To better understand the performance of a home build counter a
comparison was done with a Picotest U6200A
The two channel home build counter was setup to measure the frequency of
the 10MHz output from a Rb on one channel and the 10MHz output from a
not so good OCXO on the other channel.
The ratio between the two frequencies was measured with a 1 second gate
time, multiplied by 1e+7 and send to Timelab.
The U6200A had the Rb output as 10MHz reference and the 10MHz from the
OCXO into channel 1. Gate time was also set to 1 second.
In Timelab the data from the Counter under test and the U6200A where
recorded simultaneously over a 1000 second period
The recorded data was saved and adjusted for the difference in start
time of the measurements and loaded back into Timelab.
U6200A TIM file http://athome.kaashoek.com/time-nuts/U6200A.tim
Own counter TIM file http://athome.kaashoek.com/time-nuts/tinyGTC_2.tim
A first performance check was done by plotting the unwrapped linear
residue of the phase of both measurements. (see:
http://athome.kaashoek.com/time-nuts/tinyGTCvsU6200A_Phase.png ) . The
measurements did show some differences in instantaneous phase but the
differences where small and even at 980 seconds the two measurements
agree rather well.
A second performance check was done using the frequency plot. (see
http://athome.kaashoek.com/time-nuts/tinyGTCvsU6200A_freq.png ). Overall
the two measured frequencies agreed with sometimes up to 1e-10
difference. The difference in gate time of the two counters was very
visible as a gradual shift. As the measurements where aligned in time at
the end of the measurement the time difference at the start was about 3
seconds.
A detailed plot of the measured frequencies over the last 100 seconds
(see
http://athome.kaashoek.com/time-nuts/tinyGTCvsU6200A_freq_detail.png )
showed an occasional difference between the frequency measurements of
the two counters up to 2e-10
Questions:
1: Is this setup meaningful in assessing performance differences? If
not, how to improve?
1: How can one compress or expand a TIM file to correct for the
difference in gate time? A better approach would be to ensure gate times
where identical.
2: How can one use the two TIM files to calculate the RMS of the
differences in frequency? My hope is to use this RMS calculation as a
single number quality indicator.
Erik.
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