length and 130 mm diameter.
There are two calibration systems, one for each waveguide. The output power from the WR-10 power source, for example, is divided into two components by a directional coupler. One component is sent to a monitoring power meter that is connected to the sidearm of the directional coupler. The other component is sent to the WR-10 test port via two series of isolators with 55 dB isolation and the straight section. The isolators, the straight section, and the double cylindrical jacket are enclosed in aluminum housing to improve the thermal stability.
OPERATING PRINCIPLES
A. RF-DC substitution
The power of an incident electromagnetic wave entering the test port is measured by substituting it with a calibrated DC power. This substitution is carried out exploiting the TFS. First, a DC power of approximately twice the expected incident RF power Ps is applied to chip resistors attached on the AM. Note that the RF power has not yet been applied. A thermoelectric element simultaneously cools the AM. Then, the TFS controls the DC power so that the temperature difference between the AM and the temperature reference block becomes zero. After the DC power is measured under this condition, the RF power is applied to increase the temperature of the AM. Then, the TFS decreases the DC power to make the temperature difference zero again. Finally, an approximate value of the RF power Ps0 is obtained by subtracting the second DC power from the first DC power. This TFS greatly increases the measuring speed. The typical reading time per frequency is less than 10 min. The measurement of a power of 0.2 mW is possible owing to the small amount of noise in the system. B. Correction factor
Ps0 includes numerous uncertainty factors. However, some of the uncertainty factors can be corrected to reduce the total uncertainty after the measurement of Ps0. Applying such a correction, Ps is given by the following formula