Series 8650A Universal Power Meters - Giga-tronics

3Theory of Operation13.1 8650A Power Meter Theory of OperationThe power meter is basically a specialized microvoltmeter which measures the RF detector diode’svoltage. A patented calibration system ‘linearizes’ the diode sensors while EEPROM stored data (in thesensors) is used to apply frequency related correction factors.3.1.1 General TheoryThe 8650 power meter is used with a dual Schottky diode detector sensor module. It measures RF powerup to 40 GHz depending on which sensor is used. All Schottky diodes operate in the square law regionbelow –30 dBm. This means they give a DC voltage output proportional to the RF input voltage squared(P=V3/R). The detector is a good 50 Ω termination. At higher power levels the diodes no longer give alinear output directly proportional to input power. The detected voltage is measured with a known RFpower from –30 dBm to +20 dBm. The built in calibrator generates this accurate RF power at 50 MHzand optionally at 1 GHz. This data is used to correct the meter readings.The detector sensors are measured over their frequency range during manufacturing. Frequencycorrection factors are stored in EEPROM in each sensor. When the front panel (or GPIB) FREQ switchis pressed and the signal frequency is entered, the frequency correction factor is automatically calculatedin the displayed reading.InChopperChopperX1X8X64X8 X8 X8 MuxADCTrimGain SelectDACFront Panel/DisplayMain ProcessorDSPFPGACalibratorIEEE 488RS-232Single Channel MeterShownFigure 3-1: 8650A Meter Block DiagramManual 32089, Rev. A, December 2000 3-1