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Entry  Wed Aug 27 13:55:43 2008, rana,jenne, Update, PSL, PMC Servo Board 
    Reply  Fri Aug 29 02:40:43 2008, rana,jenne, Update, PSL, PMC Servo Board pmclp-07.pdfpmclp_40m_080824.mpmclp.m
Message ID: 895     Entry time: Fri Aug 29 02:40:43 2008     In reply to: 892
Author: rana,jenne 
Type: Update 
Category: PSL 
Subject: PMC Servo Board 

Quote:
Board is back in. PMC is locked.


This entry has details about the low pass filter after the PMC mixer. This filter has a few purposes:

1] Remove the beat signal (at 2*f_mod) between the PD RF signal at f_mod and the LO signal at f_mod.
2] Remove the beat signal (at f_mod) between the PD RF signal at 2*f_mod (which comes from the
beating of the upper and lower RF sidebands) and the LO signal at f_mod.
3] Remove other RF signals from non-ideal behavior of the LO drive signal and distortion in the RF PD pre-amp.


So its important to have a very good rejection at 35 MHz and higher. I used the Hartmut LC network design which is
installed on H1, H2, & L1. Since there is a high gain in the audio amps right after the mixer we have to get rid of
the RF or else we'll get slew rate limited or otherwise rectified downconversion of the RF signal into our audio band.

Of course, what everyone immediately realizes from the above 3 points, is that this filter can't protect the PMC
noise performance from homodyne mixing (e.g. 2*f_mod in the LO and 2*f_mod in the RF PD). To get around that, we're
ordering some filters from Mini-Circuits to remove the 2f from those signals by ~30 dB. As long as we install
the same filters on the RF and LO legs, there should be no significant phase shift in the demodulated signal.

The attached 2 page PDF shows the calculated before and after TFs of this filter. The 2 attached .m files
calculate the TF's and have ascii art which shows how the filter works.

Here's a comparison of the attenuation (in dB) of 2 candidate Mini-circuits filters:

f(MHz)SLP-30SLP-50
31 0.5 0.4
35 1.3 0.4
38 6.1 0.4
40 10.8 0.42
61 46.3 14.8
71 60 29
91 76.9 48
10780 60

We don't have tabulated data at the same frequencies for both filters so I just made up some of the points by eye-balling the
plots from the catalog - but you get the idea: we can get away with using the SLP-30 at 35 MHz since it only attenuates the
signals by ~1.5 dB. So if someone can find 4 of these then Steve doesn't have to order any from Mini-Circuits.
Attachment 1: pmclp-07.pdf  36 kB  | Hide | Hide all
pmclp-07.pdf pmclp-07.pdf
Attachment 2: pmclp_40m_080824.m  1 kB  Uploaded Fri Aug 29 03:43:06 2008  | Hide | Hide all
% PMCLP is a TF of the IF filter after the PMC mixer
%       

% Mixer_Voltage -- Rs -- L1 --- L2 ---------Vout
%                            |      |   |
%                           C1     C2   Rl                   
%                            |      |   |
%                           GND    GND  GND
%

... 58 more lines ...
Attachment 3: pmclp.m  1 kB  Uploaded Fri Aug 29 03:43:21 2008  | Hide | Hide all
% PMCLP is a TF of the IF filter after the PMC mixer
%       

% Mixer_Voltage -- Rs -- L1 --- L2 ---------Vout
%                            |      |   |
%                           C1     C2   Rl                   
%                            |      |   |
%                           GND    GND  GND
%

... 57 more lines ...
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