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Entry  Thu Feb 24 18:21:24 2011, kiwamu, Update, Green Locking, in-loop and out-of-loop measurements green_lock_setup.pngmeasurement1.pngTRX_REFL11.pngmeasurement2.png
    Reply  Thu Feb 24 19:59:25 2011, kiwamu, Update, Green Locking, whitening filter for ALS ACDC.png
Message ID: 4353     Entry time: Thu Feb 24 19:59:25 2011     In reply to: 4352
Author: kiwamu 
Type: Update 
Category: Green Locking 
Subject: whitening filter for ALS 

I forgot to mention about the whitening filter for the ALS digital control system.

As usual I used a whitening filter to have a good SNR against ADC noise, but this time our whitening scheme is little bit different from the usual our systems.

I used two ADC channels for one signal and put a digital summing point  and digital filters to keep good SNR over the frequency range of interest.

It's been working fine but it's still primitive, so I will study more about how to optimize this scheme.


     The diagram above shows our scheme for the signal whitening.

Basically the SNR at DC is bad when we use only a whitening filter as shown on the bottom part of the diagram because the signal is quite tiny at DC.

On the other hand if we take raw signal into ADC as 'DC path'  shown above, the SNR is better at DC but not good at intermediate frequencies (30 mHz - 1kHz).

So the idea to keep the good SNR over the frequency range of interest is to combine these 'DC path' and 'AC path' in a clever way.

     In our case, the 'DC path' signal is not as good as the 'AC path' signal above 30 mHz, so we cut off those high frequency signals by using a digital low pass filter.

In addition to it, I put a gain of 1000 in order to match the relative gain difference between 'DC path' and 'AC path'.

Then the resultant signal after the summing point keeps the good SNR with a flat transfer function up to 1 kHz. 


Two different measurement have been performed for a test of the green locking last night.

Everything is getting better. yes. yes.

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