Difference between revisions of "CDC readout requirements"

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(Created page with " Aim to establish how many bits are needed for each quantity, then later fit the quantities into the words. <h3>Time</h3> 11 bits, 0-2047, firm (minimum) I expect max drift t...")
 
 
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Aim to establish how many bits are needed for each quantity, then later fit the quantities into the words.
 
Aim to establish how many bits are needed for each quantity, then later fit the quantities into the words.
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First word has (header +) 15 bits available for data
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Second word has 31 bits for data
  
  
 
<h3>Time</h3>  11 bits, 0-2047, firm (minimum)
 
<h3>Time</h3>  11 bits, 0-2047, firm (minimum)
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I expect max drift time of 155 samples, to record time in tenths of samples, I need max value of 1550 which requires 11 bits, 0-2047.
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I expect max drift time of 155 samples. To record time in tenths of samples, I need max value of 1550 which requires 11 bits, 0-2047.
  
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I am using units of sample/10 because the upsampling works in units of sample/5 and it is straightforward to find the threshold crossing and then interpolate using units of sample/10. Multiplying this x 1.25 to give ns is possible but it makes the output number larger without adding precision, and it uses more clock cycles (ie the calculation takes longer), I think it is better to keep the firmware to the minimum and then convert to ns later on somewhere else. If the FDC needs better precision than 0.8ns then I could have it interpolate further but this takes yet more clock cycles.
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(Garfield calcs for 2.24T, 2.1kV, 47% Ar 53% CO2, 10mm radius straws give max drift time 1.19us = 150 x 8ns from max radius 9.5mm.
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Garfield calcs for 2.0T, 2.1kV, 47% Ar 53% CO2, 10mm radius straws give max drift time 1.20us = 150 x 8ns from max radius 9.6mm and 1.17us from 9.5mm.)
  
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<h3>Integral</h3> 16 bits, 0-65536
 
  
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Scale integral x 1/2 to fit into 16 bits.  This is ample, enough for 130 samples at 1000, scaled x 1/2.
 
  
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<h3>Pedestal</h3> 8 bits, 0-255 (OK to scale this down by factor of 4 to take up 6 bits instead).
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I am using units of sample/10 because the upsampling works in units of sample/5 and it is straightforward to find the threshold crossing and then interpolate using units of sample/10. Multiplying this x 1.25 to give ns is possible but it makes the output number larger without adding precision, and it uses more clock cycles (ie the calculation takes longer), I think it is better to keep the firmware to the minimum and then convert to ns later on somewhere else.  If the FDC needs better precision than 0.8ns then I could have it interpolate further but this takes yet more clock cycles.
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No scaling; output 255 for 255 and higher.
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<h3>Max amplitude</h3>
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Could be useful for dedx; also MIP gives a handle on gain shifts
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<h3>QF time</h3> 1 bit (firm)
 
<h3>QF time</h3> 1 bit (firm)
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Count up to 6 (and indicate 7 or more) overflow samples; this info might also be deduced from the integral if it maxes out.
 
Count up to 6 (and indicate 7 or more) overflow samples; this info might also be deduced from the integral if it maxes out.
  
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First word has (header +) 15 bits available for data
+
 
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Second word has 31 bits for data
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<h3>Integral</h3> 14 bits, 0-16383
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Scale integral x 1/16 to fit into 14 bits.  Need to cover min ~80 to max ~ 200,000
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Output max value if it is exceeded.
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<h3>Pedestal</h3> 8 bits, 0-255
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No scaling; output 255 for 255 and higher.
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Expect pedestal width < 20.  Set pedestal height at 4sigma=80 and expect it to vary between 1sigma=20 and 7sigma=140.
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<h3>Max amplitude</h3>
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Scale full range 0-4095 x 1/8 to fit into 9 bits
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MIP gives a handle on gain shifts; also amplitude might be useful for dedx.

Latest revision as of 14:19, 8 April 2014

Aim to establish how many bits are needed for each quantity, then later fit the quantities into the words.

First word has (header +) 15 bits available for data Second word has 31 bits for data


Time

11 bits, 0-2047, firm (minimum)

I expect max drift time of 155 samples. To record time in tenths of samples, I need max value of 1550 which requires 11 bits, 0-2047.

(Garfield calcs for 2.24T, 2.1kV, 47% Ar 53% CO2, 10mm radius straws give max drift time 1.19us = 150 x 8ns from max radius 9.5mm. Garfield calcs for 2.0T, 2.1kV, 47% Ar 53% CO2, 10mm radius straws give max drift time 1.20us = 150 x 8ns from max radius 9.6mm and 1.17us from 9.5mm.)


I am using units of sample/10 because the upsampling works in units of sample/5 and it is straightforward to find the threshold crossing and then interpolate using units of sample/10. Multiplying this x 1.25 to give ns is possible but it makes the output number larger without adding precision, and it uses more clock cycles (ie the calculation takes longer), I think it is better to keep the firmware to the minimum and then convert to ns later on somewhere else. If the FDC needs better precision than 0.8ns then I could have it interpolate further but this takes yet more clock cycles.

QF time

1 bit (firm)

1 bit to indicate that less accurate time is being returned

QF overflow

3 bits, 0-7

Count up to 6 (and indicate 7 or more) overflow samples; this info might also be deduced from the integral if it maxes out.


Integral

14 bits, 0-16383

Scale integral x 1/16 to fit into 14 bits. Need to cover min ~80 to max ~ 200,000 Output max value if it is exceeded.

Pedestal

8 bits, 0-255

No scaling; output 255 for 255 and higher. Expect pedestal width < 20. Set pedestal height at 4sigma=80 and expect it to vary between 1sigma=20 and 7sigma=140.

Max amplitude

Scale full range 0-4095 x 1/8 to fit into 9 bits MIP gives a handle on gain shifts; also amplitude might be useful for dedx.

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