We investigated the OMC VQPDs noise including the QPD driver circuit.
This work was performed after the klog31015
I would like to ask Aso-san about the detail of the cable noise subtraction in the QPD driver.
Summary :
- We turned the R13 resistance value, that determine the gain of the cabling noise subtraction.
* 39 kohm for QPD1
* 30 kohm for PQD2
- But, noise spectrum was better when we turned off the cabling noise subtraction
* When we turned on the cabling noise subtraction, two narrow peak glitch (~2.5 s) detected frequently
+ Ondotori glitch was about 2 s (klog23213)
- But but, when we backed to the OMC0 rack, various glitches appeared and spectrum became much worse
* QPD driver circuit may be affected by the environment(Temperature, ground and so on?), and we need more investigation to recognize the origin of the circuit glithes.
Preparation :
- We moved the QPD driver (JGWDoc11459) from the OMC0 rack to tempral working space(Fig.1.)
- At this timing , to expand the cable, we added the 1.5 Dsub-9 m cable for input and output (This information has possibility to affect to the optimization of the noise subtraction)
- Opened the QPD drier, and prepared for changing the resistance of the R13 and jumper pin (Fig.2.)
Actual work :
- By checking the reference spectrum in klog31015, we changed the configuration
Detail :
- For QPD2, we changed the R13 resistance value.
* 10 kohm + (5.1, 3.3, 1.8 and 2.4 kohm) in parallel
* Removed the 10 kohm and set the 20, 30, 39, 62 kohm
- As a result, when we set the R13 resistance around 30 kohm, noise level became better
- For QPD1, around 39 kHz would be the smaller spectrum.
- The final spectrum was in Fig.4.
* Blue : reference
* Red : turned on the cable noise subtraction
* Green : Turned off the cable noise subtraction
Notification :
- Soon after turned on the power or some times, various types of glitches detected, the trigger condition was unknown
- We cannot understand the feelings of the QPD driver circuit...
Here is the link to the LIGO DCC for the circuit design of the QPD transimpedance amplifier.
https://dcc.ligo.org/LIGO-D1001974
KAGRA uses basically the same design as above.
The noise subtraction path can be enabled/disabled by moving the jumper pin (P1).
It was disabled before.
We can adjust the subtraction gain by changing R13 of the noise cancellation amplifier.
Since the stationary noise level is better with the noise subtraction turned off (even with the optimized values of R13), we left it turned off for the moment.
We have two issues with this circuit.
1. The stationary noise is worse with the noise subtraction enabled.
2. We see glitches. They are sporadic and elusive.
The glitch issue is more serious than the small stationary noise increase.
We have only tested long operation with the noise subtraction disabled so far. So we will enable the noise subtraction and leave it for a while to see if the glitch problem is eased by the noise subtraction or not.
Yokozawa, Aso
We turned on the noise subtraction path in the OMC QPD driver.
We also tried to isolate the chasis from the rack by adding insulation between the bracket and the rack as shown in the attachment no.3.
We confirmed that the chasis is isolated from the rack in this state with a tester.
However, when we connected cables, the chasis was electrically connected to the rack. Especially, the cables between the QPD driver and the AA chasis connect the two chasis through the shell of the D-Subs.
After resinstallation of the driver into the rack, the output of the driver moved around a lot (a few tens of counts). This is a well known behavior of this circuit, probably due to the temperature drift.
We waited for ~30min, and the strange behavior of the output signals went away.
The attachment no.4 is the QPD noise spectra after the above wait.
From 12:30 to 13:30 (JST) of yesterday, the laser was shutdown, thus OMC QPDs did not get any light.
I checked the time series of the QPD outputs during this period.
Obviously, the outputs move around in the order of 0.2 counts.
Note that these signals are normalized by the sums of the QPDs. Usually, they get more than 10000 counts in sum, but it was order of 10 when no light falls on the QPDs.
Therefore, 0.2counts of noise becomes order of 1e-4 or less counts in operation. Probably this is not a big issue for initial alignment.
Zooming into a glitch, they are pretty fast change of the offset. We can see some transient response from the circuit (probably the whitening filter).
The cause of these glitches are not understood at this moment.
