To make reliable measurements possible on very noisy signals, you may use several of the above features
simultaneously. Optimizing the input amplitude and the trigger level, using the attenuator and the trigger control, is
independent of input frequency and useful over the entire frequency range. LP filters, on the other hand, function
selectively over a limited frequency range.
4.2.1. Trigger Hysteresis
The signal needs to cross the 20 mV input hysteresis band before triggering occurs. This hysteresis prevents the input
from self-oscillating and reduces its sensitivity to noise. Other names for trigger hysteresis are “trigger sensitivity” and
“noise immunity”. They explain the various characteristics of the hysteresis.
Fig. 3-12 Erroneous counts when noise passes hysteresis window.
Fig. 3-10 and Fig. 3-12 show how spurious signals can cause the input signal to cross the trigger or hysteresis window
more than once per input cycle and give erroneous counts.
Fig. 3-13 Trigger uncertainty due to noise.
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