Keita Daniel

We were able to engage the new ISS with the most recent modifications, see alog 29570. The offset adjustment is very finicky. Trying to run with high ISS gain at low input power requires offset adjustments below the 0.1 count level as function of the gain value. We decided to leave the ISS gain fixed at 5 dB and set the offset through the 3rd loop filter module to 25.5 (or near there). This yields a unity gain frequency around 200 Hz. Ramping the power up to 50 W will then move the ugf up to about 4 kHz.

The first plot shows the transfer functions at 2.1 W (red), 50 W (blue) and 50 W with the boost engaged (green). The ISS gain was 5 dB for all measurements.

The second plot shows the PSD calibrated in RIN/√Hz. The red trace shows the error point which has not been corrected for the AC coupling, whereas the magenta trace shows the sixth PD (hooked up to channel 2). Both traces were taken at 50 W without boost. The black trace shows the error point at 2 W. The amber and dark blue traces show the RIN without the outer loop engaged. The dark blue trace is limited by ADC noise above 2 kHz. The ISS gain was again fixed at 5 dB.