Problem with Lock-in Amplifier (cannot lock the signal)

[Sotiris]

Hello, I'm using a lock in amplifier (model 5210 from signal recovery) for some experiments with photodiodes. Firstly, there is an optical chopper that "feeds" the lock in (reference signal). My input is a photodiode (in current mode). The problem is,the unlock (unlk) light is always on in the lock in amplifier, but, in the display section, i can see the reference frequency from the chopper. However, it was working well in previous experiments, as my professor stated. I tested the exiting signal from the chopper with an oscilloscope and it is the right one. Also, everything else in the lab is tested and working well. So, my question is, is it possible that something in the lock in isn't working?(i'd guess it's the mixer) I'm sorry if the question is dumb or something, I don't have any previous experience with the lock in and I'm desperate. Any other ideas would be appreciated.

 

[Charles Link]

It's been a while since I used that type of lock-in amplifier, but as I recall, there may have been a couple of buttons to select the reference input=(not sure about this, because it's been a while.) I seem to recall an "external" reference button might need to be pushed in or selected to get the reference signal to work. Anyway, that might be worth trying. If your reference signal is healthy and steady, the other possibility is that the lock-in amplifier is malfunctioning. Another thing that might be worth trying is the type of coupling to the reference signal if you have that option available on the lock-in amplifier. e.g. if the chopper reference is all positive, like it is for some models, you could try ac coupling to the reference signal if that selection is available. Also, you might try using a "T" to observe the chopper reference signal on an oscilloscope at the same time that you have it feeding into the lock-in amplifier. It is possible it is getting loaded down by a failed component in the lock-in, etc.

 

[Sotiris]

It's been a while since I used that type of lock-in amplifier, but as I recall, there may have been a couple of buttons to select the reference input=(not sure about this, because it's been a while.) I seem to recall an "external" reference button might need to be pushed in or selected to get the reference signal to work. Anyway, that might be worth trying. If your reference signal is healthy and steady, the other possibility is that the lock-in amplifier is malfunctioning. Another thing that might be worth trying is the type of coupling to the reference signal if you have that option available on the lock-in amplifier. e.g. if the chopper reference is all positive, like it is for some models, you could try ac coupling to the reference signal if that selection is available. Also, you might try using a "T" to observe the chopper reference signal on an oscilloscope at the same time that you have it feeding into the lock-in amplifier. It is possible it is getting loaded down by a failed component in the lock-in, etc.

Thanks for your reply. The external button is active because the lock in is showing the chopper's frequency as reference. Also, the chopper's output is already ac. The last suggestion was interesting, i tried it and the signal was indeed healthy and steady, but the output of the lock in is either random noise, or some crazy lines, or a "no signal" message.

 

[Charles Link]

Thanks for your reply. The external button is active because the lock in is showing the chopper's frequency as reference. Also, the chopper's output is already ac. The last suggestion was interesting, i tried it and the signal was indeed healthy and steady, but the output of the lock in is either random noise, or some crazy lines, or a "no signal" message.

One other thing to try is the coupling of the input signal=be sure it is not saturated with a +12 volt DC signal. AC couple the input if necessary. Also, make sure you are using the "f" frequency mode and not "2f". Also, you might try putting in a stronger input (square wave from photodiode) signal and make sure you adjust the phase to maximize the output signal. It is certainly possible the lock-in amplifier has electronic problems of some kind. One other thing you could try that occasionally can be problematic is your coaxial cables=occasionally they can malfunction, especially the connectors=try interchanging them. If you still can't get the lock-in to give a good output, it is likely the lock-in has malfunctioned. One more item=make sure the frequency of the chopped signal (especially the chopper reference) is very steady=if it wavers in frequency, the lock-in might have trouble locking in on it. You might want to try using a different chopper before you conclude that the lock-in has failed.

 

[Sotiris]

For now, i know that the input is not saturated for sure, and the lock in is in "f" mode. I'll try to adjust the phase and replace the cables tomorrow. Another thing that i noticed, is that in the oscilloscope the chopper's signal is very steady, but in the lock in, many times it differs (for example, if the chopper is at 80 Hz, the lock in is showing 79.9 and 80 and this is changing frequently, but i don't know if this error is significant enough to make this kind of problem)

 

[Sotiris]

I tried again today, nothing worked. The next step is to open open the lock in in case we find something odd with the board that can be fixed. Your suggestions were thorough enough, thanks again. I'll post what happened when we find out.

 

[Charles Link]

I tried again today, nothing worked. The next step is to open open the lock in in case we find something odd with the board that can be fixed. Your suggestions were thorough enough, thanks again. I'll post what happened when we find out.

Generally you can get quite a lot of use out of the various electronic equipments, but on occasion they do fail and need to be replaced.