The significance of the EFC count
There has been considerable debate concerning the meaning of the EFC count as reported by the Z3801A.
In order to clarify my thoughts and to learn more about my own receiver, I have done some experiments.
The first experiment went something like this:
I synchronized a local standard to the running and stable Z3801A. This was done with some precision
using a frequency comparator which by a process of multiplying up and mixing down allows the frequency difference
to by magnified by 1000 times.
The Z3801A was put into holdover mode. It was then turned onto its side. This caused its frequency to move,
because of gravitational error, by about 1E-9. The difference frequency was noted. Then it was taken out of holdover,
and the EFC graph was monitored until it stabilized the frequency
as confirmed by the comparator. The new value of EFC count was noted. The process was than repeated
but this time restoring the orientation of the Z3801A to normal.
The outcome was that twelve counts of EFC, on my receiver, represented one part in ten to the eleven.
Some time passed, but the discussion still continued. So I devised another way to explore the meaning
and the possible range of values of the EFC count.
The data was obtained by the following method. The 10811
clock oscillator output was disconnected from the receiver board. It was used instead to feed the
external standard input of an HP 8660C synthesized generator. The generator output was connected
to the 10Mhz input of the GPS receiver board. This provided the means to shift the frequency
of the clock oscillator in 1 Hz increments. The GPS receiver automatically corrected for this by
moving the EFC in the appropriate direction to oppose the shift introduced by the generator. The
EFC voltage was measured at the oscillator. NOTE that the values at +4 Hz and -4 Hz should be
disregarded as the required EFC fell outside of the range of control of the receiver. For all other values
the receiver operated normally, was able to track satellites and record sensible values for EFC and TI.
During this experiment, once the receiver was locked the generator
would always produce exactly 10MHz out. The receiver was
offsetting the external reference of the generator in an equal and opposite direction to that of the
generator's frequency setting.
||Not able to lock
||Not able to lock
The results, albeit over a much greater range than the first experiment,
confirm that one count of EFC represents a frequency change of slightly less than 1E-12. The
actual exact sensitivity is of little importance to me since the oscillator's control
characteristic is not quite linear.
The results also confirm that the EFC count is a twenty bit number, and that the relative EFC range
goes from -100% to +100%, both as explained by Joe Geller.
Since the holding range appears to exceed what would be expected of a 10811E, I
recorded the approximate frequency offset as a function of control voltage.
The following table shows the relationship between
EFC control voltage at the oscillator and the oscillator frequency in open loop.
||Freq offset normalized
My conclusion is that in the case of my receiver the clock oscillator
has a greater range of control than that expected of a 10811E.
Do other Z3801As differ? Well, here is a table prepared by Steve Smith from his