A coaxial millivoltmeter/milliwattmeter for frequencies up to 1 Gc/s

Abstract

A description is given of a 50Ω coaxial millivoltmeter/milliwattmeter for the measurement of signal-generator output levels up to about 1 Gc/s in terms of equivalent source e.m.f. or equivalent available power. The instrument consists of a measuring head and an indicator unit. It is fully portable and requires no power supply. The measuring head comprises a precision 50Ω coaxial line terminated in a tractorially mounted resistor. The voltage across the resistor is measured by a pair of coaxial silicon diodes in a voltage-doubler circuit which minimizes the effect of source harmonics. The admittance of the diodes is compensated over the complete frequency range so that the voltage-standing-wave ratio (v.s.w.r.) is not less than 0.995. The inlet connector has a v.s.w.r. of not less than 0.999. The indicator unit contains the d.c. circuits for measuring the rectified output of the diodes and embodies an indicating instrument with a direct-reading linear power scale together with a number of resistors for calibration and temperature correction.Reference is made to the effect of ambient temperature on the sensitivity and excitation/response law of silicon diodes. A description is given of an electro-mechanical circuit, operated by a single control calibrated in terms of temperature, which not only corrects the sensitivity but also maintains a linear power response over a range of 20°C. The accuracy of the correction is such that the residual errors due to ambient temperature are not greater than 0.1%.The instrument is calibrated at 1 Mc/s by means of a special calibrator which is referred to d.c. standards. This has an inaccuracy of 0.25% on voltage at 1 Mc/s. Results are given of a comparison with a precision thermistor mount up to 1 Gc/s. These show that inaccuracies of 0.5% on voltage and 1.5% on power are not exceeded below about 800 Mc/s.