The AA filter for ASDC was fixed.
== 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
[none] CM Board
[none] ALS I/F board
The AA filter for the 4th section of the LSC analog electronics bank (D000076) was pulled out for the test. On the workbench, questionable CH8 was checked. It tuned out that the filter amplifier module for the 8th-order elliptic filter at 7.5kHz was not properly working and exhibited unusual attenuation. This filter module (Frequency Devices Inc D68L8E-7.50kHz) was desoldered and replaced with a module from a spare board. Note that Gautam and I had tried to use this spare board instead of the current one, but it didn't give us any signal for an unknown reason. Since the desoldering required a lot of force and had a risk of damaging the PCB, a socket was made from an IC socket (see Attached 1). This change made CH8 functioning equally to the other channels do.
I took this opportunity to ckech the performance of the AA filters. For each channel, the input signal was injected from J3 using a pomona clip. The output was taken from pin 1, 5, 9, ... of J2. This is the + side of the differential output. The - side just has the equivalent performance but the signal polarity. The digital signals for the AA bypass switches were not connected. Fortunately, this was just fine as it made the anti-aliasing filters engaged.
Attachment 2 shows the transfer functions of all the channels. All the channels showed an identical response (at least visually). The transfer function for CH1 was fitted by LISO. The ZPK values are listed here:
pole 5.2860544577k 503.1473053928m
pole 5.9752193716k 1.0543411596
pole 8.9271953580k 3.5384364788
pole 8.2181747850k 3.4220607928
pole 182.1403534923k 1.1187869426 # This has almost no effect
zero 13.5305051680k 423.6130434049M
zero 15.5318357741k 747.6895990654k
zero 23.1746351749k 1.5412966100M
Attachment 3 shows the ASD of the output voltage noise measurement. Note the input was shorted for this measurement. The nominal output voltage was found to be 0.1 uV/rtHz and the 1/f noise corner freq was about 100Hz. Only CH3 showed a deviation from the typical values. It looks like this is neither an artifact nor transient noise. Fortunately, nothing is connected to this channel right now.