[Matteo, Yutaro]

To investigate what the frequency response of PLL looks like, we measured the frequency response of the PZT of Prometheus X in high frequency region.

= How we did =
1. lock the phase-lock loop
2. inject sine wave at a certain frequency with relatively high amplitude (we injected 2 Vpp signal from EXC A)
3. monitor the shape of the beat note, which is broaden by the injection (spectrum of the beat note when excited at 210 kHz can be found in the first and second figure, without and with video filter)
4. measure the width, which is proportional to the gain of the PZT at the frequency.
(See also NAOJ log 730)

= Results =
Frequency to Gain plot can be found in the attachment.
We see the major peak at 210 kHz, and minor peaks at 100 kHz and 280 kHz.

= Note =
* We did the above measurement with the output to the PZT fluctuating around 0V.
* As Matteo mentioned that he had experienced in Virgo that the gain of the PZT of an NPRO was not balanced between when the output to PZT was positive and negative,
we did the similar measurements with the output fluctuating around +1 V or -1V.
We did not see any imbalance this time.