This function benefits from the basic
time-stamping calculation method utilized by
this series of counters to obtain consecutive
measurement results without dead time.
Every positive or negative zero crossing (de-
pending on the selected slope) up to the maxi-
mum frequency (125 kHz with interpolator
calibration ON or 250 kHz with interpolator
calibration OFF) is time-stamped. For every
new time stamp the previous value is sub-
tracted from the current value and displayed.
In VALUES mode the display is updated ev-
ery new period if the period time exceeds 200
ms. For shorter times every second, third etc.
result is displayed due to the limited updating
rate.
In STATISTICS mode the graphs and
statisti-cal data contain all periods up to
the maxi-mum input frequency (see above).
For higher frequencies the average period
time during the 4 or 8
µ
s observation time
is displayed. So, for higher frequencies the
actual function is rather Period Average Back-
to-Back.
The main purpose of this function is to make
continuous measurements of relatively long
period times without losing single periods due
to result processing. A typical example is the
1-pps timebase output from GPS receivers.
Frequency A, B
Back-to-Back
■ CNT-91(R) only
This is the inverse function of Period
Back-to-Back. In
STATISTICS mode mea-
surement time is used for pacing the time
stamps. The pacing parameter is not used in
this case.
Thus a series of consecutive frequency aver-
age measurements without dead time can be
made in order to fulfil the requirements for
correct calculation of Allan variance or
deviation. These statististical measures are,
for instance, widely used by oscillator
manufacturers to describe short-term
stability.
USER MANUAL ● CNT 9x Series ● Rev.22 February 2020
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