[time-nuts] How can I measure time-delay of a cable with HP 5370B time-interval counter?

Dana Whitlow k8yumdoober at gmail.com
Mon Oct 29 01:46:16 EDT 2018


If you can get access to a 2-channel 'scope, try it instead of a TIC
so you can get some clue as to what's happening.  I suspect that one
component of the problem is that the supposedly Hi-Z input of the
counter really isn't but has a stub of unterminated cable between the
input connector and where the end of said cable dumps into the internal
circuitry.

Best thing to do if anything like this is happening is to set both inputs
to 50 ohms and use an isolating splitter.  Or, failing that, put matched
attenuators in each output of the tee, using as much attenuation as
you can tolerate.

With sine waves, reflection problems just modify the amplitude and
phase, hard to tell what's going on.  But with fast-rise edges, you can
often sort these things out if you can see the waveform(s), thus the
desirability of using an oscilloscope.   And as has been pointed out,
having unequal trigger points can only make matters worse.

Dana


On Sun, Oct 28, 2018 at 11:47 PM Orin Eman <orin.eman at gmail.com> wrote:

> On Sun, Oct 28, 2018 at 8:31 PM Hal Murray <hmurray at megapathdsl.net>
> wrote:
>
> >
> > drkirkby at kirkbymicrowave.co.uk said:
> > > I'm feeding a sine wave from a Stanford Research DS345 30 MHz function
> >
> > Are the levels set so it triggers at the same point?  It doesn't look
> like
> > it
> > in your photo.
> >
> > Looks like you are running at 1 MHz.  If the stop trigger is a little
> > lower/earlier than the start trigger, you will have to wait a whole
> cycle,
> > so
> > 950 ns is a reasonable result.
> >
>
>
> Looked like the levels were on Preset.  So, a little difference in what
> "Preset" means and as Hal says, you wait a whole cycle.
>
> One could fiddle with the stop trigger level, but the rise time of the sine
> at these frequencies is so long, that a small change in the trigger level
> is going to make a large change in the measurement.
>
> I'd try the same experiment with a square wave instead to minimize this
> effect.
>
> Once you get a stable measurement with a square wave, then experiment with
> the trigger level controls.  Going clockwise from Preset, I usually see no
> trigger, followed by a narrow band where it triggers, followed by no
> trigger.  In the band where it triggers, assuming adjusting a stop trigger
> that is set to the rising edge, as the trigger level is increased, so does
> the time interval.
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