Hi ppl, I have a question to you. I want to connect lock-in amplifier to the circuit when I try to measure the signal during the scan. The scheme is: the signal from the photomultiplier I connect to the A-enter of the lock-in (SR530) and the second input comes from the chopper (SR540) which can spin at 400-4kHz. the output of the lock-in I connect to the input of the amplifier (SRS SR280) and the amplified signal from the SR280 enters to the input of National Instrument card that is connected to computer. When I scan I didn't get nothing. when I decrease the frequency of the chopper the signal during the scan is constant. The question is: what should I do to see the signal? Actually the system was previous without lock-in configuration and it works but with lot of noise. I wanted to improve the system (clean it from the noises) and to add a lock-in but i can't see the signal. I think maybe the reason is the chopper's frequency? What do you think?