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Model 5328A
Operation
time base in the measurement of a 10 MHz signal is 1 x 10-6 x 10? = 10 Hz. Similarly, for the
measurement of a 100 msec period, the error would be 1 x 10-6 x 10-1 = 1 x 10°? or 100 nsec.
3-43.
Trigger Error
3-44.
Noise on the input signal will cause uncertainties in the point at which the Schmitt trigger
switches. Provided the noise is not large enough to cause false triggering (i.e., cross both limits of
the hysteresis band which would produce more pulses out of the Schmitt trigger than input cycles
to it) no significant error is introduced in a frequency measurement.
3-45.
For period measurements, however, this uncertainty produces like error in the time the
gate is open since it is this signal that controls the gate. It can be shown that with essentially low
frequency noise and a signal to noise ratio of 40 dB, the resultant worst case trigger error is .32% of
the period. Thus, the trigger error in the measurement of the period of a 1kHz signal is 3.2x 103 x
10-3 = 3.2 usec worst case. For 60 dB S/N, worst case error is .032% while for a 20 dB S/N signal it is
3.2%.
3-46.
For an arbitrary waveshape (but constant slew rate through the hysteresis band), the
trigger error takes ona different expression. In Figure 3-7, it is shown that for this case, the trigger
error
is
.
+2 x peak noise voltage
signal slew rate
For a 40 dB S/N, this translates to
+.0025 psec
signal slew rate (V/psec)
3-47.
For time interval measurements, trigger error is generally negligible when compared to
the systematic error introduced by the uncertainty in the setting of trigger levels.
For an
uncertainty in trigger level of +10 mV and apeak noise voltage of 1 mV, trigger error is a factor of 5
less than the error caused by trigger level uncertainty, regardless of signal slew rate. For example,
trigger level uncertainty of +10 mV ona 100 mV/ns signal introduces an error in the time interval
measurement of +0.1ns. The trigger error for sucha signal, with 1 mV peak noise, is less than +.02
ns, a factor of five less. Averaging reduces the trigger error still further (but not the trigger level
uncertainty error). The error is reduced by WN for time intervals averaging and by N for period
averaging.
Ugo
—
INPUT SIGNAL
+ SLOPE TRIGGERING
HYSTERESIS
BAND
/
PEAK
NOISE
VOLTAGE
TRIGGER
ERROR
=
Eee
OSes
tog
ae
SIGNAL SLEW RATE
SIGNAL SLEW
RATE
Figure 3-7.
Trigger Error
3-7
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