A General-Purpose Electronic Wattmeter

A direct reading wattmeter is described which will read either positive or negative peak or average power of complex voltage and current waveforms containing components in the frequency range from dc to 71 kc. The instrument will measure up to 50 watts, covered in three ranges with an accuracy of 3 per cent. The scale may be extended by suitable resistance changes. The voltage and current inputs are mutually dc isolated to allow measurement of the current at any point between the ground and the voltage source. Multiplication of the current and voltage is accomplished by modulating a 10-mc carrier with the current, and then modulating the resultant with the voltage in a cascade of two suppressed-carrier modulators. The instantaneous product is recovered in a phase-sensitive demodulator, and then passed through an integrating network for average power, or through a peak detector for peak power. The wattmeter was designed with particular reference to the measurement of the plate dissipation in television horizontal output tubes. It was designed, nevertheless, to be a general-purpose instrument capable of accurately measuring power in any type of circuit where the voltage and current involved can be measured.