How to Reduce or
Ignore Noise and
Interference
Sensitive counter input circuits are of course
also sensitive to noise. By matching the signal
amplitude to the counter’s input sensitivity,
you reduce the risk of erroneous counts from
noise and interference. These could otherwise
ruin a measurement.
To ensure reliable measuring results, the coun
-
ter has the following functions to reduce or
eliminate the effect of noise:
–
10x input attenuator
–
Continuously variable trigger level
–
Continuously variable hysteresis for some
functions
–
Analog low-pass noise suppression filter
–
Digital low-pass filter (Trigger Hold-Off)
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 con
-
trol, is independent of input frequency and
useful over the entire frequency range. LP fil-
ters, on the other hand, function selectively
over a limited frequency range.
Trigger Hysteresis
The signal needs to cross the 20 mV input
hysteresis band before triggering occurs. This
hysteresis prevents the input from self-oscil-
lating and reduces its sensitivity to noise.
Other names for trigger hysteresis are “trigger
sensitivity” and “noise immunity”. They ex
-
plain the various characteristics of the hyster
-
esis.
Fig. 3-10 and Fig. 3-12 show how spurious
signals can cause the input signal to cross the
Input Signal Conditioning
3-6 How to Reduce or Ignore Noise and Interference
Fig. 3-10 Narrow hysteresis gives errone-
ous triggering on noisy signals.
Fig. 3-11 Wide trigger hysteresis gives
correct triggering.
Fig. 3-12 Erroneous counts when noise
passes hysteresis window.
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