2 Detector meeting, Cascina, ‘X’ or ‘+’ configuration?
3 Detector meeting, Cascina, Comparison ‘X’ and ‘+’ configuration Relative sensitivity of X configuration Virgo QD’s are used in X configuration Same signal-to-noise-ratio as + config. Couplings Horizontal/vertical error signal couplings X config: couplings for h+v miscentering Misalignment sensitivity Strong horizontal or vertical misalignment: X config may give no error signal Conclusion For new QD front ends “+” configuration desirable vs.
4 Detector meeting, Cascina, Simulation h/v couplings Simulation of horizontal/vertical coupling Normalized asymmetry fluctuates − 0.5 … (C7) ‘X’ config: 4 % coupling Θ x − Θ y ‘+’ config: none C7 Q81 (8 hours) Normalized asymmetry Power as seen by QD (horizontal) dh = dv = 0.3
8 Detector meeting, Cascina, Discussion we can work shot noise limited with present ISYS quadrants Question: Is it worth to extend LA standard for ISYS quadrants (2*4)? Main advantage: standardization Price to pay: - Extra demodulation boards - Extra cabling - Swap translation stages - rearrange optical setup? (VIRGO quadrant box larger) What about QDs for monitoring SL & PMC (nearly same as IMC & RFC) (14 MHz)? What about DC QDs (BMS, MC end mirror transmission)? at present: EGO is to provide those QDs (UDT photodiodes already bought), use temporarily PMC QDs (for BMS), and Nice QD for MC mirror ISYS Quadrant PHD (IMC, RFC, PMC, SL)
10 Detector meeting, Cascina, LA electronics TStage driver QD front end Demodulator board (2 channel) Phase shifter (2 channel)
11 Detector meeting, Cascina, Quadrant photodiode type EG&G YAG 444 sensitivity = 0.45 A/W DC power = 3 mW max transimpedance = 2 k Bias voltage = 180 V
13 Detector meeting, Cascina, Scheme of LA electronics ADC noise Preamp. noise Shot noise Low-pass filter AC: Gain 200 DC: Gain 1 diff. sig. non-diff.sig. Non-optimal treatment of DC signals dominated by ADC noise (but were not foreseen as error signals) VME QD box
14 Detector meeting, Cascina, Noise measurements after demodulator quadrant diode AC output Theoretical shot noise M. Mantovani demodulator gain
15 Detector meeting, Cascina, Influence of QD power Can gain ~ factor 10 by increasing QD powers
16 Detector meeting, Cascina, Fast quadrant centering system Couplings Coupling with longitudinal error signals Horizontal/vertical error signals are coupled if h & v miscentering Coupling with waist mismatch Coupling with longitudinal noise Better rms centering required See G. Vajente’s talk (Napoli 03/10/2005) Bad mirror motion reconstruction Not understood Fast QD centering system could remain ON all time Control signal would be used for beam stabilization Present system too noisy
17 Detector meeting, Cascina, Requirements for centering With the present centering configuration (activates every few minutes) RMS change of a factor of 4 Extrapolating from these measurements… Taking a reasonable range around the “minimum”, the maximum DC asymmetry must be less than 0.1 Even with the best centering rate now possible (every 4-5 seconds) we can’t reach this precision.