c.
Hum, Noise, and Spurious Pick-up:
When measuring low level RF voltages, precautions should
always be taken to avoid the possibility of erroneous
readings resulting from hum, noise, or stray RF pick-up.
Although all low frequency hum and noise is attenuated at
the input by 60 db, it is still possible for high level
unwanted signals to get through and cause errors. The
best test for this condition is to reduce the test signal
to zero level and note whether the voltmeter continues
to read some spurious signal level. In some cases it may
be necessary to provide extra shielding around the probe
connections to reduce stray field pick-up. Typical
heating units, diathermy machines, local radio transmitters,
grip dip meters, and amplifiers with parasitic oscillations.
d.
Magnetic Fields:
Operation of the voltmeter in strong 60 cycle magnetic
fields such as those surrounding unshielded power trans-
formers should be avoided; otherwise a standing reading
that cannot be nulled by the balance control may result.
The magnetic field induces small 60 cycle currents in the
amplifier section of the instrument which, due to the
extremely high gain at this frequency, appear as an
indication on the panel meter.
AN/URM-145
a
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