An improved meter for measuring RF power. The RF power is applied to thermocouple means which transforms the RF power into a low level d.c. signal which then passes through a chopper stage that is driven by a clamped sinusoidal input. The output of the chopper stage includes periodic pulses with a prescribed pulse spacing: pulse duration ratio, the pulses being passed through a band-pass filter at a harmonic of the chopper drive signal. The filtered signal, which is also amplified, undergoes synchronous detection with a tri-level square-wave signal which is centered in phase relative to the amplified, filtered signal when the two signals are in phase. The power meter measurements adjusted for zero drift and calibration drift by a microprocessor; based on zero measurements and calibration measurements made before and after an RF measurement is made. Measurements are stored and processed digitally to enhance accuracy. Signals through the meter can be at relatively high frequencies to reduce 1/f noise; spiking at the chopper stage and synchronous detector is virtually eliminated; and the tri-level square- wave permits greater tolerance to phase shift, all of which reduce output error.
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