Dave, The trick is to closely synchronize your 1PPS generator, whether you use the 1970's method of a string of seven '7490 decade divider chips (common reset) or a 1900's method of a sync'able MCU divider such as a picDIV (http://www.leapsecond.com/pic/picdiv.htm).
It's so simple it's irresistible. The PIC s/w is there for free. The PIC h/w is less than 1$. Happy dividing, /tvb > On Apr 26, 2015, at 8:17 AM, Dave Martindale <dave.martind...@gmail.com> > wrote: > > 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 <t...@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 >> >> _______________________________________________ >> time-nuts mailing list -- time-nuts@febo.com >> To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts >> and follow the instructions there. > _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
