Hum, Noise, and Spurious Pick-up

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

sources of spurious radiation are induction or dielectric

heating units, diathermy machines, local radio transmitters,

grip dip meters, and amplifiers with parasitic oscillations.

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