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yutaro.enomoto - 22:59, Tuesday 07 August 2018 (5735)Get code to link to this report
Modification of RFPD for Green Lock PDH

I am currently modifying one of mass-produced LSC RFPDs, to be used for green lock PDH.
The concept of this modificaiton is described in JGW-G1808442.

In reality, some hack is required due to the difference of pin assignment of PDs, such as cutting off some surface patterns, connection with jumper wires, and isolation of a PD pin using Kapton sheet (See the attachments).

The modification was basically done, but I found an op-amp (U8) seems to be broken, so the final operation check will follow the replacement of the op-amp.

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yutaro.enomoto - 12:16, Thursday 09 August 2018 (5760)

[Kiwamu, Yutaro]
(Work on Aug 8 and 9)
== One PD modification DONE (two needed) ==

I suspected that the op-amp (U8) was broken but it turned out it was not broken. And we saw the same behavior in the DC path of all the RFPD we tested, which suggested this problem was not unique to the one I was modifying, but universal to every RFPD stored in Mozumi.
Then I looked into JGWdoc S-number pages and found that a capacitor (C31) has been removed for every RFPD installed in the tunnel. Found a klog (1331) as well.

So, I removed C31 from the RFPD I was modifying, and then confirmed that it worked properly.
Attached figure shows the transimpedance of this circuit (not calibrated yet).

= NOTE =
You MUST remove C31 before using an KAGRA RFPD.

yutaro.enomoto - 16:40, Thursday 09 August 2018 (5762)

== The Second PD modification DONE (two needed) ==

I also modified an RFPD (S1808690) in the same way as the first one.
The resonant frequency was tuned to 33MHz (see the attachement).

Now we have two working RFPDs for green.