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Entry  Thu Oct 13 21:44:48 2011, Mirko, Update, CDS, Seismic BLRMS channels, new RMS calculation New_RMS_vs_old_RMS.pngOriginal_RMS_LP_slower_than_0.1Hz.png
    Reply  Mon Oct 17 14:26:22 2011, Mirko, Update, CDS, Seismic BLRMS channels, new RMS calculation 
Message ID: 5663     Entry time: Thu Oct 13 21:44:48 2011     Reply to this: 5679
Author: Mirko 
Type: Update 
Category: CDS 
Subject: Seismic BLRMS channels, new RMS calculation 

[Rana, Koji, Mirko]

We looked into the CDS RMS block c-code as described in Rolfs RCG app guide. Seems the block uses a first order LP filter with a corner freq. / time of 20k execution cycles. There are also some weird thersholds at +-2000counts in there.

I was looking into implementing a hand-made RMS block, by squaring, filtering, rooting. The new RMS (left) seems nicer than the old one (bottom right). Signal was 141counts sinus at 4Hz.

Filters used: Before squaring: 4th order butterworth BP at given freq. & (new) 6th order inverse Chebyshew 20dB at 0.9*lower BP freq. and 1.1*upper BP freq. => about 1dB at BP freq.

                       After squaring: 4th order butterworth LP @ 1Hz.

C1PEM execution time increased from about 20us to about 45us.

Made a new medm screen with the respective filters in place of the empty C1PEM_OVERVIEW. Should go onto the sitemap.


Original RMS LP is slower than 0.1Hz, see below for single LP at 0.1Hz in the new RMS. Original RMS is faster than single LP @ 0.01Hz


Some of the channels are recorded as 256Hz DAQ channels now. Need to figure out how to record these as 16Hz EPICS channls.

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