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Entry  Fri Oct 21 01:36:52 2011, frank, tara, DailyProgress, FSS, FSS debugging/ checking 6x
    Reply  Mon Oct 24 21:20:26 2011, frank, tara, DailyProgress, FSS, rf summing box setup.pngIMG_2176.JPG
    Reply  Wed Nov 2 20:05:15 2011, tara, DailyProgress, FSS, FSS debugging, harmonic line problem fixed errorpoint.pngIMG_2177.JPG
       Reply  Thu Nov 3 19:51:07 2011, tara, DailyProgress, FSS, FSS debugging, harmonic line problem fixed beat_2011_11_03.pngzoom.pngbeat_2011_11_03.fig
          Reply  Tue Nov 8 03:15:10 2011, tara, DailyProgress, FSS, FSS debugging, harmonic line problem fixed beat_2011_11_07.pngbeat_2011_11_07.fig
             Reply  Wed Nov 9 02:10:58 2011, tara, Notes, FSS, FSS debugging, harmonic line problem fixed beat_2011_11_08.png
                Reply  Mon Nov 14 21:51:07 2011, tara, Notes, FSS, FSS debugging, OLG TF measurement beat.pngbeat.figOLGTF.pngOLGTF.fig
Message ID: 712     Entry time: Mon Oct 24 21:20:26 2011     In reply to: 711
Author: frank, tara 
Type: DailyProgress 
Category: FSS 
Subject: rf summing box 

 We check modulation depth of the PDH signal to see if we have enough modulation depth or not. The result will be analyzed later.

==motivation==

Based on the design of the RF summing box we got from Peter ,

IMG_2176.JPG

the TF from 35.5 MHz input to EOM does not have a resonant frequency at 35.5 MHz. Rather, the resonance is ~ 10MHz. We are not sure if we need to have the resonance at 35.5 Mhz or not. So we decide to check if the modulation index is already large enough for the current design, and compare it with that of the modified one. If the modulation depth is already large enough, we can keep it as it is. Basically we just need a high pass for 35.5MHz input to EOM.

By changing the inductance L from 1uH to 80 nH, the peak can be moved to ~ 35.5 MHz.

==Setup & Measurement==

 We want to check the slope of the error signal when the RF power changes, so there's a small change in the setup, see fig1.

setup.png

The input power is 2mW.

Then the slope of the error signal was record, the pk-pk of the sideband and carriers were also record between the input power of 10dBm and 13dBm to the EOM.

Then the RF summing box was modified (changing L), and the measurement was repeated.

==results==

coil       |  input pwr (dBm) |  Carrier slope ( MHz/V) |   pk-pk value (mV) (carrier/side band)          

 80nH   |     10                   |     1.43                          ||   28/18

            |             13           |      1.11                          ||  38.8/22.6

1 uH    |       10                 |      0.91                          ||   49.2/27.6

            |             13          |       0.74                         ||    68.4/37.2

 

From the result, the best setup is 1uH coil with maximum side band power which provides the best slope ( maximum gain at frequency discriminator). Note that 1uH coil is the original value for the schematic, so modification to 80nH does not help.

-> Modulation depth

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