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Delaying a logic signal?

Discussion in 'Misc Electronics' started by Philip Pemberton, Jul 22, 2003.

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  1. Hi,
    I'm designing a simple (ha!) digital oscilloscope that should theoretically
    be able to sample up to 80MHz with two A/D converters (giving an effective
    frequency of around 160 MHz). I'd also like to design this device so I can
    expand it with more converters later on (up to four). What I need to know is
    how to delay a logic signal (the /SAMPLE line) so that it's 90 degrees out of
    phase. Then I need another two in addition to that - 180deg and 270deg. The
    signal needs to be as close to 50% as possible. How could I do this? None of
    my textbooks seem to cover this sort of thing.
    The idea is that I'll have multiple ADCs sampling at different times, one
    after the other. The data will end up in a FIFO, which will then be read by
    the PC via the ISA bus.
    I'm also going to need a programmable clock divider that can provide
    various frequencies to the ADCs. I guess this would be best done with a
    counter and a multiplexer?

  2. CWatters

    CWatters Guest

    Lots of ways..

    Try using a counter to steer the master Sample signal to the output of a 2
    to 4 decoder.

    This produces a pulse on each output in turn eg

  3. CWatters

    CWatters Guest

    Actually you want to be able to control the timing of the sample pulses more
    accuratly than that. Ideally you want to be able to adjust the interval
    between triggering and sampling perhaps a down to a nS?. That way you can
    sample repetitive signals (only) that are much faster than the sample rate.
    eg you can display a 200MHz sin wave even ifyou only have 85Mhz ADC's.

    Do a google for "PC oscilloscope" and read about existing designs including
    at least two kits that are available.
  4. In message <3f1eb44a$0$9628$>
    That sounds like the best (cheapest) option - get an SRAM FPGA (or an EEPLD)
    and put the glue logic inside it. I don't mind being stuck at 10nS - my
    Tek466 only goes down to .05uS anyway.
    That's going to be, er... "fun"...

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