We measured the TF of marconi using PLL loop. Marconi has flat response up to around 200kHz. This is quite good and we can certainly use it in ACAV loop.
==block diagram of ACAV loop==
- D: frequency disc
- G: servo
- A: Marconi
- H: Amplifier
The whole OLG TF was measured in PSL:... This time we looked into the marconi to see if its TF has bandwidth high enough for ACAV loop or not. We know that Marconi has lower phase noise than LIGO homemade VCO (Megan's elog), but we have not learned about its bandwidth yet.
==PLL setup for Marconi TF==
The actual magnitude TF at DC can be determined by using a voltage calibrator to inject DC signal and measure how much the frequency of the output changes. This depends on the tuning range setup on the Marconi. However, we don't know the bandwidth of the TF, so we use PLL to find out. The setup is shown below. The gain from SR560 was set to be low, so that the signal at high frequency will be the TF of the Marconi + frequency discriminator.
Since the mixer output gives 1/f response (flat in [rad/rtHz] unit), we corrected the TF by multiplying back with f to get the TF of the marconi. The magnitude on the plot below has arbitrary unit. We are only interested in its shape. We tried 100Hz, 1kHz(not shown), 10kHz tuning range. The magnitude varies with the tuning range as expected. The phase does not change that much.
If we want to have phase margin of 45 degree, assuming other components in the loop have no phase lag. The best UGF we can do is upto 200kHz, according to the phase response of the Marconi (the phase drops by 135 degree around 200 kHz). Therefore, using a Marconi as an oscillator for driving the AOM is also possible because its bandwidth is high enough for measurement up to 1kHz.
Note: we will check what is the TF of the amplifier (H) used in our setup to make sure that it is not the limiting component.
We can definitely use Marconi as a VCO in our ACAV loop.
NExt: The next step is checking the UPDH box. At a glance, we found that the TF shape of the current UPDH is not suitable for our requirement.