[Halld-tracking-hw] FDC meeting minutes posted

[Gerard Visser]

Hi Lubomir,
	We should perhaps discuss the shaping option ideas more fully in the next 
meeting, I can call in. I didn't realize this was on the agenda today.
	In my opinion it is *feasible* to support two different versions of the module 
with different shaping time. This amounts only to different values for some 
capacitors, inductors, and resistors to be used in assembly. The quantities are 
large enough that there should be no significant cost impact, except for 
probably a larger overall quantity of spares to be built. Of course, I agree it 
is simpler to have only one version.
	Presently the peaking time of the preamp-cable-ADC125 is about 35 ns I believe. 
(Maybe a bit more in the case of the cathodes if the detector capacitance 
affects it; it would make sense but I don't know really.) We might try to reduce 
it but 14 ns seems to me too small - there will be too much amplitude above the 
Nyquist zone, this has to degrade timing at some point. Maybe some compromise 
value like 25 ns would be better to try.
	Anyway the starting point for this should probably be to remove all explicit 
shaping from a channel, hook it up w/ preamp and cable and a test pulser and 
input loading capacitor, and see the pulse shape. This will exhibit the minimum 
achievable peaking time; and we can also then we can calculate the shaping time 
for the ADC board to get to the desired overall peaking time.
	Can you describe the algorithm applied to ADC data to get the timing 
measurement for page 514 work? Is there a fit here, or just level-crossing using 
the same threshold e.g. 30mV? Certainly an optimal timing algorithm will use 
more than 2 datapoints from the ADC, i.e., is not simply just a level crossing 
and linear interpolation between two points.
	Is the discriminator simulation 'perfect' or does it include real-world 
distortions such as time walk (dispersion)?
	Sincerely,

		Gerard


Lubomir Pentchev wrote:
> The minutes of the last FDC meeting were posted at:
> 
> http://www.jlab.org/Hall-D/software/wiki/index.php/Minutes-9-16-2010
> 
> Regards,
>      Lubomir