For most frequency measurements, the optimal triggering is obtained by positioning the mean trigger level at mid amplitude, using either a narrow or a wide hysteresis band, de­pending on the signal characteristics.

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 ap­plied 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.

However, stable readings are not necessarily correct; harmonic distortion can cause errone­ous yet stable readings. Sine wave signals with much harmonic distor­tion, 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.

Fig. 3-16 Variable sensitivity.
Fig. 3-17 Harmonic distortion.