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Entry  Thu Apr 28 15:23:11 2016, ericq, Update, LSC, Green PDH demod lowpass 
    Reply  Thu Apr 28 18:53:05 2016, rana, Update, LSC, Green PDH demod lowpass 
       Reply  Fri Apr 29 16:13:36 2016, ericq, Update, LSC, Green PDH demod lowpass IMG_3186.jpgpdh_lpf.pngelp_liso.zip
          Reply  Fri May 6 16:42:12 2016, ericq, Update, LSC, Green PDH demod lowpass pdh_lpf.pngAUX_X_PDH_OLG_May6.png
             Reply  Fri May 6 19:08:52 2016, rana, Update, LSC, Green PDH demod lowpass 
                Reply  Sat May 7 09:40:40 2016, ericq, Update, LSC, Green PDH demod lowpass 
                   Reply  Sun May 8 08:39:21 2016, rana, Update, LSC, Green PDH demod lowpass 
Message ID: 12097     Entry time: Thu Apr 28 15:23:11 2016     Reply to this: 12098
Author: ericq 
Type: Update 
Category: LSC 
Subject: Green PDH demod lowpass 

The 2F product out of the mixer is a natural concern when demodulating. However, I think this isn't so big of a deal in our green PDH servos; 420kHz isn't so high of a frequency that the servo amplifiers are bandwidth or slew-rate limited. Furthermore, the amplitude of this line is supressed by the loop somewhat, since it arises from the same field product that the loop is acting on. Measuring the Y end mixer output with a high impedance probe and the AG4395 shows it to be something like -50dBm. 

In fact, the main thing that the pomona LPFs are accomplishing right now is filtering the 1F content of the mixer output that arises from the second order sideband creating a signal at 2F, and beating with the LO at (2F-1F)=1F. This line is something like -30dBm (5mVrms) at the mixer output; I can reproduce this amplitude with a back-of-the envelope calculation using a modulation depth of 0.3, 8V out of the PD at DC when unlocked, the mixer datasheet, and the nominal cavity parameters. 

The nice thing about this is that we don't need to filter this after the mixer, we can use a [bandpass/lowpass/notch] filter before the mixer (as is done in the LSC demod boards) to filter out the 2F (420kHz) content of the PD signal, which will only introduce some small amount of linear time delay to the PDH loop, instead of the wicked phase loss from the current post-mixer LPF. We can then replace that 70kHz filter with something of lower order or higher corner frequency to win a good deal of phase in the PDH loop. 

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