Evan Stefan Daniel
The second EOM driver was installed in the CER using the 9MHz control and readback channels. The first attached plot shows the DAQ readback signals. Both drivers show the similar noise levels for the in-loop and out-of-loop sensors. They are also coherent with each other as well as ASC-AS_C! The in-loop noise is clearly below which would indicate that the signal is suppressed to the sensor noise. The measured out-of-loop noise level is also a factor of 4 higher than the setup in the shop.
The second plot shows the same traces but this time the ifr is feeding the EOM driver in the CER. As expected its out-of-loop noise level is now consistent with measurements in the shop and no longer coherent with the unit in the PSL.
We were starting to suspect that we are looking at down-converted out-of-band noise...
Using a network analyzer, we took the following measurements:
- rf spectrum of 45.5 MHz from harmonic generator + distribution amplifier in CER,
- rf spectrum of 45.5 MHz from IFR in CER,
- rf spectrum of 45.5 MHz from spare harmonic generator in the EE shop,
- rf spectrum of 9.1 MHz from OCXO + distribution amplifier in CER,
- audio-band spectrum of out-of-loop RAM readback of EOM driver, using harmonic generator as 45.5 MHz source, and
- audio-band spectrum of out-of-loop RAM readback of EOM driver, using IFR as 45.5 MHz source.
The first four of these are shown in the attached plot [the OCXO has been multiplied by 5 in frequency for the sake of comparison]. The message is that the 45.5 MHz in the IFO distribution system has huge, broad wings out to 2 MHz away from the carrier. These are not seen on the IFR, the harmonic generator on the bench, or the 9.1 MHz in the distribution system.
Although the EOM driver still works to suppress some of the RFAM below 50 kHz, the broad wings still contribute significantly to the rms; most of it is accumulated above 200 kHz offset from the carrier. This is shown in the second attachment.
I looked again at some rf spectra in the CER.
These peaks appear on every output of the harmonic generator, even when it is not driving any distribution amplifiers (just a network analyzer).
These peaks also appear even when the harmonic generator is driven by +12 dBm of 9.1 MHz from an IFR (not from the OCXO + distribution amplifier).
This suggests we should focus on the harmonic generator or its power supply.