TM 11-6625-524-14-2
local radio transmitters, grid dip meters, and
noise is attenuated at the input by 60 db, it is still
amplifiers with parasitic oscillations.
possible for high level unwanted signals to get
through and cause errors. The best test for this
in strong 60 Hertz (Hz) magnetic fields, such as
condition is to reduce the test signal to zero level
-
t h o s e surrounding unshielded power trans-
and note whether the voltmeter continues to read
formers, should be avoided. The magnetic field
some spurious signal level. In some cases, extra
induces small 60 Hz currents in the amplifier
shielding may be necessary around the probe
section of the instrument which, due to the ex-
connections to reduce stray field pickup. Typical
tremely high gain at this frequency, appear as an
dielectric heating units, diathermy machines,
indication on the meter.
3-4
