On 05/07/2011 02:06 AM, Bob Bownes wrote:
Thanks to all for the discussion, but I'm still not sure why I have a
(now) consistent 1.495 hz frequency difference between the thunderbolt
and the VE2ZAZ FLL.
On a similar note, forgive my ignorance, but is there a simple
explanation why there are different frequency readings on my 5370B
when selecting frequency with a 1s gate vs frequency with 100k
samples? Not a few uHz, 1sec gate reading of the VE2ZAZ is
10,000,001.4946 Hz (+/- noise in the last digit) while 100k sample
reading is 9,997,322.2 Hz +/- noise in the last four digits.
You can have time-biases and voltage-biases which comes into the
measurement.
Recall, that the estimated frequency is
f = eventcount / time
and that an average over 100k samples will mainly average to higher
resolution mainly in the time numbers and to a smaller amount in the
event-count number.
Voltage-biases is due to difference in the trigger voltage of the start
and stop comparators on the input. By use of a calibrator you can
identify these can remove the effect, HP has patents and app-notes
relating to their calibrator. Similar results can be achieved using
other methods. Consider for instance that the frequency should stay the
same regardless if the start and stop is on the rising or falling edges,
but voltage biases can skew the time to create a time-bias.
Some counters, such as the SR-620, also has built-in time-biases in
their frequency mode which needs to be calibrated. This is due to delay
differences between start and stop events.
Now, with these skewed times going into the frequency formula above, it
should not be strange if these systematic errors will cause a systematic
skewing of the value on which no averaging is able to defeat. These
biases needs to be handled up-front.
A classical setup to identify this type of bias is to measure the
frequency of the counters reference output. That way you have completely
removed any issues relating to frequency difference.
A useful method to disclose if you have a bias is to shift time-base
length. If you have 1 second time-base and shift to 100 ms then the
eventcount value will become one tenth and 900 ms will be removed from
time, but the time-bias is as large as before but now has 10 times
higher impact. If you see that, you can be quite sure that you have a
bias and the best way to defeat it is by adjusting your trigger points.
When you have the counter under computer control, and pull out the
event-counter and time-values separately you can do more intricate
tricks naturally.
There are a way for a counter-designer to avoid voltage biases to
inflict on the frequency measure, and that is to take both the start and
stop events from the same comparator. Only very small biases of voltage
nature would remain, but the time-difference from that comparator to the
start and stop detectors would remain.
TIC-based comparision between two frequencies works well to measure
relative frequencies of fairly small differences in a way that biases
cancels on a first-degree level.
Measuring frequency has some intricacies in it.
Cheers,
Magnus
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