By Mingyuan Tara
We measured the FFT of the demodulated signal from chopping technique. We did not see much. The background noise is still too high.
With everything ready, we used chopping technique to measure crackling noise. We measure the PSD from the demodulated signal between a) the mirrors being driven at 2Hz, and b) background noise, when the system was at rest, no driving force applied to the mirrors. We did this to check if we can see any signal due to crackling noise/ rubbing noise/ pzt noise or any noise originated from the driving mechanism or not. The result is not quite clear, we see a few peaks from the driven system around 40 Hz, but we have yet to confirm and identify them.
The setup is shown in the diagram below. For each bandpass through SR560, we added the gain to the signal as much as possible without railing the signal. Note that in this setup we did not bandpass the signal from PD after we square it , as shown in previous entries. Because Mingyuan did not understand why would we need to and I could not answer him properly, so I agreed to let him have it his way.
When we measured the data from the driven system (red curve in the plot), the setup is as shown in the diagram. However, for background measurement (blue curve in the plot), we want to keep the DC supply provided by the function generator to the pzt so that the sensitivity of the signal remain the same. Hence, we used a second function generator to send in similar driving voltage to the squaring box, while the first function generator was set to the dc output voltage to supply the pzts, no sinusoidal output. (We made a mistake by just unplugging the Vdrive to the pzt and to the squaring box, and the noise level dropped so much.)
The red and blue curve shows the psd of the demodulated signal when the blades were driven, and the static case respectively. The peak at 4 Hz that presents in both cases are from the square of the driving signal at 2Hz.
There are a few peaks around 36 - 40 Hz when the blades were driven. We could not see this in the SR785 monitor because the monitor was so faint. I just saw this after I plotted the data. The peaks might come from some resonances in the setup. We expect crackling noise to be more broad band. We will confirm and identify the source of the peak to make sure that we can see some signal from the driving (it can be rubbing between metal, pzt noise, crackle.)
We will repeat the same measurement, and try changing driving frequency/ amplitude, to see if the signal changes or not.