Figure 5-11. Measurement errors as a
function of input-signal harmonic distortion
13. The triple-section, low-pass filter at the input to the Null Detector
attenuates ac signal components.
At frequencies below 100 KHz the filter
becomes less effective. When the frequency is close to a multiple or sub-
multiple of the 84 Hz chopper frequency, the meter needle will oscillate at
the difference frequency. If error-causing ac components are encountered,
additional filtering will be required.
For single frequency signals, a
twin-T filter is effective and has the advantage of low total series
resistance. For variable frequency signals, an ordinary low pass filter may
14. Most ac sources are not stable enough to be measured in the most
sensitive null range. For example, if 1.0 volt is measured with the RANGE
switch set at 1 and the NULL switch set to 0.0001, the null detector
sensitivity is 100 V full scale. Since 100 V is 0.01% of 1 volt, an ac
source with a stability less than 0.01% will cause meter fluctuations. In
addition, when using the most sensitive null range, converter noise may
cause meter deflection if the input signal is below 1 mV.
Learning Event 4:
DESCRIBE THEORY OF OPERATION OF THE 887AB/AN DIFFERENTIAL VOLTMETER
of an unknown voltage to a calibrated reference voltage.
The principle is