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Entry  Mon Jan 24 02:58:46 2011, kiwamu, Update, Green Locking, X arm locked ! time_series.pnggreen_one_arm.png
    Reply  Mon Jan 24 09:33:08 2011, rana, Update, Green Locking, X arm locked ! 
       Reply  Mon Jan 24 13:08:07 2011, kiwamu, Update, Green Locking, Re: X arm locked ! e_pll_oltf.png
    Reply  Mon Jan 24 10:19:21 2011, Koji, Update, Green Locking, X arm locked ! 
       Reply  Mon Jan 24 14:27:13 2011, kiwamu, Update, Green Locking, Re: X arm locked ! masslock_oltf.png
Message ID: 4195     Entry time: Mon Jan 24 13:08:07 2011     In reply to: 4192
Author: kiwamu 
Type: Update 
Category: Green Locking 
Subject: Re: X arm locked ! 

Quote: #4192

Also, the PLL diagram seems to show that you have a 1/f^2 loop: 1/f from the SR560 and 1/f from the Hz->rad conversion ??

Well, the diagram I drew is true. I also have been confused by this 1/f^2 issue in our PLL.

As Rana pointed out, the open-loop TF should become 1/f^2 over most of the frequency range, but it still remains 1/f above 5kHz for some reasons. 

 Need more investigations.

e_pll_oltf.png

 At the beginning I tried phase-locking the VCO to the beat note without any external filters (i.e. SR560 see here), but I never succeeded.

It was because the hold-in range of the PLL was not sufficiently wide, it could stay locked within frequency range of less than +/- 1MHz from the center frequency of 80 MHz.

This is obviously not good, because the beat note typically fluctuates by more than +/- 3MHz in time scale of 1 sec or so.

  So I decided to put an external filter, SR560,  in order to have a larger DC gain and a higher UGF.

Somehow I unconsciously tuned the SR560 to have a pole at 1Hz with the gain of 2000, which shouldn't work in principle because the open-loop will be 1/f^2.

However I found that the PLL became more robust, in fact it can track the input frequency range of +/- 5MHz.

The open-loop TF is shown above. For comparison I plotted also the open-loop TF wehn it's without the SR560.

I checked the frequency of the VCO output when it was phase-locked to a Marconi, it was healthy (i.e. the same frequency as the input signal from Marconi).

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