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GHz counter

Discussion in 'Electronic Design' started by helgeb, Jul 11, 2005.

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  1. helgeb

    helgeb Guest

    Hi all,
    I am looking for a giga hertz counter for a laser distance measurement
    project.
    How to measure the time between signal out and back?
    Is there an existing project?

    Thanks
    Helge
     
  2. I gues there is no public domain project.
    As to the counter, there is the MC100EP016A, an 8bit
    counter doing 1.3GHz.

    Are you sure you're having sufficient light to
    trigger the counter ? What kind of laser with
    what power, and what distances are you aiming at ?

    Rene
     
  3. Mike Monett

    Mike Monett Guest

    Noise will be a problem. You can eliminate most of it with a new
    technique that is much faster than averaging. See

    http://www3.sympatico.ca/add.automation/sampler/intro.htm

    Mike Monett
     
  4. Tim Shoppa

    Tim Shoppa Guest

    How to measure the time between signal out and back?
    What distances is it going to be used at?

    For times in the picosecond to hundreds-of-nanosecond range and low rep
    rates, a "Time to amplitude" converter (basically using a capacitor as
    an integrator) followed by an ADC is very simple technology. Dates
    back to WWII radar and probably earlier. There are gotchas with
    switching the current source on and off rapidly and resetting the
    capacitor back to zero charge, but these are lessened somewhat when you
    are in control of the repetition rate/sync.

    For longer times, a combination of a counter with a TAC is very
    appropriate. The counter won't have to run in the GHz if you do it
    right.

    There are some clever things that can be done with
    multiple-approximation switchable delay lines, too. Not very hi-tech
    (this would be the solution from 40 years ago) but it does work if you
    control the repititions.

    All that said, I would highly encourage you to look at the waveforms on
    a scope before going into too much deail with measuring sub-nanosecond
    times. You'll have to work with discriminators on your sensor, for
    example, and adjusting their threshold depending on path loss (I
    suppose for very short paths this may be a non-issue, the laser
    rangefinding I did was in the several km range) or adjusting laser
    output power to compensate for path loss.

    Tim.
     
  5. Dave Garnett

    Dave Garnett Guest

    I am looking for a giga hertz counter for a laser distance measurement
    One common way of doing this is to continuously amplitude modulate the laser
    signal (much easier than switching on or off quickly). Then look at the
    relative phase of the received signal. This will give you an ambiguous
    result of course.

    If you modulate with a few well chosen frequencies then there will be a
    unique solution for the actual distance.

    Another system uses the received signal to modulate the transmitter. The
    system will oscillate of course, and the frequency will tell you the
    distance ...

    Have fun !
    Dave
     
  6. helgeb

    helgeb Guest

    Thank you for opinions:

    Rene:
    the MC100EP016A is nice! I did not know it.
    I wanted to use a laser with some 100mW (1440 nm) or a 10Watts at 808 nm.
    The distance is around 100 meters or more.
    I will use a telescope/laser device. (Maybe I can use it like ceilometer...)
     
  7. helgeb

    helgeb Guest

    Hi Tim, thank you!

    around 100 m
    do you mean a switched integrator?
    What is a TAC? I was thinking about 1 Ghz regarding 30 cm resolution...
    I never heard from.
    You created a range finder?
     
  8. Focusing a diode laser is not that trivial, as there
    are two different divergence angles. Usually a
    cylindical plus a spherical is used.
    You have to limit the pulse. For CW laseres, the
    upper limit for careless pointing is 5mW. More
    power requires the site to shut and cleared.

    There is a disadvantage when the beam is not seen.
    Where is it pointing at (for a human pointer) ?

    Rene
     
  9. helgeb

    helgeb Guest

    I will use a collimator with 1 mrad divergence.

    I wanted to send out a powerfull puls (10 Hz ?) and start counting internal
    1 GHz frequency.
    After detecting the puls I finish counting and read out the result.
    Also I could use a switched integrator. I will see.

    You are right, a non visible beam is more dificult to handle.
    I will measure height of clouds...

    Helge
     
  10. Ken Smith

    Ken Smith Guest

    This brings to mind another attack on the problem:

    Build what is basically the front end of a simplified sampling scope that
    feeds a digitizer. The digitized values can then be examined by a micro
    to find the peak.

    This changes a "measuring a delay" problem into a "making a ramped delay"
    problem.

    Since the situation being measured is changing fairly slowly, you may be
    able to use only one sample per laser pulse and still get a good enough
    representation of the real curve.
     
  11. Tim Shoppa

    Tim Shoppa Guest

    You created a range finder?

    Not exactly an optical rangefinder (are many MeV gammas optics?!), but
    it was my business for a while to do sub-nanosecond time-of-flight to
    find energy, mass, etc.

    As sort of an extrapolation beyond "if you don't know how to do it, you
    don't know how to do it on a computer", please look at the waveforms by
    eye using a good fast scope before attempting to automate measurement
    with a counter. I suspect that you are underestimating the importance
    of the discriminator as range varies, and the response time of your
    detector/electronics chain.

    Tim.
     
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