Frequency Noise in Virgo

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Frequency Noise in Virgo
by Matt Evans
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The Actors

• Noise Sources
• Input Mode Cleaner length noise
• Sensing noise on IMC lock
• Frequency Servo
• Servo topology
• Gain limitations
• Sensing noises
• Coupling to the Dark Fringe
• Common Mode Rejection Ratio
• Fluctuations

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Sources IMC Length Excitation

• Injection bench alignment sensor noise below 15Hz
• Seismic excitation
• between 15 and 20Hz
• also possibly higher problem with suspension?
• Actuator noise
• IB coil-driver/DAC noise
• Mode cleaner end mirror DAC noise

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Sources Seismic Excitation
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Sources IB transfer function

• Dihedron resonace at 80Hz

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Sources Actuator Noise

• Dihedron resonace at 80Hz
• Couples also through offsets (FmodErr)

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Frequency Servo
MC path 100Hz
Fast Loop 250kHz
SSFS 25kHz
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Servo Fast Loop

• Noise properties not known

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Servo SSFS
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Servo SSFS
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Servo MC path
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Coupling to the Dark Fringe

• Line match at 444 and 1111Hz
• Modulation spreads line power
• The coupling fluctuates quickly
• Coherent analysis is difficult
• Noise projection is approximate

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Coupling to the Dark Fringe
Line match
Feature match
B1 Shot Noise
B5 Shot Noise
ALL VERY ROUGH!!!
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Left to be Done

• Tune SSFS
• Increase gain between 1 and 5kHz
• Improve phase margin by removing unnecessary
  delay in cables, OpAmps, ...
• Understand fast loop noise
• Electronics
• Scattering on external bench
• ND filters, clipping
• Other ideas?
• Stabilize coupling (alignment?)