D
RF Detectors
D.1 Introduction
The detectors used with the Series 8500A Peak Power Meters (PPM) are dual diode. They contain a
preamplifier/line driver, a correction PROM with an address counter, and a thermistor to measure the
temperature of the detector mounting. Since diode detection is used, there is an inherent non-linearity that
must be compensated before the detector can be used for power measurements. Compensation is done
automatically by a special calibration routine performed by the PPM that steps the power output of an
internal RF source from -30 dBm to +20 dBm, and then records the resultant output voltage of the
detector at each step. A conversion table is automatically calculated and stored in non-volatile RAM so
that the detectors will remain calibrated when the PPM is turned off and back on again. This process
takes less than a minute and can be performed at any time.
The detectors are either high or low speed. The high speed has a rise time of 15 ns and a minimum
frequency of 750 MHz for accurate measurements. The low speed detector has a rise time of 750 ns and a
minimum frequency of 30 MHz. The difference between the two is totally in the diode element. The low
speed element contains an additional filter capacitor that slows the pulse response time and allows lower
RF frequency operation.
The diode element is field replaceable. After replacement, the correction PROM must be replaced with
one that has been programmed with the diode’s new video resistance and frequency characteristics.
Manual No. 20790, Rev C, November 1998
D-1
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