[time-nuts] What is the best counter for a Time Nuts?
Ulrich Bangert
df6jb at ulrich-bangert.de
Wed Oct 8 14:10:22 UTC 2008
Gentlemen,
up to now I have been thinking that I am pretty well informed about
current counter technology but some experiences of the last days make me
doubt. The following applies to frequency measurements of a 10 MHz
signal.
I have received some data measured with an Agilent 53131 counter from
Time Nuts member James Miller. That data shows a resolution of 3 digits
after the decimal point for a frequency measurement of 1 s "Gate time"
and 4 digits for a 10 s "Gate time".
This is perfectly in line with what I thought to know about modern
counters: Internally EVERY type of measurement is reduced to a time
interval measurement. Since a frequency measurement with 1 s "Gate time"
equals a time interval measurement of an 1 s interval and the counter
has an single shot resolution of 500 ps the relative statistical error
due to this resolution is 500 ps / 1 s = 5*E-10. As the frequency is
computed from the time interval measurement is has the same relative
error and for that reason displaying 3 digits after the decimal point is
a good choice because 0.001 Hz (1E-10) is the least digit of
significance. Using a digit more would suggest a resolution that is not
available, using one digit less would decrease the numerical resolution
below the measurement resolution.
Up to this point my statement of beliefs is in harmony with Agilent.
Now comes the strange part: In the course of a discussion between Time
Nuts member Bernd Neubig of AXTAL Germany and me I received data
measured with an Agilent 53132 which is the 53131's big brother with a
better single shot resolution of 150 ps.
I have been prepared to see this better resolution in the data but what
I really have seen SHOCKED me a lot. Not only does the 53132 generate 5
(!) digits after the decimal point for a 1 s "Gate time" (with 0.00001
equal to 1E-12) a sigma tau diagram of the data revealed that the last
digit was not only "noise" but seemed to be of real significance. I had
been presuming that perhaps the counter's statistics had been enabled
for that but as it turned out this was not the case. From that data one
must make the conclusion that the overall jitter of the 53132 including
the resolution quantification, trigger errors and everything else is
<=5E-12 !!!!!!
Big question: If the single shot resolution is only 3-4 times better
that that of the 53131 and much inferior to the 20 ps of my SR620, how
does the 53132 manage to be THAT GOOD A PERFORMER? Magic? Black art?
I immediatly made the same measurement with my SR620 to receive a
counter noise floor of 6E-11 for 1 s frequency data, pretty much a
decade worse of the 53132. Must we throw our 5370s and SR620s out of the
window and settle for second hand 53132 from eBay to make precise
stability measurements ???
TIA for your suggestions
Ulrich Bangert
www.ulrich-bangert.de
Ortholzer Weg 1
27243 Gross Ippener
More information about the time-nuts
mailing list