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
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
82 | Operating the Meter |