Fluke PM6690 - Harmonic Distortion

high sensitivity (narrow hysteresis band) to re
-
duce the trigger uncertainty. Triggering at or
close to the middle of the signal leads to the
smallest trigger (timing) error since the signal
slope is steepest at the sine wave center, see
Fig. 3-15.
When you have to avoid erroneous counts due
to noisy signals, see Fig. 3-12, expanding the
hysteresis window gives the best result if you
still center the window around the middle of
the input signal. The input signal excursions
beyond the hysteresis band should be equally
large.
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Auto Trigger
For normal frequency measurements, i.e.
without arming, the Auto Trigger function
changes to Auto (Wide) Hysteresis, thus wid-
ening the hysteresis window to lie between
70 % and. 30 % of the peak-to-peak ampli-
tude. This is done with a successive approxi-
mation method, by which the signal’s MIN.
and MAX. levels are identified, i.e., the levels
where triggering just stops. After this
MIN./MAX. probing, the counter sets the trig-
ger levels to the calculated values. The default
relative trigger levels are indicated by 70 %
on Input A and 30 % on Input B. These values
can be manually adjusted between 50 % and
100 % on Input A and between 0 % and 50 %
on Input B. The signal, however, is only ap
-
plied to one channel.
Before each frequency measurement the coun
-
ter repeats this signal probing to identify new
MIN/MAX values. A prerequisite to enable
AUTO triggering is therefore that the input
signal is repetitive, i.e., ³100 Hz (default).
Another condition is that the signal amplitude
does not change significantly after the mea
-
surement has started.
NOTE: AUTO trigger limits the maximum mea
-
suring rate when an automatic test sys
-
tem makes many measurements per
second. Here you can increase the
measuring rate by switching off this
probing if the signal amplitude is con
-
stant. One single command and the
AUTO trigger function determines the
trigger level once and enters it as a
fixed trigger level.
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Manual Trigger
Switching to Man Trig also means Narrow
Hysteresis at the last Auto Level. Pressing
AUTOSET once starts a single automatic
trigger level calculation (Auto Once). This cal
-
culated value, 50 % of the peak-to-peak am
-
plitude, will be the new fixed trigger level,
from which you can make manual adjustments
if need be.
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Harmonic Distortion
As rule of thumb, stable readings are free
from noise or interference.
However, stable readings are not necessarily
correct; harmonic distortion can cause errone-
ous yet stable readings.
Sine wave signals with much harmonic distor-
tion, see Fig. 3-17, can be measured correctly
by shifting the trigger point to a suitable level
or by using continuously variable sensitivity,
see Fig. 3-16. You can also use Trigger
Hold-Off, in case the measurement result is
not in line with your expectations.
Input Signal Conditioning
3-8 How to Reduce or Ignore Noise and Interference
GOOD
BAD
Fig. 3-16 Variable sensitivity.
Fig. 3-17 Harmonic distortion.