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Entry  Fri Feb 4 11:10:27 2011, Suresh, Update, Green Locking, VCO PLL Frequency noise VCO_calibration.pngVCO_PLL_Servo.pngVCO_Gain.pngVCO_Transfer_Functions.pngVCO_PLL_Freq_Noise.png
    Reply  Mon Feb 14 00:39:21 2011, rana, suresh, Update, Electronics, VCO Frequency Noise Measurement with the MFD 
       Reply  Thu Feb 17 23:11:40 2011, rana, Update, Electronics, VCO Frequency Noise Measurement with the MFD vco.png
Message ID: 4248     Entry time: Fri Feb 4 11:10:27 2011     Reply to this: 4281
Author: Suresh 
Type: Update 
Category: Green Locking 
Subject: VCO PLL Frequency noise 

This measurement pertains to the BL2002 VCO PLL unit.

 

Our goal is to measure the frequency fluctuations introduced by the VCO. 

 

First the VCO calibration was checked.  It is -1.75 MHz per volt.  The calibration data is below:

VCO_calibration.png

 

 

 

Next we measured the Transfer function between points A and B  in the diagram below using the Stanford Research System's SR785.  This measurement was done with loop opened just after the 1.9MHz LPF and with the loop closed.

VCO_PLL_Servo.png

 

The TF[open] / TF [closed ] gave the total gain in the loop.  As shown below:

VCO_Transfer_Functions.png

Green curve is the Transfer Function with the loop open and the red with that of the loop closed.

Gain Shown below is the quotient TF[open]/TF[closed]

 

VCO_Gain.png

 

 c) As can be seen from the graph above the loop gain is >>1 over 0.1 to 300Hz.  And hence the frequency noise of the VCO is just the product of the voltage noise and the VCO calibration factor over this range,

d) the noise power at the point B was measured and multiplied by the VCO calibration  factor to yield dF(rms)/rtHz:

VCO_PLL_Freq_Noise.png

The green line with dots are the data

The blue line is the rms frequency fluctuation.

This corresponds to a arm length fluctuation of about 20pm.

 

 

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