Question: What Should I Do to See Signal w/ Lock-in?

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Discussion Overview

The discussion revolves around the integration of a lock-in amplifier into a measurement system involving a photomultiplier tube (PMT) and a chopper. Participants explore issues related to signal detection, noise reduction, and the proper configuration of the lock-in amplifier within the circuit.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Experimental/applied

Main Points Raised

  • One participant describes their setup involving a PMT, lock-in amplifier (SR530), chopper (SR540), and National Instruments data acquisition card, noting that they are unable to see the signal during scans.
  • Another participant suggests that the lock-in amplifier requires a synchronous reference input from the chopper and emphasizes the importance of setting the lock-in to "external reference." They also mention that an additional amplifier should be placed before the lock-in, not after.
  • A participant confirms that they have connected the reference input correctly and questions whether the chopper's frequency is the issue, seeking advice on what frequency to use for optimal signal detection.
  • One participant asserts that the chopper frequency should not be problematic and inquires whether the lock-in shows any signal changes when the input signal is varied, as well as the effects of changing the range and time constant settings on the lock-in.
  • Another participant expresses uncertainty about the signal observed on the lock-in, noting that it does not seem to correlate with expected results and raises the possibility that incorrect time constants may be filtering the signal.
  • There is a suggestion that the connection order of the devices may be relevant, with one participant questioning whether the lock-in should be placed after the amplifier instead of before.
  • One participant states that the output from the amplifier does not require further amplification if set to an appropriate range, reiterating that any necessary amplification should occur before the lock-in.

Areas of Agreement / Disagreement

Participants express differing views on the configuration of the lock-in amplifier and the role of the chopper frequency in signal detection. There is no consensus on the optimal setup or the cause of the signal visibility issue, indicating ongoing uncertainty and exploration of the problem.

Contextual Notes

Participants mention various settings and configurations for the lock-in amplifier, including reference input connections, time constants, and signal ranges, but do not resolve the implications of these factors on the overall system performance.

Dimani4
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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?
 
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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 into "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?
 
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.
 
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?
 
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)
 
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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