(Richard M, Fil C, Ed M, Daniel S)
ECR E1600192.
Split Whitening Chassis S/N S1101627:
- On the interface chassis remove R7, R8, R9 and R10, R17 and R18 (disconnects in-vac cables and Z-switch).
- On the interface chassis jumper 220 pF capacitors on J3 between pins 1-3, 9-11, 2-4 and 10-12 (adds one pole of high pass at 7.2 kHz).
- On the 2nd whitening board remove C37, C38 and C39 (removes both poles of low pass).
AA Chassis S/N S1102788 & S1202201:
- On channels 15 and 16 remove C1, C6 and C9 (two poles of 10 kHz low pass).
- On channels 15 and 16 remove C7 (one pole of 10 kHz low pass).
- The twin-T notch was left in. It is fairly wide and attenuates ~17 dB at 50 kHz.
The attached plots show the transfer functions of
- whitening chassis: OMC DCPD_A: same as before
- whitening chassis: OMC PI DCPD_A: modified
- whitening chassis: OMC DCPD_B: same as before
- whitening chassis: OMC PI DCPD_B: modified
- AA chassis: channel 15
- AA chassis: channel 16
Whitening chassis S1101603 was removed from ICS-R5 U18. New chassis S1101627 was reinstalled with modifications listed above. New unit is the split variant.
[Kiwamu, Carl]
First indications are that the DCPD HF channels are behaving as expected. With the OMC locked on an RF sideband the DCPD A is compared to the new DCPD HF A. The transfer function between them at low frequency has a 'f' relation which transistion fto f^3 at 10kHz as expected from the AC coupling change and the removal of 2 poles at 10kHz. Coherence is lost at about 20kHz.
I have changed the whitening gain of the AHF DCPD path (A and B) to 15dB. This pushed the noise floor in the 15-29kHz region to a factor ~2 above what I guess is ADC noise. In the attached plots the original DCPD can be seen to reach a common noise floor with the AHF DCPD path with no whitening gain at about 17kHz. Turning the whitening gain up we can get some coherence with the shot noise of the DCPD through original electronics.
A forest of new peaks is visible, mainly in the 17-29kHz region. There are 80 peaks in teh 25-26kHz band. I stepped the ETMX ring heater at 7:01 up by 0.5W and down again at teh end of lock at 7:22. This may give some mode identificatiion data.
This morning we removed the Twin T notch on the AA board by removing C3, C4, C5, C10, C11 leaving in the 100 Ohm resistors in place.