Matteo and Yuhang

In elog2236, before CCFC demodulated, its SNR was reported to be about 40dB with taking ~50% FC_ref . And the signal was ~-25dBm. So it was clear that SNR is large enough. Nevertheless, when we check the demodulated CCFC on the oscilloscope, the signal seems not to have an SNR of 40dB. So we thought the noise couples through the demodulation process, which could be solved by amplifying the CCFC signal.

The TAMA RF PD was used to acquire CCFC. And it was investigated when we used it to acquire CC1 from OPO reflection. That investigation proved that amplification was not effective to improve SNR. However, recently Matteo found that the resistor was chosen inappropriately during that time. In the first attached figure, the resistors used to amplify RF signals are marked. As shown in the attached figure 2, these resistors need to be 10Ohm and 100Ohm to have the best performance. However, it was chosen to be 1kOhm and 10kOhm when it was used for CC1. So we put the recommended 10Ohm and 100Ohm in TAMA RF PD.

Then the modified TAMA PD was placed back to the reflection of FC with ~50% FC_ref going inside. We checked again amplifier noise, TAMA PD noise ,and CCFC sideband. The test result is shown in the attached figure 3. There is also a zoom-in of the CCFC sideband shown in the attached figure 4. We can see that the signal is amplified by ~10dB while noise is amplified by ~20dB. A factor of 10 of voltage amplification should corresponds to a factor 100 power amplification. So the 10dB signal amplification seems to be limited somehow.

But anyway, we demodulated this signal and checked it on oscilloscope. The signal didn't become much cleaner.