Zero crossing detector for high frequencies

Discussion in 'The Projects Forum' started by carlio, Oct 10, 2008.

  1. carlio

    Thread Starter New Member

    Oct 10, 2008

    I am currently looking for a way to make a zero crossing detector (i.e. a detector that generates a TTL signal each time an input sine or square signal crosses the zero axis). The signals that I want to process have a very large bandwidth (from 0 to 100 MHz), so I need the generated TTL signals to correspond with a good precision to the real zero crossing points. I found a website showing ZC detectors based on XOR gates or comparators, but I was told that for high frequencies (above several MHz) some of the components are too slow and do not give reliable results.
    Does someone have an idea on the way to make it?

    Another challenge is that I would like also to be able to count (or to make a signal that is proportional to) the elapsed time between two successive zero crosses. This gives the time duration (i.e. a frequency measurement)for each frequency cycle. I do not know if there exists a circuit that is able to do that for such high frequencies.

    Thanks a lot for your help.
  2. beenthere

    Retired Moderator

    Apr 20, 2004
  3. Wendy


    Mar 24, 2008
    The only practical use for period style measurements I know of is slow waveforms, something in the realm of 1Hz or slower. That way you can get a frequency measurement in one pulse. Fast frequencies, where you can get 6 digits with 1 second samples, don't really need period measurements.

    Given you can calculate accurately period or frequency if you have the other I guess I don't see the point. Most high end counters will measure a pulse width, which is pretty much the same thing.

    Most folks already know what I've said, just asking the quesion, why this way?
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    If you are hoping to reach 100Mhz in TTL you need to use the 74XXS series, none of the others are fast enough including 74XXLS.

    Also be prepared to supply plenty of juice, speeders are thirsty.

    As an alternative you could bone up on your FM radio theory. There were a few pulse counting discriminator designs in the past.
  5. Ron H

    AAC Fanatic!

    Apr 14, 2005
    I suggest you use ECL or PECL logic. With 100MHz signals, your pulse width even with 50% duty cycle is 5ns. A meaningful zero crossing pulse will be around 2ns wide. I believe this is beyond the capability of most (all?) TTL logic-level families, as well as TTL-output comparators.
    Here is a way to do it with ECL.

    MAX9691 datasheet.
    MC100EL07 datasheet.