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Entry  Wed Dec 16 11:21:20 2009, Alberto, Update, ABSL, Universal PDH Box Servo Filters NewandOlfFilterTF.pngNewandOlfFilterTF.png
    Reply  Wed Dec 16 11:55:47 2009, rana, Update, ABSL, Universal PDH Box Servo Filters 
Message ID: 2421     Entry time: Wed Dec 16 11:21:20 2009     Reply to this: 2423
Author: Alberto 
Type: Update 
Category: ABSL 
Subject: Universal PDH Box Servo Filters 
Yesterday I measured the shape of the servo filter of both the old and the new Universal PDH boxes.
Here they are compared.


The way the filter's transfer function has been measured is by a swept sine between the "SERVO INPUT" and the "PIEZO DRIVE OUTPUT" connection on the box front panel. The spectrum analyzer used for the measurement is the SR785 and the source amplitude is set at 0.1V.

The two transfer functions are clearly different. In particular the old one looks like a simple integrator, whereas the new one already includes some sort of boost.

That probably explains why the new one is unable to lock the PLL. Indeed what the PLL needs, at least to acquire lock, is an 1/f filter.

I thought the two boxes were almost identical, at least in the filter shapes. Also the two schematics available in the DCC coincide.

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