A thermal millivoltmeter for measuring radio-frequency voltages

The paper describes a radio-frequency millivoltmeter which is novel in principle and design. Developed for calibrating high-output levels of a signal-generator, it measures voltages of the order of 100 mV, the measuring element being a commercial thermocouple milliammeter or vacuo-junction. The input impedance, controlled by means of a turret switch, is variable over a range of values, in order that the output voltage of a source may be measured under any conditions of resistive loading. In the first place the instrument was designed to operate at frequencies up to 50 Mc/s, but an assessment of its accuracy, based on impedance measurements, shows that the error is less than 1% for purely resistive 70-ohm sources up to 100 Mc/s. For a reactive source with a resistive component of 70 ohms and a phase angle of arc tan ¿, the error at 100 Mc/s is less than 5%.