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Entry  Thu Jan 27 17:12:02 2011, Aidan, Joe, Summary, Green Locking, Digital Frequency to Amplitude converter Screenshot.pngSwept_sine_F_to_A.png
    Reply  Thu Jan 27 23:21:50 2011, rana, Summary, Green Locking, Digital Frequency Discriminator 
       Reply  Fri Jan 28 09:03:38 2011, Aidan, Summary, Green Locking, Digital Frequency Discriminator 
          Reply  Tue Feb 8 10:23:02 2011, Aidan, Summary, Green Locking, Digital Frequency Discriminator - reference 
    Reply  Fri Jan 28 10:27:46 2011, Aidan, Joe, Summary, Green Locking, Digital Frequency Discriminator - calibration 
Message ID: 4213     Entry time: Thu Jan 27 17:12:02 2011     Reply to this: 4216   4218
Author: Aidan, Joe 
Type: Summary 
Category: Green Locking 
Subject: Digital Frequency to Amplitude converter 

Joe and I built a very simple digital frequency to amplitude converter using the RCG. The input from an ADC channel goes through a filter bank (INPUT), is rectified and then split in two. One path is delayed by one DAQ cycle (1/16384 s) and then the two paths are multiplied together. Then the output from the mixer goes through a second filter bank (LP) where we can strip off twice the beat frequency. The DC output from the LP filter bank should be proportional to the input frequency.

Input Channel: C1:GFD-INPUT_xxx

Output Channel: C1:GFD-LP_xxx

Joe compiled the code and we tested it by injecting a swept sine [100, 500]Hz in the input filter bank. We confirmed that output of the LP filter bank changed linearly as a function of the input frequency.

The next thing we need to do is add a DAC output. Once that's in place we should inject the output from a 4kHz VCO into the ADC. Then we can measure the transfer function of the loop with an SR785 (driving the VCO input and looking at the output of the DAC) and play around with the LP filter to make sure the loop is fast enough.

The model is to be found here:

/opt/rtcds/caltech/c1/core/advLigoRTS/src/epics/simLink/c1gfd.mdl

The attached figures show the model file in Simulink and a realtime dataviewer session with injecting a swept sine (from 500Hz to 100Hz) into the INPUT EXC channel. We've had some frame builder issues so the excitation was not showing on the green trace and, for some reason, the names of the channels are back to front in dataviewer (WTF?), - the lower red trace in dataviewer is actually displaying C1:GFD-LP_OUT_DAQ, but it says it is displaying C1:GFD-INPUT_OUT_DAQ - which is very screwy.

However, the basic principle (frequency to amplitude) seems to work.

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