[time-nuts] Once again about counter calibration

Dave Martindale dave.martindale at gmail.com
Sun Apr 26 11:17:16 EDT 2015

Yeah, I considered saying that.  But if you don't have a TI counter, you
need some way of resetting the divide-by-1e7 chain so the two 1 Hz pulses
are close enough in time that you can see them on the scope at some
reasonably fast sweep rate.  Yes, you can used delayed sweep, but how
stable is the delay?  If you do have a TI counter, then the accuracy of the
counter's time base also factors into the reading (though you don't really
care about absolute timebase frequency, just drift).

A compromise method might be to divide the 10 MHz down to 10 kHz or 1 kHz.
Then the nearest adjacent "wrong" integer multiple of 1 Hz where the drift
would be zero is 1 part in 10,000 or 1 part in 1000 off the nominal
frequency.  Any decent crystal is unlikely to start out 50 PPM or more off
frequency, and really unlikely to be 500 PPM off frequency, so this mostly
eliminates the wrong ratio problem.  Yet you get one cycle of the scope
input signal every 0.1 or 1 ms, giving a reasonable chance for one of those
edges to drift close enough to the 1 PPS reference to measure the drift at
a fast sweep rate.

- Dave

On Sun, Apr 26, 2015 at 5:58 AM, Tom Van Baak <tvb at leapsecond.com> wrote:

> > The problem with using a 1 Hz reference when looking at a nominal 10 MHz
> > signal is that you will get a stable scope display with no drift when the
> > input is *any* integer number of cycles/sec.  So 10,000,000 Hz will give
> a
> > stable display, but so will 9,999,999 Hz and 10,000,001 Hz.  Unless you
> > know that your 10 MHz signal is already within 0.5 Hz of the correct
> > frequency, the drift method is likely to cause you to adjust to the
> nearest
> > integer number of Hz, not exactly 10 MHz as you want.
> One solution to this problem is to divide the 10 MHz to 1PPS and then
> compare the two 1PPS signals, using a 'scope or a TI counter.
> The horizontal sweep of your 'scope and your patience will determine the
> resolution of the measurement. For example, at 1 ns/div you can easily
> resolve a 1e-11 frequency difference within a minute.
> /tvb
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