[time-nuts] Once again about counter calibration

Tom Van Baak tvb at LeapSecond.com
Sun Apr 26 05:58:09 EDT 2015

> 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.


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