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Simple pulse stretcher

Discussion in 'Electronic Design' started by George Herold, Mar 7, 2013.

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  1. Hi guys, I need a little digital pulse stretcher. I've got pulses
    that are 5ns to 25ns in width, and I'd like to bump them all up to all
    be at least 10ns. I've got a few spare 74HC14 inverters. I was
    thinking of just feeding one through a diode followed by an R/C to
    ground. Any other ways? There might be an extra AND gate too.
    (not my circuit so I'm not quite sure.)

    Thanks

    George H.
    (and who ordered the second t in stretcher)
     
  2. Joerg

    Joerg Guest

    The diode thing is what I usually do. But 10nsec through a 74HC14? Now
    that's a strrrrretch.
     
  3. OK, maybe I should stretch to 20 ns.

    George H.
     
  4. OK thanks, I'll do 20ns then.

    George H.
     
  5. Joerg

    Joerg Guest

    Still a bit of a stretch but if you need pulses this short there are
    faster versions of the HC14.
     
  6. Yeah I was thinking of something like that too. The diode thing was
    an easy 'hack' and seems to be working just fine. (I stuck in 10k
    and 10pF and have about a 50ns minimum pulse.) Life is good.

    George H.
     
  7. Joerg

    Joerg Guest

    Diode plus Schmitt is the better solution. The main reason is that doing
    it with piped-in gates relies on their prop delays. I have seen that
    blow up in production when people did it. All it takes is one or more
    manufacturers improving a semiconductor process, things become faster,
    suddenly the fix ain't working no more. Or only sometimes. Datasheets
    typically do not specify a guaranteed lower limit on prop delay. For
    example, there is no entry for it in the "min" column for the 74HC14
    from TI:

    http://www.ti.com/lit/ds/symlink/sn74hc14.pdf
     
  8. Jamie

    Jamie Guest

    I think you're going to have a little problem with that inverter in
    regards to the speed you are referring to.

    Jamie
     
  9. Guest

    The 5ns is way out of line for HC. You can apply the pulse directly to an emitter follower loaded with parallel RC and then to HC to stretch it to anywidth you want. The BE junction is your diode. HC will not work reliably across VCC and temperature with anything less than about 25ns.
     
  10. Yeah there's already ~20ns of latency between the input pulse and the
    output so a bit extra ain't too bad. I'll knock the 10pF down to 4.7

    George H.
     
  11. Whee... fun!

    George H.
     
  12. Yeah in this case there's certainly already some delay. But it
    doesn't really matter.
    OK it matters.. it all adds to the built in latency of the device.
    But at the moment there's some 'software' latency that's ~x10's as
    bad.
    So I can burn 10's of nanoseconds with abandon.

    George H.
     
  13. In this case I'm timing a pulse stream. But absolute time doesn't
    matter as much as relative time. So as long as the prop. delay
    doesn't stop more than one pulse from traveling down the signal chain,
    then it may not matter.

    George H.
     
  14. Guest

    Please link to a datasheet- all I'm seeing is 20ns nominal performance on both Tpd's and minimum pulse widths into edge activated inputs like clk's, R's, and S's.
     
  15. Yeah Jim the 5ns pulses weren't getting counted....
    Sorry I missed a reply to you. There's a comparator on the input and
    I put the diode R/C thing between it and the first 74HC14 gate...

    George H.
     
  16. Cool 25 ns is just what I'd expect if I cut my C to 4.7pF

    Thanks Fred.

    George H.
     
  17. Guest

    Okay, you know what you measured, but you need to be real careful with layout using that parts family. It used to be a point of advantage for a logic family that it was unresponsive to full logic level transitions of durationless than the fastest edge rates in the system. Interwiring capacitive coupling into a nice high impedance input converts the transition into a nice square pulse there. I think Lenin covered that one.
     
  18. Guest

    A *triggered* pulse generator.
     
  19. Hi Fred, As Vlad said the HC series was really doing just fine with
    10ns pulses and perhaps only 'missing' 1% of the 5 or 6 ns ones. I
    don't have a commercial pulser that will do anything less than ~20ns,
    so it's hard to test. I had a pulse swallower circuit on a piece of
    copper clad, but I might have ripped it up to make something else...
    Anyway I don’t see any reason not to play it safe and make the minimum
    pulse 25ns.

    My ‘scope is only 200MHz, so I’m not really sure I even believe the
    5ns... what’s that rule of thumb for ‘scope bandwidth/ rise time? tau=
    1/(3*BW) ?

    George H.
     
  20. The tinylogic one-shot that John mentioned is guaranteed to trigger
    off 2.5ns pulses (with a 5V 'Vcc'). Available in a nice friendly
    0.65mm pitch 8-pin package (also a 0.5mm pitch or 1 x 2mm BGA if you
    are in need of pain).
     
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