Design and testing of a software feedback loop for RF power leveling

We have developed a software feedback loop that stabilizes the RF power input to less than 30 ppm during calorimetric measurements. In this system, a bolometric sensor with a type IV power meter is used to detect the power. Feedback is provided to the amplitude modulation (AM) input of an RF generator through a computer-controlled voltage source. The software feedback loop replaces an analog feedback loop that used an NBS type II power measurement system to provide the AM input signal. The new system removes a potential error in the DC substituted power caused by loading of the type IV when connected to the type II. The new leveling system is more flexible than the previous system. Both the desired leveling accuracy and realtime feedback rate can be readily modified. The design and evaluation of the software leveling system are presented. It has been tested with a new WR-15 microcalorimeter which will be the national power standard in the US from 50 to 75 GHz.