Dan Chen, Darkhan Tuyenbayev

2020/12/1

We investigated noise of XPcal.

We measured noise at error monitor of OFS circuit box. (FFT analyzer has -120dB noise)
Case A: Test IN = short, PD = connected, Excitation = short, Offset = short => -120dB
Case B: Test IN = open, PD = connected, Excitation = short, Offset = connected => -80dB
Case C: Test IN = open, PD = connected, Excitation = connected, Offset = short => -60dB
Case D: Test IN = open, PD = connected, Excitation = connected without ground connection, Offset = short => -60dB
Case E: DGS + 40m cable (Excitation) => -100dB
Case F: DGS + 40m cable (offset) => -120dB
Case G: Test IN = short, PD = connected, Excitation = short, Offset = short, 40m cable housing and cable housing from OFS are connected => -120dB(?)
Case H: Test IN = short, PD = connected, Excitation = short, Offset = short, 40m cable housing and cable housing from OFS are not connected => -120dB(?We need to check this exactly value from the data but the difference from case G was small.)
Case I: Test IN = short, PD = connected, Excitation = short, Offset = short => noise at error monitor point and to AOM point has not so much difference. ~4dB

From these measurements we think:
- Signal from DGS through 40m cable is clean.
- OFS circuit itself is also clean.
- But somehow when it connect, noise goes up.
- It seems ground connection between 40m cable and cable to OFS circuit does not make the noise worse.
- It seems a variable gain just after the error monitor point does not make the noise worse.