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Re: Shaping time Setting



Hi Mitch and Gerard,

    I agree with Mitch that we need to determine if we really need the 
back-termination resistors with a real test. We also need to keep in 
mind that the power dissipation in the detector area (FDC) is already 
troublesome.

    For the moment, we don't have the 56 Ohm resistors on hand and it 
would be perhaps the end of the week before we could ship out the preamp 
boards. Because we really don't know if we are going to use the 
resistors or their values, I will remove the back-termination resistors 
that are installed, remove the resistor for the shaping time, as 
suggested by Mitch, and ship the preamp boards today. All the preamp 
boards will be configured in the same way.

    The preamp boards can be re-configured, if necessary, once we get 
all the tests done. Please, let me know if you have any objections.

Thanks,
Mitch





Mitch Newcomer wrote:
> Yes 18 Meters is a significant distance, but the track signals are far 
> from pristine. It sound like a good time to  make some  in situ  
> (track) signal observations.  They are sure to make Fe55 signals  look 
> like the Taj Mahal. of pulses.  My real motivation for responding 
> however, is to remind you all that  there is   1K to Vdd and 1K to Gnd 
> on chip so you are terminated with  500 ohms into  Vdd/2.   This allow 
> termination across the differential lines at the far end.   It  should 
> hold the common mode at the far end to a reasonable value without 
> requiring external resistors. No I didnt try it yet,  yes I would keep 
> the back termination option at least on the board. -- Mitch 
> Visser, Gerard wrote:
>> 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
>>>     
>
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