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Entry  Fri Aug 29 22:57:48 2008, Yoichi, Update, PSL, FSS loop transfer functions OPLTFs.pdfOPLTFsScaled.pdf
    Reply  Mon Sep 1 04:34:00 2008, rana, Update, PSL, FSS loop transfer functions n.pdfFSSLO.PNGFSSLO-Spec.pngPC25.pngpc.pdf
       Reply  Tue Sep 2 07:58:34 2008, rana, Summary, PSL, FSS & PMC LO trends for 2 years psl-lo-trend.png
Message ID: 907     Entry time: Mon Sep 1 04:34:00 2008     In reply to: 905     Reply to this: 909
Author: rana 
Type: Update 
Category: PSL 
Subject: FSS loop transfer functions 
I started from 6th item in Yoichi's todo list.

1) Increased the setpoint of the thermostat next to the framebuilder from 73F to 79F. Its freezing over there
in the room with the drill press. Steve's illegal mercury thermometer is reading 19 C.


2) Looked the RFPD's output spectrum using the 20 dB coupled output from the coupler that's in-line.
The first attached PDF file (n.pdf) has several plots:
page 1: 0-500 MHz anomolous peaks at 138 & 181 MHz but nothing too crazy
page 2: 0-100 MHz 80 MHz peak is RF pickup from the VCO Driver - not on the light
page 3: 10-30 MHz totally nuts
page 4: 18-25 MHz that's just wrong

The RF spectrum should only have some action around 21.5 MHz and a little peak at 2x 21.5 MHz. All that extra
junk means that something is broken!


3) To see if I could rid of any of the 80 MHz signal or any of that other trash from 18-25 MHz, I wound the RF cable
around a large toroidal ferrite core. This should have given us many uH of inductance for any signals common to
both the center and shield of the cable with no effect on the differential RF signals. There was no effect.


4) Next went to look at the 21.5 MHz Crystal Oscillator Reference card (D980353...I bet you can't figure out how
this one works). These have the Mini-Circuits SMA 30 MHz low pass (SLP-30) filters on both the LO and EOM outputs.

FSSLO.PNG shows the waveform after 20 dB attenuation going into a scope terminated with 50 Ohms.
FSSLO-Spec.png shows the spectrum of this signal - its pretty distorted. Here's the levels
   f (MHz) |  before filter (dBm) | after filter (dBm)
   ---------------------------------------------------
     21.5  |       -12.8               -13.1       
     43            -24                 -46
     64.5          -50               < -80
     86            -64               < -80

This would be OK after the filter, but the level is very low. Only 7dBm (accounting for my 20 dB att) !!
The FSS uses a JMS-1H mixer which needs, as everyone knows, a +17 dBm LO signal. Que lastima.

There seems to be something wrong already, but wait...


5) PC25.PNG shows the output signal going to the EOM from 0 - 25 MHz. The step that's visible there at
around 10 MHz is just something inherent to the analyzer (??). But see all that crap there down below
5 MHz ? That is NOT supposed to be there.

pc.pdf shows on the first page the comparison in EOM drive with 2 different slider values on the
RF AM adjust screen for the FSS. But page 2 is the punchline of this long entry: There is a bunch of
excess junk on the drive signal going to the FSS's phase modulator.
The FSS is then trying to handle
this extra frequency noise and getting into trouble.

We have to fix this board. I have also ordered a few SBP-21.4 from mini-circuits (SMA bandpass around 21.4 MHz)
just in case. Another option is to just replace this thing with a Marconi and an RF amp.






Attachment 1: n.pdf  26 kB  | Hide | Hide all
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Attachment 2: FSSLO.PNG  11 kB  | Hide | Hide all
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Attachment 3: FSSLO-Spec.png  2 kB  | Hide | Hide all
FSSLO-Spec.png
Attachment 4: PC25.png  2 kB  | Hide | Hide all
PC25.png
Attachment 5: pc.pdf  16 kB  | Hide | Hide all
pc.pdf pc.pdf
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