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RE: Shaping time Setting (fwd)



Hall D Electronics:


FYI

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---------- Forwarded message ----------
Date: Fri, 30 Nov 2007 21:17:30 -0500
From: "Visser, Gerard" <gvisser@indiana.edu>
To: Mitch Newcomer <mitch@hep.upenn.edu>,
     Fernando J. Barbosa <barbosa@jlab.org>
Cc: Yves Van Haarlem <yvhaarle@ernest.phys.cmu.edu>,
     Daniel S. Carman <carman@jlab.org>, Simon Taylor <staylor@jlab.org>,
     Curtis Meyer <cmeyer@ernest.phys.cmu.edu>,
     Elke-Caroline Aschenauer <elke@jlab.org>,
     Armen Stepanyan <armenak@jlab.org>, Chris Cuevas <cuevas@jlab.org>,
     Mark Taylor <taylorw@jlab.org>
Subject: RE: Shaping time   Setting

Hi Mitch,
  Our cables will be about 18m long, I think that counts as "long" and source termination is probably going to be useful. Besides, it is only these resistors which set the common-mode termination of the outputs, i.e., the load end of the cable has only a floating differential termination. So they cannot be omitted entirely. But you are right, we could possibly save some power by increasing the value of the source termination resistors to at least the point where reflections on the cable are barely within the error budget (probably 1%, I say that on the basis of the likely precision of the cable frequency equalization i.e. tail cancellation - but this, I mean practical achievable "error budget" for the pulse tail, is being studied right now, I don't have a final determination yet). We should keep it in mind, to use higher values of these resistors possibly.
  Thanks,

    Gerard

p.s. I forgot about the internal 500 Ohm resistor to ground on each output... That means the preamp boards should have been stuffed with 56.2 Ohm resistors not 51.1 Ohm to get a matched termination here. Oops. Sorry Fernando!


________________________________________
From: Mitch Newcomer [mitch@hep.upenn.edu]
Sent: Friday, November 30, 2007 8:26 PM
To: Fernando J. Barbosa
Cc: Yves Van Haarlem; Visser, Gerard; Daniel S. Carman; Simon Taylor; Curtis Meyer; Elke-Caroline Aschenauer; Armen Stepanyan; Chris Cuevas; Mark Taylor
Subject: Shaping time   Setting

I would suggest leaving out the  resistor  that switches in additional
shaping time (GT on the silk screen bottom side  near output
connector).  You could also install it so that it goes to 0V having the
same effect. Using  a pulser shaped to look like an Ar gas signal we see
a peaking time of 14ns with the resistor removed ( defaults to 0V)  and
17.5ns  with it in.   Our board is stuffed with back termination which
we resistors have removed.  Since our load is 25 ohns per side when we
use a  mini-circuits RF transformer to go from the differential outputs
to a single ended 50 ohm resistor going into the scope.   An easy way to
increase the observed gain is to increase the back termination resistor
values.   I'm not convinced they will be needed unless you are using
long cables.   In this case the cable is discharged more quickly with
back termination.  Increasing the size of the back termination has the
added value of removing
heat from the detector area.   If you remove them completely you will
still measure an impedance of  ~1K between outputs or 500Ohms to Gnd
due to the internal
resistance on the outputs.   -- Mitch

PS  We have measured  an equivalent input noise of  3300e with  a  ~15pF
input capacitance.   The uncertainty is on the amout of capacitance on
the preamp card and various connectors.  I should say  3300e is measured
when adding 10pF to the existing capacitance of board and connectors....



Fernando J. Barbosa wrote:
> Hello everyone,
>
>    The preamp cards have now been assembled and functionally tested. I
> will be shipping/delivering the preamp cards, power cables, signal
> cables and interposer boards, as appropriate, on Monday so that you
> will have them early next week. I will also email you some documentation.
>
>    Gerard, please send me your address for delivery by Fedex. I have
> Mitch's and Yves but please confirm.
>
>    I should mention that our techs, Armen and Mark, did a superb job
> in assembling and ordering all these parts in such short notice and
> with limited resources. Thanks so much!
>
> Regards,
> Fernando