Agilent Technologies 34970A, User Manual, Page: 299 / 356

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Chapter 7 Tutorial
Measurement Fundamentals
4
7
Sources of Error in Frequency and Period Measurements
The internal DMM’s AC voltage measurement section performs input
signal conditioning. All frequency counters are susceptible to errors
when measuring low-voltage, low-frequency signals. The effects of both
internal noise and external noise pickup are critical when measuring
“slow” signals. The error is inversely proportional to frequency.
Measurement errors will also occur if you attempt to measure the
frequency (or period) of an input following a DC offset voltage change.
You must allow the internal DMM’s input DC blocking capacitor to fully
settle before making frequency measurements.
If the external noise pickup becomes large enough to exceed the
hysteresis of the measuring circuitry, the frequency function may
actually become unusable. External shielding and low-pass filtering may
help.
Input Random Noise (RMS)
1 mV 2 mv 3 mv
10 mv, 10 Hz
10 mv, 100 Hz
100 mV, 10 Hz
1%
0.5%
Input Err
or
34970A Refresh UG.book Page 299 Wednesday, February 17, 2010 12:34 PM