I've been trying to measure the ring heater transfer function (current to emitted power) by sweeping the supply voltage and measuring the emitted power with a photodector positioned right next to the ring heater.
Last night the voltage was sweeping with a 1000mV setting on the SR785 which was fed into the Voltage Control of the Kepco Bipolar Operational Power Supply/Amplifier which was biased around 10V.
The results are very, very strange. The magnitude of the transfer function decreases at lower frequency. I'll post the data just as soon as I can (ASCII dumps 13 and 14 on the disk from the SR785).
The circuit looks like this:
SR785 drive ----> Amplifier ----> Ring Heater : Photodetector ---> SR560 (5000x gain) ----> SR785 input