ELOG - Excess Noise in Digital Filtering

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Fri Nov 25 18:45:31 2011, Den, Rana, Summary, SUS, Excess Noise in Digital Filtering

Fri Nov 25 19:45:36 2011, Mirco, Den, Summary, SUS, Excess Noise in Digital Filtering

Message ID: 6008 Entry time: Fri Nov 25 19:45:36 2011 In reply to: 6007

Author:	Mirco, Den
Type:	Summary
Category:	SUS
Subject:	Excess Noise in Digital Filtering

Quote:

We are now trying to understand why the coherence between SUS-X_SUSPOS_IN1 and SUS-X_SUSPOS_OUT is lost below 1 Hz for X = MC1, MC2, MC3, BS, ITMX, ITMY, ETMX, ETMY, SRM. It is OKEY only for PRM but the different filteres are used there. For PRM - 30:0.0 and Bounce Roll, for all others - 30:0.0 and Cheby. The transfer functions between these two signals plotted in foton and fft tools are also not the same.

If we switch off all the filters between these 2 signals, than the coherence is one. If one of the filters is switched on, everything is also fine. But if there are several present, than they filter the signal in unexpected way.

Moreover it seems that the coherence is dependent on the input signal. The coherence is better with local dumping than without.

We have done the following on the c1sus and c1lsc computers : provided excitation, let the signal pass through filters 30:0.0 and Cheby and plotted the coherence between in and out signals.

c1sus computer - coherence is corrupted

c1lsc computer - coherence is not corrupted

Attachment 1:

sus_coh-crop.pdf 28 kB | Hide | Hide all

Attachment 2:

lsc_coh-crop.pdf 23 kB | Hide | Hide all

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