(Sheila, Alexa)

We had thought that maybe the 1/f noise seen in COMM came from the PFD. Today we have ruled that out. We did this by replacing the PFD with the spare IQ demod channels of the IMC. In this confirgaution we measured the beatnote signal to be 250mVpp, and set the COMM PLL Input gain to 31dB. This resulted in a loop with -18dB less gain than the nominal configuration. We then used the control signal to measure the noise.

I have attached a DTT with the following traces:

• REF 2-3: signal with demod and input 1 gain 31dB (corresponds to UGF of ~3kHz)
• REF 4: signal with PFD and input 1 gain 27dB (nominal configuration corresponds to UGF ~30kHz)
• REF6: signal with PFD and input 1 gain 9dB (corresponds to UGF of ~3kHz)
• REF10: signal with PFD and input 1 gain 18dB

Clearly, the PFD does not seem to be the culprate. These measurements also suggest that we should lower the UGF of the COMM PLL loop.

With the demod in place and the input gain at 31dB, I also measured the amplitude spectrum out of the IMON of the demod. I have attached the data.