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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: 12101     Entry time: Fri Apr 29 16:13:36 2016     In reply to: 12098     Reply to this: 12110
Author: ericq 
Type: Update 
Category: LSC 
Subject: Green PDH demod lowpass 

We can get as much, if not more, attenuation of the 1F line in the mixer output that we get from the post-mixer LPF from using the following passive filter between the PD and mixer RF input:

There should still be some kind of LPF after the mixer, but I haven't yet determined what it should be; this will determine how much phase the PDH loop wins. At most, this should win around 25 degrees at 10kHz.


The filter was designed by referencing the "Handbook of Filter Synthesis" by Zverev, looking for an elliptic filter for matched source and load impedences, 40dB min attenuation in the stopband, a stopband frequency that starts at twice the corner frequency, and minimizing the VSWR between the PD and filter in the passband.

In terms of the tables in the book, this means: n=5, rho=2%, theta=30deg, K**2 = 1.0. The dimensionless component values were scaled by the corner frequency of 200kHz, and reference impedence of 50 Ohm. (The corner is a little lower than the real modulation frequency, since the nonzero resistance of the inductors pushes the frequency up a bit)

The ideal capactior values do not correspond to things we have in hand, so I checked our stock and chose the closest value to each one.Unsurprisingly, due to these component substitutions, and the fact that the coilcraft inductors have a resistance of about 7 Ohms, the predicted TF of the realizable filter does not match the design filter exactly. However, the predicition still looks like it will meet the requirement of 40dB of supression of the 2F line in the PD signal. (Since we have tunable inductors, I've used the ideal inductor values in generating the TF. In practice I'll inspect the TF while I tune them)

  Desired Realizable
C1 8.28 nF 10 nF
C2 1.39 nF 1.5 nF
C3 19.6 nF 22 nF
C4 4.22 nF 4.7 nF
C5 6.08 nF 6.8 nF
L2 43.1 nH 32-48 nH + 7 Ohm
L4 34.4 nH 32-48 nH + 7 Ohm

[In this TF plot, I've multiplied the real response by 2 to account for the voltage division that occurs with ideal 50 Ohm impedance matching, to make 0dB the reference for proper matching]

The filter's phase delay at the modulation frequency is just about 180, which as a time delay of 5usec works out to 9 degrees of phase loss at 10kHz in the PDH loop. According to some old measurements, the current LPF costs something like 35 degrees at 10k, so this wins at most around 25 degrees, depedent on what LPF we put after the mixer.

LISO source both traces is attached!

Attachment 3: elp_liso.zip  1 kB  Uploaded Fri Apr 29 17:14:59 2016
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