Quadrant diode configuration

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Quadrant diode configuration
X or configuration? ISYS quadrant diodes
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X or configuration?
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Comparison X and configuration

• Relative sensitivity of X configuration
• Virgo QDs are used in X configuration
• Same signal-to-noise-ratio as config.
• Couplings
• Horizontal/vertical error signal couplings
• X config couplings for hv miscentering
• Misalignment sensitivity
• Strong horizontal or vertical misalignment
• X config may give no error signal
• Conclusion
• For new QD front ends
• configuration desirable

vs.
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Simulation h/v couplings

• Simulation of horizontal/vertical coupling
• Normalized asymmetry fluctuates - 0.5 0.5
  (C7)
• X config 4 coupling Tx - Ty
• config none

Q81 (8 hours)
dh dv 0.3
Normalized asymmetry
Power as seen by QD (horizontal)
C7
Normalized asymmetry
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ISYS quadrant diodes
Overview
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ISYS Quadrants PHD (IMC, RFC) (Preliminary
results)
Quadrant photodiode specifications - Centronic
QD40-4x, silicon photodiode (EGG YAG 444-4) -
7.98mm diam, 200microns gap, 0.4 A/W (11.4 mm,
0.45 A/W) - 120V bias (IMC RFC) (180 V) -
10mA peak current (3 mA) - geometrical
configuration (X configuration) Quadrant box
specs. - 6 MHz 22 MHz, demodulation done
onboard (RFC, IMC respectively) - IMC RFC
involved in servo-loops - vertic/horiz error
signals (AC DC) provided onboard, BNC
connectors (unipolar) - R_ac 50 kohm -
4quadrants, shot noise limited for 16 mA,
should be 0.6mA, (need TBC and check reason
for extra noise) (in that case, S/N would be
equivalent to Frascati quadrants)
Frascati QDs
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ISYS Quadrants PHD (IMC, RFC, PMC, SL)
Centering electronics
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ISYS Quadrant PHD (IMC, RFC, PMC, SL)

• Discussion
• we can work shot noise limited with present
  ISYS quadrants
• Question
• Is it worth to extend LA standard for ISYS
  quadrants (24)?
• 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

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End
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LA electronics
TStage driver
Demodulator board (2 channel)
Phase shifter (2 channel)
QD front end
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Quadrant photodiode

• type EGG YAG 444
• sensitivity 0.45 A/W
• DC power 3 mWmax
• transimpedance 2 k?
• Bias voltage 180 V

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QD electronics
Quadrant diode box
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Scheme of LA electronics
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Noise measurements
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Influence of QD power
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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. Vajentes 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

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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 cant reach this precision.