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Message ID: 719     Entry time: Wed Jul 23 01:42:26 2008
Author: rana 
Type: Configuration 
Category: PSL 
Subject: FSS RFPD: Examined, "repaired", and re-installed 
Rob said that there might be something wrong with the FSS RFPD since the loop gain is so low.
Next time we should just use the Jenne laser on it in-situ and compare with our reference.

We had a 24.5 MHz LSC PD which Rob got from Sam. Sam got it from Rai. I gave it to Rai in Livingston
because it seemed suspicious. Seems fine now. This black box PD had a lower overall response than
the goldbox one we already had. The 2001-2005 era diodes which we got from the Canadian Perkin-Elmer
all had high capacitance and so that's not a surprise.

So the goldbox one was not broken totally.

I found that the offset came from a cracked capacitor. C25 was a yellow thru-hole ceramic 0.1 uF.
Its a surface mount board...don't know why this was like this but there's also no reason it should
have cracked unless it was soldered on with too much heat. I replaced it with a 0.47 uF ceramic
surface mount. Also R24 was a 20 Ohm resistor and L3 was not stuffed?? Removed R24 and put a 1 uH
inductor into L3. This is there so that the input to the MAX4107 is AC coupled.

However, the DC signal that Rob saw was actually because of the cracked C25. It had shorted and was
making a 25 mV signal at the input to the MAX4107 which has a gain of 10. This was producing ~165 mVdc
into a 50 Ohm load and so it could have saturated most mixers. The FSS board, however, has an overly
monstrous level 21 (I think) mixer and so this should not have been an issue. Maybe.

I was able to lock with the 24.5 MHz black box PD but it was not too hard to repair the gold box one
so I did. I tuned it so that the notch is truly at 43 MHz (2x the FSS 21.5 MHz modulation) but because
someone has done this using a hacky cap in parallel with the main PD, I am unable to get the resonant
peak to line up at 21.5 MHz. Its at 23 MHz instead. This loses us ~2 dB in signal. Since the frequency
is so low, we can increase the gain in the MAX4107 by another factor of 3 or so in the future.

So the PD is not our problem. Still worth verifying that the cable is good -- its around 10 miles long!!
And loops around in there with a bunch of other cables. We have an electronic phase shifter so this seems
totally misguided.

The other bad problem is that the mode matching is pretty horrible. Something like 1/3 of the carrier
power doesn't go into the cavity.
1) Check cable between RFPD and FSS box for quality. Replace with a good short cable.

2) Using a directional coupler, look at the RFPD output in lock on a scope with 50 Ohm term.
   I suspect its a lot of harmonics because we're overmodulating to compensate for the bad
   mode matching.

3) Purchase translation stages for the FSS mode matching lenses. Same model as the PMC lenses.
   Fix the mode matching.

4) Get the shop to build us up some more bases for the RFPDs on the PSL such as we have for the LSC.
   Right now they're on some cheesy Delrin pedestals. Too soft...

5) Dump the beam reflected off the FSS RFPD with a little piece of black glass or a razor dump.
   Anodized aluminum is no good and wiggles too much.

The attached PDF shows photos of the old and new style PDs. One page 3 there's a wire that I soldered on
as a handle so that we can remove the RF can (occasionally people claim that soldering to the lid screws
up the magnetic shielding magic of the lid. use this as a litmus test of their electronics know-how; its
a tin can - not an orgone box). Pages 4 & 5 are the circuit before I soldered, page 6 the cap after I
tried to remove it, page 7 is the circuit after I put in the new cap, and page 8 is the schematic with
the mark up of the changes.
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