Dramatic decrease in the LVDT spurious coupling was achieved

Today, I continued with the replacement of the PRM LVDT driver.
With the new LVDT driver, the spurious coupling at high frequencies is dramatically suppressed (40dB or more).
This will make the damping of tower modes at the BF stage much easier.

Corrections to the table reported yesterday

First of all, there were several mistakes in the table I reported yesterday for the calibration of the old LVDT channels at 2Hz.

The correct table is shown below:

Single frequency TF of the new LVDT driver

After taking note of the calibration values for the old LVDT driver, I adjusted the demodulation-phase tuning resistors of the new LVDT driver so that the DC value of the LVDT output signal becomes maximum.

Unfortunatelly, the demodulation phase cannot be set to the optimal one due to the limited adjustment range. By turning the trimmer of the resistor all the way from zero to maximum, the LVDT signal changed from negative to positive but never saturated. This means, the optimal phase is out of the tuning range. Anyway, I turned the trimmers maximally to the right, where the signal is maximum within the tuning range.

Then I measured the single-frequency TF at 2Hz with the new driver.

The gain was increased by about 6dB from the old driver. This is partially due to the increased demodulation gain by fixing the grounding problem of AD630. Another contribution comes from the demodulation-phase adjustment to maximize the signal gain. This increased gain could reduce the working range of the LVDTs. Currently non of the BF-LVDTs are close to saturation. In this case, the increased gain will give us more noise margin. So I will just leave as is.

Changing the calibration factors

In order to compensate for the increased hardware gain of the LVDT driver, I changed the gain values in K1:VIS-PRM_BF_LVDTINF_{H1, H2, H3, V1, V2, V3} so that the output from the filters should have the same gain as before. The old and new values are summarized here:

The calibration factor of V2 is computed in a different way from the others, because we don't know the single-frequency TF value of the old driver (V2 was broken).

For this channel, I excited the SF keystone LVDT at 2Hz. Assuming that the BF moves up and down without tilting, we can measure the relative gains between V channels. The transfer function values at 2Hz from the excitation amplitude to the LVDT output signals are the following:

From this, one can deduce that the gain of V2 is lower than V1 by 3.7dB. Therefore, the calibration factor of V2 should be 3.7dB(=1.53 times) larger than that of V1. This is how the calibration factor for V2 was computed. 

Self-TFs with the new LVDT driver

Finally I measured the transfer functions from an LVDT actuator to the readout of the same LVDT channel. The results can be found in the attached PDF. There is a dramatic improvement in eliminating the spurious coupling at high frequencies. This will allow us to increase the bandwidth and gain of the BF damper loops, which may be crucial in taming down the suspensions on a stormy day.

We should implement this new driver in all the BF-LVDTs. We may also want to replace the drivers to new ones for the LVDTs which do not share the actuation and readback coils. Even for those "normal" LVDTs, magnetic coupling between the actuation coil and the readback coil may create spurious couplings.