Chapter 8 Tutorial

Measurement Fundamentals

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.

Input Error

1%

0.5%

10 mV, 10 Hz

10 mV, 100 Hz

100 mV, 10 Hz

1 mV

2 mV

3 mV

Input Random Noise (RMS)

 

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.

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