[Halld-tracking-hw] Tracking Meting Tomorrow

[Fernando J. Barbosa]

Hi Gerard,

I believe reflections would be less. Say you have a 10% impedance 
mis-match at the fADC125, i.e. a reflection coefficient of 0.05. The 
reflected signal travels back to the preamp through 18 m of cable with a 
total attenuation factor of 0.57. At the preamp, without the terminating 
resistors, the reflection coefficient is 0.82. This reflected signal 
travels back to the fADC125 through the cable with the same attenuation  
factor. So, I expect to see reflections of the order of 
(0.05)(0.57)(0.82)(0.57)=0.013 or ± 1.3%.

The observed amplitude value will be less than this because of 
dispersion in the cable, even though the rise time is reasonably long at 
the end of the cable (~15 ns). I also think that a 10% impedance 
mismatch at the fADC125 is excessive. So, I would expect reflections of 
the order of ± 1% or less.

I will send Naomi a CDC preamp w/o terminating resistors and I will send 
you two preamps (CDC and FDC) with the terminating resistors and you can 
remove the resistors (all or some channels) from the CDC version (the 
resistors are already on).

Best regards,
Fernando

Gerard Visser wrote:
> Hi Fernando,
>     This is certainly a good idea for testing. I would expect to see 
> reflections at the ~+/-5% amplitude level after a round trip cable 
> delay after any signal pulse... That should be completely ok for 
> cosmic ray tests, but I'm not sure it is what you would want to do for 
> real in a high rate situation. The double-termination does serve a 
> useful purpose!
>     Certainly we can try it and evaluate the magnitude of the 
> reflections and then decide if it is something that we can live with.
>     Perhaps a more 'robust' way to accomplish the same goal would be 
> to put a series resistors at the ASIC output on the FDC preamp card 
> (the boards are not yet in production, right?) and set the ADC125 gain 
> as may be decided for the CDC. Then on the FDC preamp only, the 
> resistors are used for attenuation of the signals. In this way the 
> double termination can be maintained, reflections will be (5%)**2 i.e. 
> about zero. The downside is of course that the ADC125 gain will be 
> higher so more sensitive to any external pickup on the cables. But 
> this is hopefully not going to be an issue at all.
>     Sincerely,
>
>         Gerard
>
> On 4/13/2011 7:47 PM, Curtis A. Meyer wrote:
>> Hi Fernando -
>>
>> please do send us one of these preamp cards -- thanks .. curtis
>> On 4/13/11 5:22 PM, Fernando J. Barbosa wrote:
>>> Hi Gerard,
>>>
>>> OK, I will send you two preamps (CDC and FDC).
>>>
>>> There is an easy way to double the pulse amplitude at the input of 
>>> the ADC
>>> which is to remove the back-terminating resistors on the preamp 
>>> card. This
>>> would not change the preamp response . This change would not require 
>>> any
>>> changes to the fADC125 and one version would still fit both 
>>> detectors as this
>>> is already the consensus at JLab. I can send a preamp to Naomi in this
>>> configuration for tests.
>>>
>>> Best regards,
>>> Fernando
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