Fig. 3-15 Timing error due to slew rate.
When measuring LF sine wave signals with little noise, you may want to measure with a high sensitivity (narrow
hysteresis band) to reduce 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.
Auto Trigger
For normal frequency measurements, i.e. without arming, the Auto Trigger function changes to Auto (Wide) Hysteresis,
thus widening the hysteresis window to lie between 70 % and. 30 % of the peak-to-peak amplitude. This is done with a
successive approximation 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 trigger 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 counter 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., >100 Hz (default). Another
condition is that the signal amplitude does not change significantly after the measurement has started.
NOTE: AUTO trigger limits the maximum measuring rate when an
automatic test system makes many measurements per second. Here you
can increase the measuring rate by switching off this probing if the signal
amplitude is constant. One single command and the AUTO trigger function
determines the trigger level once and enters it as a fixed trigger level.
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 calculated value, 50 % of the peak-to-peak amplitude, will be the
new fixed trigger level, from which you can make manual adjustments if need be.
Harmonic Distortion
As rule of thumb, stable readings are free from noise or interference.
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