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Message ID: 3930     Entry time: Tue Nov 16 09:02:54 2010
Author: Aidan 
Type: Update 
Category: Green Locking 
Subject: PID loop - calibration of SR620 output 

 [Aidan, Kiwamu]

Kiwamu and I roughly calibrated the analogue output from the SR620 frequency counter yesterday. The input channel, intuitively named C1:PSL-126MOPA_126MON, now reads the measured frequency in MHz with an error of about 0.1MHz - this is, I think, due to the bit noise on the D/A conversion that Kiwamu discovered earlier. That is, the output range of the SR620 corresponds to around 100MHz and is digitized at 10-bit resolution, and ...

100MHz/(10^2) ~= 0.098MHz. [Sad Face]

Calibration:

We set the EPICS range to [-100, 100] (corresponding to [-5V, 5V]), connected a Marconi to the Freq Counter, input a variety of different frequencies and measured the counts on the EPICS channel.

The linear fit to the calibration data was F = 2.006*EPICScount - 0.2942. From this we worked out the maximum and the minimum for the range settings that give the channel in MHz: EGUF = -200.8942 and EGUL = 200.3058. The previous range was [-410, 410]

 

Calibration of SR620 analogue output
Input Frequency (MHz) Measured EPICS Value
10  5.191
20  9.98
30 15.21
40 20.00
50 25.18
60 29.99
70 35.18
71 35.565
72 35.9894
73 36.3861
74 37.17
75 37.576
76 37.9669
77 38.3575
78 39.166
79 39.5691
80 39.978
   

 

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