Calorimetric rf power meter with dynamic zeroing and constant

Abstract

The effects of differential heating inside the calorimetric bridge are reduced by first heating the bridge externally and controlling the resulting temperature. The bridge is also heated internally by an amount that varies according to the applied power to be measured. This acts to eliminate the principal source of thermal gradients in the first instance. By applying this heat as a common mode input signal to the two terminating loads of the calorimetric bridge the bridge itself does not become unbalanced, and therefore does not attempt to measure that common power. Actual input power to be measured does initially unbalance the bridge, and is still measured as usual. A constant power circuit is response to the indication of measured power and acts to keep the sum of measured power (plus the equal amount of feedback power that balances the bridge) and the common mode power at a constant value corresponding to a full scale measurement. This eliminates internal thermal transients within the bridge that lengthen response time. The feedback signal to balance the calorimetric bridge is produced by an integrator. To allow operation of the integrator output closer to zero for measurement of small applied signals, the applied input to be measured is periodically removed and a suitable slight offset from the true balance condition is introduced into the servo loop. The bridge is then exactly balanced with a separate sample and hold control loop. 'Periodically' can mean once a second, ten times a second, or as is preferred, at the start of each measurement.