

  7. Thomas Magma

    Thomas Magma Guest

    Can you multiply by 1/N instead?

    Ah...yes?... I don't quite know what you mean here. I can see how they are
    equal mathematically, but I don't know how to achieve this with electronics.
  8. Look at OnSemi, under high performance counters, you'll find ones that
    go 1.4GHz. You will pay for this performance :)
    If a few hundred Mhz is OK, then look at any modern 32 macrocell CPLD
    [ Xilinx / Lattice ], and they are closer to $1
  9. Thomas Magma

    Thomas Magma Guest

    Ha. Let me give you a bit of back ground then. I am really pushed for space.
    I need different clock frequencies thru-out my board. My highest frequency
    is a Fox RFXO running around a GHz. I would like to derive all other clocks
    from this one if possible. Next lowest frequency is around 80 MHz. Then a
    few after that. I hate PLLs and I don't want a bunch a xtals on my board. So
    I thought I would just divide down from my highest frequency. One chip
    solution would be the best.

  10. Chris Jones

    Chris Jones Guest

    The important number is what is the minimum divide ratio that you need. If
    the minimum N is a reasonably large number then you can use the
    conventional approach used in RF synthesisers, where you have a fast
    prescaler (e.g. divide by 8 or 9) and then a slower block of logic that
    decides when to switch the prescaler into divide by 8, and when to use
    divide by 9.

    You might be able to find a RF synth chip where the divider output is
    available at a pin. Look at the Analog Devices ADF4111 for example which
    can mux out the divider output to a pin.

  11. Jerry Avins

    Jerry Avins Guest

    It's a good approach, and the reasoning seems sound, but I was struck by
    the very large spread between "need" and "want".

  12. On Tuesday, in article <[email protected]>
    Have you looked at ICS <>. You might find this useful
    or not


    Upto 2GHz input and various divide outputs 1,2,4,8,16

    They also do other dividers and buffers. Usually I find the costs, very
    low in small quantities. However distributing all the clocks divided around
    the board will be fun. ICS also do small PLL chips that work well and are
    cheap, small and could run off of one lower clock drive. Unless of course
    you all the clocks synchronised to the highest frequency.
  13. John Fields

    John Fields Guest

    You should be ashamed of youself! <G>

    When you ask for a super-fast divider around here, (excuse me for
    seeming to be territorially authoritative) without specifying that
    it's a clock frequency divider, you raise the spectre of a machine
    which can grind out quotients in arbitrary timeslots, the ideal
    being loading the dividend and the minuend on one edge of the clock
    and then spitting out the quotient on the next edge.

    Clock dividers? Tell us what frequencies you need, your source
    frequency and the skew you can stand, and you'll get an an answer
    soon enough. :)
  14. Mac

    Mac Guest

    There are DDS's that can run off of a 1 GHz clock. The Analog Devices
    AD 9858, I think it is. That might be massive overkill for what you are

    Instead, you could divide the 1 GHz down by 4 or 8 using some kind of
    prescaler, and then feed the resulting 250 MHz clock to a CPLD or
    something which could implement whatever kind of divide-downs you want,
    and produce multiple copies.

    You don't say anything about jitter.

    I dropped comp.dsp, because it seemed to have more to do with hardware

  15. Jon Harris

    Jon Harris Guest

    I think he is talking about dividing a clock signal down by N, down dividing a
    digital value by N (e.g. in a DSP). I missed that point the first time
    too--reading this in a DSP group, I assumed it was a DSP question, but notice
    the cross-posts. It wasn't until I read some of the responses that I realized
    what he was trying to do.
  16. MC100EP016A :
    3.3V ECL 8bit synchroneous Binary up counter,
    operating frequency > 1.30GHz, LQFP32, [email protected]

  17. While a divide-by-N could be implemented with an up-counter and using
    some gating to detect N and asynchronously reset the counter, this
    will usually have some timing glitches. Usually a divide-by-N is
    implemeted with a presettable down counter, when "0000" is detected, N
    is loaded into the counter.

    If you do not need the divide by 1, some synchronous presetting could
    be used, i.e. the gating detects the "0001" state and let the next
    clock pulse do the actual presetting.

    This could be done with up-counters, but now the gating would have to
    detect the N-1 condition to perform a synchronous reset at the next
    clock pulse.

  18. How about using the carry as synchroneous load ?

  19. Guest

    Thomas Magma skrev:
    I'd think that whenif you find something that will divide 1GHz to
    80MHz, it'll be bigger more expensive and more trouble than just adding
    a 80MHz oscillator ...

  20. Jerry Avins

    Jerry Avins Guest


    Nice to hear from you again! I think the OP might want a coherent
    division. Maybe not.

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