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 Mon Dec 27 05:38:56 2021, rana, Update, Control System, cavity locking loop shape Mon Jan 24 10:24:49 2022, shruti, Update, Control System, cavity locking loop shape Wed Jan 26 17:28:53 2022, shruti, Update, Control System, cavity locking loop shape Mon Jan 31 19:03:43 2022, shruti, Update, Control System, cavity locking loop shape - pomona box TF
Message ID: 2843     Entry time: Wed Jan 26 17:28:53 2022     In reply to: 2841     Reply to this: 2849
 Author: shruti Type: Update Category: Control System Subject: cavity locking loop shape

### Some observations while measuring the noise spectra [Rana, Shruti]:

• The cantilever mode at 45 Hz is the dominant source of noise in the measured noise spectra. Attachment 1 shows the noise spectrum at lower frequencies and Attachment 3 shows this signal in the PDH signal, and cavity transmission and reflection.
• The contribution of the noise of the second order (90 Hz) is roughly 35 dBV smaller than the fundamental, and the third harmonic at 135 Hz seems virtually non-existent visually.
• There are present many peaks from 1 kHz  to tens of kHz. The smaller spacing between them seems to be around the cantilever resonance. (Attachment 2). Although very similar looking peaks do seem to be present in noise spectra measured before we added the cantilever (such as in elog 2776).

In order to suppress these noisy peaks, following the suggestion in elog 2838, I made the pomona box filter shown in attachment 4. The $R_{in}$ is the output impedance of the LB1005 servo and $R_{out} = 10 k\Omega$ is the input impedance of the analog modulation port of the ITC502 current driver being used.

[Update 31st Jan 22] $R_{in}$ is shown incorrectly in Attachment 4 - it is supposed to be in series, not in parallel to the remaining circuit.

 Attachment 1: ctrl_pdh_5kHz_Screenshot.png  878 kB  Uploaded Thu Jan 27 14:02:56 2022
 Attachment 2: ctrl_pdh_10kHz_Screenshot.png  1.258 MB  Uploaded Thu Jan 27 14:03:06 2022
 Attachment 3: scope_red.pdf  146 kB  Uploaded Thu Jan 27 14:20:21 2022
 Attachment 4: filter_red.png  418 kB  Uploaded Thu Jan 27 14:22:49 2022
 Attachment 5: ctrl_pdh_100kHz_Screenshot.png  1.372 MB  Uploaded Thu Jan 27 14:31:11 2022
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