Yuhang and Aritomi

Last week, we found we were using FC lock mixer in an inappropriate way. Then we talked with Matteo and Matteo said that he and Marco did it on purpose.

So I checked what is the difference between amplifying LO and amplifying RF. The result is shown in the attached figure 1 and 2. It is obvious that, by amplifying RF, both signal amplitude(10 times larger) and signal to noise ratio become better.

Then I tuned FC locking servo input attenuator and gain, finally, both 7 seems a good choice(see attached figure 3 and 4). The measurement of the open-loop transfer function is shown in figure 5. Now we have a unity gain frequency of 22kHz and phase margin of 55 deg. But there is a very broad peak after unity gain frequency, maybe we should make unity gain frequency a bit lower.