NARDA 8718B, User Manual

Page: 83 / 137

82 Operating the Meter
Causes of Zero Drift
All sensors have a change in output that is a function of
temperature. In a probe, it is both the thermal coefficient of the
detector and the gain of the probe amplifier that determine how
much a probe drifts over temperature. Thermal compensation
circuitry in the probe is used to minimize the impact of
temperature change. Thermocouple detectors have the lowest
thermal coefficient but their very low level output requires much
more amplifier gain than diode sensors, especially in electric
field probes. Therefore, Narda probes with the greatest amount
of thermal drift are the microwave- band electric field probes,
which all use thermocouple sensors. The lower frequency
electric field probes use diodes and require much less gain.
Narda magnetic field probes use thermocouples but the circuit
requires lower gain than the electric field probes.
The other possible cause of zero drift can occur when a probe is
subjected to a very high field level and then is moved to an area
with a very low field. The high field heats the sensor and can
cause drift. However, this type of drift normally corrects itself
within a few minutes.
Many competitive diode sensor probes exhibit drift for another
reason - reaction to infrared light on the diodes. Narda probes
do not have this problem . Narda uses special diodes with an
integral light shield, photographic paper inside the probe cover,
and a combination anti-static shield and blackout coating on the
inside of the probe cover.
Checking for Zero Drift
There are two indications that the probe may have drifted
enough to require correction.
•
Whenever the negative zero drift exceeds 0.1% of full
scale of the probe, the words “ RE-AUTO-ZERO ” will
flash on the top line of the display. This will replace the
bar graph.