The Measurement of Peak Voltage at a Frequency of 600 Mc/s by means of a Modified Probe Circuit

If the probe current of a coaxial-line standing-wave detector is passed through a pair of rectifiers, a simple relation may be obtained between the mean current through either of the rectifiers, the capacitance of the probe to the inner conductor, and the frequency and peak amplitude of the voltage on the line. At a frequency of 600 Mc/s, errors are introduced into voltage measurements made with this circuit, owing mainly to the shunting of the rectifiers by stray capacitance. These errors may be reduced to 5¿10% by using low-resistance silicon crystal rectifiers at their maximum current rating, and the magnitude of the errors can be deduced from a simple test made at low frequency. The shunting effects of stray capacitance across the rectifiers may be compensated, for to a large extent by a variable inductive stub, tuned to resonate with the stray capacitance at the operating frequency. The compensation is not perfect, however, and the use of a stub is a disadvantage with large rectifier currents. Measurements of 600-Mc/s pulsed voltages made with this probe circuit were found to agree, within the limits of experimental error (±3%), with the corresponding values deduced from measurements of the power dissipated in a calorimeter which terminated the coaxial line.