Keita Marc Fil Daniel
We realized that we can implement the required compensation for the second loop by using the whitened monitor signals (TP10) from the transimpedance board as the inputs to the sum of PD1-4 and PD5-8, respectively—instead of the unwhitened outputs from the transimpedance amplifiers.
The required modifications are:
-
Add a 220K resistor in parallel to C29 and C30 (4x channels on 2x boards)
This limits the AC coupling and gives a DC gain around 1. -
Replace C25 and C26 (4x channels on 2x boards) with 47 pF.
This gives the signals a wider bandwidth. -
Lift the input side of R48/R49/R51/R52 and jumper it to TP10 (4x channels on 2x boards)
This rewires the inputs of the sum amplifiers. -
Change R38 and R39 to 4.87K (from 4.53K).
This brings the poles better in agreement with the ones form the second board.
During testing we noticed that the input transimpedance amplifiers (eight AD797s), were all oscillating between 10-15 MHz with an amplitude of about 500 mVpp. Adding capacitance to the feedback network seemed to have little effect, so we swapped all of them with TLE2027. This solved the oscillation. Using 400 Ohms transimpedance, the input referred noise of a channel is about 25 pA/√Hz between 10 Hz and 10 kHz. This is maybe a factor of 2 below shot noise at high laser input power. The electronics noise is dominated by the Johnson noise of the first 4.87K resistor in the whitening stages.
The output SMA connector which was shortening the negative leg to the chassis has been removed. Instead, we drilled a new hole for an isolated TNC connector.
With the upfront whitening gain the fast monitor points now have too much gain and are saturating. We removed the gain of 50 from ERR1 and ERR2 by replacing R60 with a 4.53K (from 220K). This also removed the 2.7 kHz pole in this path. The OUTPUT channel was also modified for a flat transfer function with a fixed gain of 1. It now looks like the other two. In detail, C52 and C53 were shorted out, and R60 and R61 were changed to 4.53K (from 45.3K).
ISS Outer Loop AC Coupling
The transfer function of the ISS outer loop AC coupling is attached. As implemented it should be unconditionally stable with a ugf of 10 Hz. With a gain of ~500 at 10 Hz in the overall outer loop servo, the AC coupling point would be around 0.1 Hz.
Outer Loop Readbacks
Here is a spectrum of the outer loop readbacks at 2W and 50W input power, respectively, with the ISS second loop open. The AC coupling is on. At 50W the third loop is also on. The ERR readbacks are very near saturation at the higher power. Since the error signal is followed by a fixed gain of 3, the output was saturating.