Jackson Labs FireFly-1A GPSDO, User Manual

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FireFly-1A User Manual
36 © 2011 Jackson Labs Technologies, Inc.
5.4 Interpreting the Data
Figure 5.5 shows the data acquired by the FireFly-1A unit over a period of more than 200 hours The
red trace is EFC (crystal frequency control voltage). The crystal is aging (becoming faster in
frequency over time). This requires the control voltage to be lowered to maintain 10.0MHz exactly.
A drift of ~2mV is visible over 200 hours. On the left side of the screen the EFC range over this 200
hour plot is displayed vertically as 0.00193V. This means the drift of the EFC voltage due to aging is
~88mV per year. The EFC sensitivity of the crystal is about 8Hz per volt, so the crystal ages at:
8Hz/V * 0.088V/Year = 0.704Hz/Year drift.
At 10MHz:
0.704Hz / 10MHz = 7.04E-08 aging rate per year.
This is the same as 0.2ppb drift due to aging per day.This crystal aging is fully compensated by the
firmware with and without GPS reception of course.
The OCXO heater current is shown in turqoise. We can see it ranges from 0.135607A to 0.178146A.
The OCXO current jumps lower every 24 hours because the unit is sitting next to a window, and the
sun shines onto the OCXO in the evenings, heating it up, and thus making the unit use lower power
during that event.
In Figure 5.6 , which is a zoom of Figure 5.5 , we can see the phase offset error of the internal OCXO
to the UTC GPS reference. We can see the maximum drift is -77ns to +93ns. The average is (TI
av=-0.03ns). The standard deviation over the 200 hour plot is sd=11ns. This means the average error
of the 10MHz phase of this unit over 200 hours is only +/-11ns rms. Or, in other words the average
jitter (wander) over 200 hours of operation is:
11ns / 200Hrs = 1.528E-014
or in other words the unit performs as well as a high quality Cesium Atomic reference clock over
long periods of time. The unit disciplines its internal 10MHz reference to within less than +/-80ns
peak to peak of UTC at all times, which is less than one complete clock cycle at 10MHz.