An unusual frequency counter effect

An unusual frequency counter effect

A vintage rack-mount Racal (Sulzer) double ovenized laboratory quartz frequency standard was slowly warming up. I was monitoring it with a hp 53132A frequency counter. Gate time was set to 10 s and statistics were enabled (N = 30). So one measurement per 10 seconds with statistics every 300 s (5 minutes). The external reference for the counter was a 10 MHz atomic standard.

The measurements were captured via the talk-only RS232 serial port, MJD timestamped, and logged by a PC. Timestamping the readings is optional but very useful when you look back at data years later. So as a habit I timestamp any data from any counter or sensor that I use. Not only does it tell you when the data was collected, but also the sample rate. Any timestamping method works (local time, UTC, unix ctime or time_t, JD) but I always use MJD.

Sample raw data looks like this:

...
53530.612558 5,000,000.062649  Hz
53530.612674 5,000,000.062590  Hz
53530.612789 5,000,000.062598  Hz
53530.612905 5,000,000.062664  Hz
53530.613021 5,000,000.062640  Hz
53530.613137 5,000,000.062576  Hz
53530.613252 5,000,000.062607  Hz
53530.613368 5,000,000.062659  Hz
53530.613484 5,000,000.062637  Hz
53530.613600 5,000,000.062576  Hz
53530.613715 5,000,000.062613  Hz
53530.613843 5,000,000.062672  Hz
53530.613958 5,000,000.062631  Hz
53530.614074 5,000,000.062571  Hz
53530.614190 5,000,000.062639  Hz
53530.614306 5,000,000.062673  Hz
53530.614421 5,000,000.062614  Hz
53530.614537 5,000,000.062580  Hz
53530.614653 5,000,000.062661  Hz
53530.614769 5,000,000.062655  Hz
53530.614884 5,000,000.062599  Hz
53530.615000 5,000,000.062595  Hz
53530.615116 5,000,000.062651  Hz
53530.615243 5,000,000.062654  Hz
53530.615359 5,000,000.062585  Hz
53530.615475 5,000,000.062582  Hz
53530.615590 5,000,000.062659  Hz
53530.615706 5,000,000.062653  Hz
53530.615822 5,000,000.062576  Hz
53530.615937 5,000,000.062607  Hz
53530.615937 N      : 30
53530.615937 STD DEV: 34.2 uHz
53530.615937 MEAN   : 5,000,000.0626221 Hz
53530.615937 MAX    : 5,000,000.062673  Hz
53530.615937 MIN    : 5,000,000.062571  Hz
53530.616053 5,000,000.062672  Hz
53530.616169 5,000,000.062632  Hz
53530.616285 5,000,000.062570  Hz
...

This is a very stable frequency standard and the frequency drift is very low. So it accidentally makes a fine slow sweep test of the precision and accuracy of a frequency counter.

The 6-day frequency plot is typical of an oscillator still slowly drifting after power-up

Here the plot is normalized to show 2.5 ppb drift over 6 days (= 4e-10 / day)

The corresponding phase plot is a nice parabola, indicating roughly linear frequency drift

Here's a plot of stdev from the hp statistics every 5 minute

Something unusual near day 1 and day 5; zooming to the 5th day

Switching to Excel to plot both frequency and stdev in the same chart

Same y-scale but changing x-scale from 24 hour span to 8 hour span

Now changing y-scale for a better look

Excel plots: 2005-racal-sdev.xls

Raw data: log19816.txt.gz

Experiment dates:

C:\tvb\2022-racal$ gzip -d < log19816.txt.gz | grep -v -e ": " | mjdrate
53530.609873 ... 53536.529815 5.92 days (511483.0 s = 142.079 h),   50777 lines, 10.073322 seconds/line,  0.099272 Hz

C:\tvb\2022-racal$ mjd 53530.609873
2005-06-09 14:38:13 UTC (DOY = 160, Thu, STOD = 52693) = JD 2453531.10987 = MJD 53530.609873
Thu Jun 09 07:38:13 2005 (UTC-7 Pacific Daylight Time)

C:\tvb\2022-racal$ mjd 53536.529815
2005-06-15 12:42:56 UTC (DOY = 166, Wed, STOD = 45776) = JD 2453537.02982 = MJD 53536.529815
Wed Jun 15 05:42:56 2005 (UTC-7 Pacific Daylight Time)

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