If it's really effective to reduce DAC+HPCD noise, C-code of coil driver BIO controller should be modified.
I modified C-code and check the improvement on the standalone system before a new code would be installed to KAGRA system.
In conclusion, Ushiba-kun's prediction seems to be correct and there was an improvement in output noise.
And also hacked C-code works fine. So we can install the new code in the next maintenance day.
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For measuring the output noise of HPCD, input port of HPCD was shorted and output port was connected to the whitening filter on which 36dB gain and 2 stages of whitening were engaged. Black curve in Fig.1 shows the ADC, AA and whitening filter noise with 36dB gain and 2 stages of whitening. This noise seems to be limited by the noise of whitening filter not ADC (see also klog#31763).
Output noise of coil driver without any de-whitening filters are shown as dark green curve. Brown, light green, and blue curve represent the output noise with one stage of de-whitening at the 1st, the 2nd, and the 3rd stage of de-whitening respectively. We can see the improvement by engaging a more latter stage of whitening filters. In the case of engaging the 3rd stage, whitening filter noise limits this measurement. So we couldn't know actual noise level in the case of engaging the 3rd stage. Anyway some improvement can be seen by engaging the 3rd stage instead of the 1st or the 2nd stage. So Ushiba-kun's suggestion seems to be effectively to reduce the coil driver noise.
In the cases of engaging 2 stages of de-whitening are shown in Fig.2. Again, we can see that the 3rd stage filter being engaged is important for noise reduction e.g. blue or green curves vs. brown curve.
In order to engage a latter stage first, I modified the C-code for the coil driver BIO controller. This hack seems to work fine as shown in Fig.3 (disengage all de-whitening), Fig.4 (engaging 1 stage of de-whitening), Fig.5 (engaging 2 stages of de-whitening) and Fig.6 (engaging all 3 stages of de-whitening). These 4 states which are used during the lock acquisition are assigned as 1-4 on the state request numbers. For various future test, 5-8 on the state request numbers are also defined in C-code. List of states are as follows.
1: LP1:OFF, LP2:OFF, LP3:OFF
2: LP1:OFF, LP2:OFF, LP3: ON
3: LP1:OFF, LP2: ON, LP3: ON
4: LP1: ON, LP2: ON, LP3: ON
5: LP1:OFF, LP2: ON, LP3:OFF
6: LP1: ON, LP2:OFF, LP3:OFF
7: LP1: ON, LP2:OFF, LP3: ON
8: LP1: ON, LP2: ON, LP3:OFF
