Thermoelectric power meters

Abstract

Thermoelectric power meters take several forms. At frequencies below 1 GHz instruments based on the traditional thermoelement may be used. Thermoelements are indirectly heated evacuated wire thermocouples which form the basis of many RF/DC substitution instruments. Their RF/DC substitution error is usually small below 30 MHz but becomes appreciable above this frequency, so that it is necessary to apply correction factors. Above 300 MHz these corrections become large as a consequence of resonances. Directly heated wire thermocouples are potentially capable of operation up to higher frequencies than thermoelements because of their comparative simplicity, but their development was cut short by the relative success of wire barretters, which are even simpler and can be made with smaller diameters if the Wollaston wire technique is used. Consequently thermoelectric power meters did not become common for frequencies above 1 GHz until the development and application of thin film technology, which resulted in the appearance of commercial thin film instruments in the 1960s. A further advance was made in the 1970s, when instruments employing a combination of thin films and microelectronics techniques first appeared. Since that time thin film thermoelectric power meters have shown a steady increase in popularity, largely at the expense of the thermistor. Nevertheless they possess a number of disadvantages which have only partly been overcome. The main problems are that the thermocouple output voltage is not exactly proportional to the rf power level, which can lead to errors of several percent if not corrected for, and the sensitivity is dependent on ambient temperature. In addition, the variation of sensitivity with frequency is larger for thermoelectric devices than for thermistors. The difficulties are reduced by the use of microprocessor based instruments which store and automatically apply correction factors, but such corrections are usually only approximate and, although they add greatly to the convenience of the instruments, the necessity for them makes precision measurements more difficult. Both indirectly heated and directly heated thin film thermoelectric power meters exist, but most instruments in common use are of the directly heated type. These cannot be used for RF/DC substitution measurements, as they incorporate a DC blocking capacitor which separates the input signal from the thermocouple output voltage. Instead, they often incorporate a reference oscillator which supplies a known rf power at a fixed frequency and which enables changes in sensitivity with time to be detected and corrected for.