nutsinee.kijbunchoo@LIGO.ORG - posted 14:37, Tuesday 01 October 2019 - last comment - 15:39, Tuesday 01 October 2019(52246)
LHO Homodyne modified to have lower RF gain
Daniel, Marc, Nutsinee
Following ECR E1900262 we modified the currently installed homodyne (S1800623, circuit diagram D1300671) to have lower RF gain for the upcoming CLF power stabilization servo installation (E1900231). U11 has been replaced with LT6200CS8, R46 removed, and R48 replaced with 0Ohm resister. We sent in 0.2Vpp of 3.125MHz signal into the test input (gain = 18mA/V) and measured 151mV rms at RF2 OUT (213.5 mVpk). Taken into account a factor of 2 (50Ohms at the output) we have 427mV before U11. 0.1Vpk input corresponds to 0.18mA. Thus,
Transimpedance = 427mV/1.8mA = 237 Ohms.
According to the ECR we should have 2kOhms.
I wouldn't trust the test input too much for that. It wasn't re-engineered for the low 3.125MHz resonance. When it is on, a high pass capacitor is on the test injection input (an possibly inductance from transformer coupling, can't remember) and it detunes the RF circuit. The 3.125MHz tuning is particularly sensitive. It needs to be calibrated optically. Either use a modulated LED source (cross calibrated from a reference diode like a PDA10CS), or calibrate from the real CLF (the DC outputs are plenty sensitive to get the power level accurately).