Manual - 8500A Series Peak Power Meter - Giga-tronics

Theory of Operation
discussion. The output from this feedback system controls the Q1 oscillator so that the total power in
RT1 is maintained at a constant level.
Following the thermistor leveling circuit is a series of pin diode attenuators comprising three stages of
attenuation (10 dB, 10 dB, and 20 dB) for a total capability of 40 dB. These stages are switched in and
out by control signals from the GPIB/Cal Control board. In the first 10 dB stage, if the current through
the attenuator is flowing in the direction required to turn on the series diode, CR2, then the attenuator is
bypassed and minimum attenuation occurs. If the control current is in the other direction, CR2 is turned
off and CR1 and CR4 will conduct so that 10 dB of signal will be lost. The other stages are similarly
switched in and out to allow attenuation from 0 to 40 dB in 10 dB stages. The output then goes to the
CAL connector.
R3 supplies current from the -10 to -13 V control voltage source to the emitter of Q1, and R1 and R2 are
bias resistors to provide a -10 V bias to the base of Q1.
Manual No. 20790, Rev C, November 1998 4-29
Superceded by Revision D, March 2009