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Re: Cathode Films from Sheldahl
Hi,
Just a further remark on this, I was motivated to put together a quick
SPICE simulation. It confirms that the total resistance of the longest
cathode strip should optimally be kept less than about 10-20% of the
characteristic impedance of the cathode strip as a transmission line.
That value should be in the range 50 - 100 Ohms, surely. So for instance
the 0.5 um of copper (sheet resistivity 34 mOhm/square, total 8.3 Ohms)
should be ok. But that's about the limit.
To analyze this more carefully, the following steps ought to be taken.
1. Line impedance (& velocity, right now I assume it has no dielectric)
should be determined from E&M calculation (probably numerical
calculation). I don't have the relevant software to do this. Maybe Fernando?
2. The preamp front end, and the interconnect capacitance, and the
protection diode network, should all be put into the SPICE model. I can
do these things, but of course it should be a lower priority for me than
the ADC board, I think.
3. Actual chamber induced current pulse shape needs to be put in the
SPICE model - this is trivial, I'll do it sometime soon.
4. Tail cancellation and shaping need to be put in (a simple model will
suffice) so that we can see how much practical pulse amplitude is lost.
If these four steps are done, we can see the effect of cathode strip
resistance on the pulse amplitude.
5. Actually someone should also look at crosstalk simulations, this
requires a much more complicated "coupled lines" E&M calculation... And
SPICE (or other) transient simulation. I doubt we'll actually do all
that. Perhaps at least some back-of-envelope estimates.
Let's follow up on this further next week when the next hurdle is passed.
- Gerard
Elke-Caroline Aschenauer wrote:
> On Tue, 25 Mar 2008, Gerard Visser wrote:
>
> Dear Gerard,
>
> funny our mails must have crossed in cyber space, but I agree with you
> 1000% this needs a bit more time and thoughts before we can jump on some
> new option.
>
> cheers elke
>
>
>> Date: Tue, 25 Mar 2008 12:04:02 -0400
>> From: Gerard Visser <gvisser@indiana.edu>
>> To: Simon Taylor <staylor@jlab.org>
>> Cc: Fernando J. Barbosa <barbosa@jlab.org>,
>> Daniel S. Carman <carman@jlab.org>, Roger Flood <flood@jlab.org>,
>> Elke-Caroline Aschenauer <elke@jlab.org>,
>> "halld-tracking-hw@jlab.org" <halld-tracking-hw@jlab.org>
>> Subject: Re: Cathode Films from Sheldahl
>>
>> Hi Simon,
>> I think you may be overlooking something... -->
>>
>> resistivity of gold
>> 2.2E-8 Ohm*m
>>
>> surface resistivity @ 900 Angstroms
>> 2.2E-8 Ohm*m / 9E-8 m = 0.244 Ohm (per square)
>>
>> length of longest strip
>> about 242 squares
>>
>> characteristic impedance
>> who knows - not simulated - but surely on the order of 100 Ohms at the
>> most
>>
>> signal loss (at the most optimistic) 1-exp(-242*0.244/100)=45%
>>
>> not so good, I think.
>> and this simple analysis did not mention the noise contribution of this
>> resistance. probably nonzero.
>>
>> I think this all needs to be considered more carefully. A real
>> calculation of the cathode line impedance and attenuation and the noise
>> contributed by it, should be done.
>>
>> - Gerard
>>
>> p.s. Yes I do agree with the concern about handling (and connecting to)
>> any super-thin layers whether copper or gold.
>>
>>
>> Simon Taylor wrote:
>>> Hi, Fernando,
>>>
>>> As far as thickness goes, 900 Angstroms of Gold should be okay:
>>> For each gold layer I get 900 x 10^-10 m x 19.3 g/cm^3 / (6.4 g/cm^2) =
>>> 2.7 x 10^-5 X0,
>>> compared to 1.3 x 10^-4 X0 for 2 microns of Copper. My concern for such
>>> a thin layer of conductor is handling: I can well imagine it could be
>>> quite easy to cause breaks in the strips due to scratches... Also, we
>>> would probably have to do some selective plating near where the strips
>>> need to be connected to the outside world.
>>>
>>> Simon
>>>
>>>
>>> Fernando J. Barbosa wrote:
>>>> Sheldahl has a series of gold deposited materials that can be used for
>>>> the cathodes. These materials are normally specified as thermal
>>>> barriers in satellites but can be used for the FDC cathodes. Part #
>>>> 146437 (see below) is a standard product with 900 angstroms of Au on
>>>> 25 micrometer Polyimide and can be delivered in 48 inches wide rolls.
>>>> The left column below is for standard products (available) and they
>>>> range from 8 to 127 micrometer Polyimide.
>
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