We have made another round of modifications to the ISS second loop board to have a bit of more gain at 1kHz.
Boost 1 zero was increased by a factor of 2, BOOST 2 was added, and a bit of phase compensation for high frequency to allow us to push the UGF to 20kHz-ish (details of the mod will be attached to this entry).
In the attached, black is the measurement from yesterday at 50W, 7dB of VGA gain, boost on.
Blue is today with 13dB of VGA gain, first boost on.
Red is today with 13dB of VGA gain, second boost on.
All in all, the modifications bought us about 13dB at 1kHz.
If we really need more, we should be able to push the UGF even higher, but that needs some serious thinking.
Here is a measurement of the open loop transfer function with SR785 at the floor. The UGF was at 18 kHz and the phase margin was roughly 50 deg.
The data, plotting script and figure are attached as a zipped file.
Boost 2 and boost 3 have been rewired in series to yield a double integrator. Each of them has a 50Hz pole and unity gain at 2kHz. The model has been changed to enable stages 2 and 3 simultaneously with the boost 2 switch. This boost does not work without boost 1. It would generate a notch near 2kHz but together with boost 1 the two zeroes become complex. Therefore engaging boost 2 will enforce boost 1.
At 1kHz the higher bandwidth increases the gain by 2. The higher frequency knee in boost 1 adds another factor of 2. The second boost adds 1-2dB at 1kHz and and additional 20dB at 100Hz.
A new filter modules has been added after the integrator of the AC coupling, called DRIVE. This allows the an easy hold of this output, when the AC coupling is off.
