[Koji]

The logic chips 74ALS573 were replaced. And now the gain sliders are working properly.

== Test Status ==

[done] Whitening gain switching test
[done] AA enable/disable switching
[0th order] LO Det Mon channel check
[none] PD I/F board check
[done] QPD I/F board check
[done] CM Board
[none] ALS I/F board

Last week we found that the logic chip for the REFL1 gain switching was not transmitting the input logic. I went to Downs and obtained the chips. After some inspection some other latch chips were suspicious. Therefore U46, U47, and U48 (#1, #3, and #4 from the top) were replaced. After the replacement, the gain measurements were repeated. This time the test for the AO gain was also performed. Now all three slideres show the gain as expected except for the consistent -0.2dB deficit.

Note that the transfer functions for the REFL gains were measured with the input at IN1 or IN2 and the output at TESTA1. The TFs for the AO gain was measured with the excitation at EXC B, the input at TESTB2 and the output at the SERVO output. The gain and phase variantions for the AO gain at low frequency is the effect of AC coupling existing between the excitation and the servo output.

[Update on Oct 14, 2019]

The measured transfer functions show the phase delay determined by the opamps involved. The phase delay well below the pole frequencies can be represented well by a simple time delay (a phase delay linear to the frequency). Attachment 7 shows the time delay estimated by LISO for each gain setting of each gain stage. REFL2 has particularly large phase delay because of the use of OP27s. The delay is even larger when the gain is high presunmably because of the limited GBW.