The current PD for the 160 MHz beat signal is 120MHz. We use a 2 GHz PD to compare the results between two PDS,
and there is no difference. 120 MHz PD seems to be working fine for us. However, the beat signals at freq above ~5 kHz we have seen so far are not real signal from cavities' noise.
We have checked several parts on the PSL setup to search for excess noise, we have not checked the PD for beat signal, so
we try this measurement.
We use a 2 GHz PD to see the beat signal from another port of the BS. The attached figure has
two traces of the beat signal, the one on top from 120MHz PD and the one on the background from 2GHz PD .
The results are similar up to ~10 kHz.
The difference at high f comes from different bandwidth and gain setup for PLL loop, because
it changes with gain setup on SR560. So, the beat noise results shown so far are valid only up to 10kHz.
At higher f, it just the PLL loop.