Fluke PM6690 - Controlling Measurement Timing; The Measurement Process; Resolution as Function of Measurement Time

Controlling Measurement
Timing
The Measurement
Process
Basic Free-running Measurements
Since these counters use the reciprocal count-
ing technique, they always synchronize the
start and stop of the actual measuring period
to the input signal trigger events. A new mea-
surement automatically starts when the previ-
ous measurement is finished (unless HOLD is
on). This is ideal for continuous wave signals.
The start of a measurement takes place when
the following conditions have been met (in or
-
der):
–
The counter has fully processed the previ
-
ous measurement.
–
All preparations for a new measurement
are made.
–
The input signal triggers the counter’s
measuring input.
The measurement ends when the input signal
meets the stop trigger conditions. That hap
-
pens directly after the following events:
–
The set measurement time has expired (ap-
plies to Frequency and Period Aver-
age measurements only).
–
The input signal fulfils the stop trigger
conditions, normally when it passes the
trigger window the second time.
Resolution as Function of
Measurement Time
The quantization error and the number of dig
-
its on the display mainly define the resolution
of the counter, that is the least-significant digit
displayed.
As explained on page 4-10 under Reciprocal
Counting, the calculated frequency f is:
f
n
t
g
=
while the relative rms quantization error
E
q
= ±100ps/t
g.
The counter truncates irrelevant digits so that
the rms quantization resolution cannot change
the LSD (least-significant digit) more than ± 5
units. This occurs when the displayed value is
Measurement Control
5-4 The Measurement Process