Sample-Hold; Time-Out; Measuring Speed - Fluke PM6690 Operator's Manual

Measuring Functions
tual gate time (tg) and the number of cycles
(n) that occurred during this gate time.
Thereafter, the counter calculates the fre-
quency according to Mr. Hertz's definition:
n
=
f
t
g
The '90' measures the gate time, tg, with a res-
olution of 100 ps, independent of the mea-
sured frequency. Consequently the use of
prescalers does not influence the quantization
error. Therefore, the relative quantization error
is: 100 ps/tg.
For a 1-second measurement time, this value
is:
100 ps
-
= ´
12
100 10
1 s
Except for very low frequencies, t
measurement time are nearly identical.

Sample-Hold

If the input signal disappears during the mea-
surement, the counter will behave like a volt-
meter with a sample-and-hold feature and will
freeze the result of the previous measurement.

Time-Out

Mainly for GPIB use, you can manually select
a fixed time-out in the menu reached by press-
ing SETTINGS ® Misc ® Timeout Time.
The range of the fixed timeout is 10 ms to
1000 s, and the default setting is Off.
Select a time that is longer than the cycle time
of the lowest frequency you are going to mea-
sure; multiply the time by the prescaling fac-
tor of the input channel and enter that time as
time-out.
4-10 Theory of Measurement
-
= ´
10
1 10
and the set
g
When no triggering has occurred during the
time-out, the counter will show NO SIGNAL.

Measuring Speed

The set measurement time determines the
measuring speed for those functions that uti-
lize averaging – Frequency and Period
Avg. For continuous signals,
1
»
Speed readings/s
+
t 0 2 .
g
when Auto trigger is on and can be increased
to:
1
»
Speed readings/s
+
t 0 001 .
g
when Manual trigger is on, or via GPIB:
1
»
Speed
+
t 0 00012 .
g
n
Average and Single Cycle
Measurements
To reduce the actual gate time or measuring
aperture, the counters have very short mea-
surement times and a mode called Single for
period measurements. The latter means that
the counter measures during only one cycle of
the input signal. In applications where the
counter uses an input channel with a prescaler,
the Single measurement will last as many cy-
cles as the division factor. If you want to mea-
sure with a very short aperture, use an input
with a low division factor.
Averaging is the normal mode for frequency
and period measurements when you want to
reach maximum resolution. There is always a
tradeoff between time and precision, however,
so decide how many digits you need and use
as short a measurement time as possible to ar-
rive at your objective.
readings/s