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Question about lock-in

  1. Sep 19, 2010 #1
    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?
    Last edited: Sep 20, 2010
  2. jcsd
  3. Sep 20, 2010 #2
    The Lock in should have a "reference input". This is where you would have to connect the signal that is synchronous to the chopper. If it has a differential input, you cannot connect the chopper to the b input, as this is just for differential signals not for the reference. You should also set the lock in to "external reference". I would be surprised if it should help to have an amp after the lock in. If you use an additional amp, then it should be before the lock in. What do you scan and why do you scan?
  4. Sep 21, 2010 #3
    Thanks for reply,

    I've connected :input 1 (A) my signal and reference input I've connected the chopper (lock-in sees that input,i.e. when I change the frequency of the chopper the frequency in the reference input of the lock-in changes like in the chopper). The signal from the lock-in (output) enters to the NI (National Instruments) board card (CB-68LP) and then it enters to the NI DAQ card in the computer. This card convert the analog signal to digital and the sampling rate of that card I think 200kS/s. So, I guess the problem in the chopper's frequency. What frequency should I give to see the signal? Or problem lies in something else? The system works without a lock-in but the signal is little bit noisy so I try to improve the signal to connect the lock-in into the system.
  5. Sep 21, 2010 #4


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    The chopper frequency shouldn't be a problem, the ref channel on a SR530 work up to about 100 kHz or so.
    Do you see the signal on the lock-in? I.e. if you change the signal does the indicators on the front panel change?

    Also, what happens when you change the range and the time constant on the lock-in?
  6. Sep 21, 2010 #5
    Thanks for reply,

    Yeah, i understand that the lock-in can lock on the high frequencies, but when I try to scan I can't see anything resemble to what should I get. I see some signal on the lock-in but seems to me it doesn't refer to the real signal. When I try to shut off the laser the signal on the lock-in doesn't change a lot. I didn't try to change the time constants, maybe the signal is just being filtered because of wrong time constants?

    And if it's matter how I connect the lock-in: now I've connected it right after the PMT, (PMT->lock-in->amplifier). Maybe I should connect it after the amplifier? (PMT->amplifier->lock-in)
  7. Sep 22, 2010 #6


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    The signal coming out from the amplifier does not need to be amplified (it will be in the volts range if you setthe range to a sensible value), if you need an amplifier it should come BEFORE the lock-in.
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