We measured the phase noise of the low noise VCO. We used the COMM PLL which deploys a single stage frequency-difference divider (FDD). The PLL was locked to a fixed frequency oscillator (ifr locked to GPS running at 78.95 MHz). The output filters of the PLL were: boost engaged, generic disabled, VCO comp off and low pass off. We added a SR560 with 3 kHz low pass and a gain of 10 to clean up the high frequency part. We then looked at the error point of the common mode board (H1:LSC-REFL_SERVO_ERROR). The calibration is as follows:

25kHz/V (VCO sensitivity) / 25 (boost gain) * 5 (gain divider in VCO output path) / 10 (SR560 gain) / 100 (gain in ERROR readback) * 1V/3200cts = 0.0016 V/cts

For the SSB (single-sided sidebands) we divided by an other sqrt(2). This plot matches nicely to the high frequency SSB curve measured in T0900451. Above 100 Hz we see some excess phase noise which is most likely due to the ifr. The two curves together characterize the low noise VCO all the way from 1 mHz up to 100 kHz. To get the phase noise of the VCO without the FDD we can just multiply the attached curve by 10.

The rms frequency noise of the VCO was previously measured in alogs 6972 and 6865. The numbers were 2 Hz rms for a VCO with FDD and 16 Hz for a VCO without. This is in good agreement with the current measurement.