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.

Fig. 3-13 shows that less noise still affects the trigger point by advancing or delaying it, but it does not cause erroneous counts. This trig­ger uncertainty is of particular importance when measuring low frequency signals, since the signal slew rate (in V/s) is low for LF sig­nals. To reduce the trigger uncertainty, it is de­sirable to cross the hysteresis band as fast as possible.

Fig. 3-14 Low amplitude delays the trig¬ger point

Fig. 3-14 shows that a high amplitude signal passes the hysteresis faster than a low ampli­tude signal. For low frequency measurements where the trigger uncertainty is of importance, do not attenuate the signal too much, and set the sensitivity of the counter high.

In practice however, trigger errors caused by erroneous counts (Fig. 3-10 and Fig. 3-12) are much more important and require just the op­posite measures to be taken.

To avoid erroneous counting caused by spuri­ous signals, you need to avoid excessive input signal amplitudes. This is particularly valid when measuring on high impedance circuitry and when using 1 MW input impedance. Under these conditions, the cables easily pick up noise.

External attenuation and the internal 10x attenuator reduce the signal amplitude, includ­ing the noise, while the internal sensitivity control in the counter reduces the counter's sensitivity, including sensitivity to noise. Re­duce excessive signal amplitudes with the 10x attenuator, or with an external coaxial attenuator, or a 10:1 probe.