[time-nuts] Evaluating counter performance, seeking feedback.

[Erik Kaashoek]

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.