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David Notebook: Difference between revisions
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The value that the lock-in displays is actually slightly different from what it outputs. When viewing the signal on an oscilloscope, the mean voltage better matches what I would have expected yesterday (about 8.6 V for a 2 V peak to peak square wave--closer to the 9.0 V it should be than what the screen shows). Looking at the output of the function generator on another oscilloscope revealed that the output is slightly noisy--this might account for the difference in the expected and measured values. I'm going to just assume that the lock-in is operating on square waves according to the formula from yesterday. It's certainly close.
The photodiode is definitely too slow for the chopper. The rise time is plenty fast, but it seems there is a fairly long decay time--the waves were asymmetric, with the first half looking good and the second half looking like an exponential decay. They got worse with increasing chopper frequency. I switched to a [[Thorlabs PDA10A Si Amplified Fixed Detector|Thorlabs PDA10A]], which has no problem even at 1000 Hz. At 633 nm, it has a listed responsivity of about .38 A/W.
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