I have set up diaggui which monitors not only below 8 kHz but also the frequencies above up to 30 kHz for the ALS PLL feedback signals.

The xml file now resides at

/users/Commissioning/data/ALS/PLL/ALS_PLL_XY_SuperNyq_20211004.xml

It looks at the test point channels in the associated IOP process at 64 kHz. This means that the data will not be something we can look back. The aim is to pull up this particular diaggui when we see the mystery peak (17659) developing again to see if there are something suspicious above 8 kHz.

Background

The causes for the mystery peak at around 300 Hz in the feedback signals of ALS PLL X has remained unknown. In order to characterize what that is, I decided to monitor the frequency components above the Nyquist frequency of 16 kHz data.

The setup

The screenshot below shows how it looks like. Currently, I see no peaks at around 300 Hz in the X signals.

The relations between the TP channels and RT channels are such that

• IOP-ALS0_MADC0_TP_CH10  for ALS-PLL_X_SLOW_DAQ
• IOP-ALS0_MADC2_TP_CH10 for ALS-PLL_Y_SLOQ_DAQ

An identical calibration in the frequency domain is applied to both TP channels using the diaggui's calibration function. It is

• zpk([100, 100], [10, 10], 620)

A factor of 620 came from the product of 20 V/215 cnts,  1/2 (single-ended cable) and 2e6 Hz/V (laser piezo efficiency).