[Halld-tracking-hw] FDC meeting minutes posted

[Lubomir Pentchev]

Hi Gerard,

Thanks for your response. For the results at page 514 and 513 I used 
simple linear interpolation to find the threshold crossing. I convoluted 
the detector signals with:

560*(4.07*t/tpeak)**12*exp(-t*12/tpeak), tpeak is the peaking time.

The coefficient in front gives me the correct gain of 0.6mV/fC. I 
simulated the noise by varying the threshold with a sigma of 20%. As for 
the discriminator it was "perfect" in my simulations. I did 
these studies to compare the FDC resolution for different gases and also 
to estimate the deterioration of the resolution if we decide to use 
fADC125 instead of F1TDC for the wires. Therefore, the conclusions I made 
are valid only for the timing resolution. Yes, I agree 11 or 14ns 
peaking time might be too small given the 8ns sampling, but you don't have 
to reconstruct the whole signal, just the threshold crossing. I have not 
looked yet at the resolution of the charge reconstruction.

Let's discuss this in the next FDC meeting. I'll repeat the simulations 
with 25ns peaking time. Also I can present studies of the timing 
resolution with different interpolations using the real cosmic data when 
the fADC125 was connected the FDC prototype.

Best regards,
   Lubomir

On Thu, 16 Sep 2010, Gerard Visser wrote:

> 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
>