RE: RF Interference of GM Instruments

[IBODEP@ip3gate.usa.com]

RF fields do interfere with both the detector and the electronics.

In a nut-shell, there are a huge number of physical and electronic variables
that determine what reaction a RF source in relatively close proximity to an
electronic instrument will have.

Also, the op-amps used at the front end of most systems have a very high
impedance and significant gain capabilities. A small amount of RF energy
captured by these circuits can have a significant effect. 

The reactive characteristics (feed-back circuits, couplers) of these  circuits,
especially if not shielded, can cause the meter to peg low or high, depending
on the individual situation.

Also, bear in mind that at relatively close proximity , harmonics to the
main carrier can have a significant effects as well. Physical re-orientation of
the antenna in relation to the instrument can have a significant effect on a
particular component's (read harmonic) ability to resonate within the
counter (probe, cable, counter). 

If your interested (read; if you have the luxury of extra time and resources) ,
do an experiment.

Modulate a RF carrier with a steady ~1khz tone. 

Using an o-scope, stage your leads to measure the probe's cable (probe
connected) while hooked up to the energized counter (the o-scope lead 
itself may pick up some interference depending on the quality of your leads
and equipment, so identify this component)

Then disconnect the probe's cable and do some signal tracing within the
instrument.

Look for the 1 khz signal, the carrier, and any measurable harmonics of the
carrier.


Paul Bode
HP Training Program Administrator
ibodep@ip3gate.usa.com