This is my outline of how a pulse height readout using the LeCroy
MQT300A charge to time converter IC would operate.
  Comments on whether these deadtimes, etc. are O.K. and explanations of
how the required gate would be generated would be appreciated.

John

John Schaapman                     Ph: 780-492-3043
Centre for Subatomic Research     Fax: 780-492-3408
University of Alberta            NOTE: new area code
Edmonton, AB
CANADA       T6G 2N5

		Pulse Height Readout



	A pulse height readout system is required to help decide

whether a muon has stopped in the target.

	The proposed 128 channel system would use a splitter on the

chamber service board to provide ten percent of the chamber signal

to a second VTX preamp board. The following 16 channel 

postamp/charge to time converter CAMAC module consists of an

inverting opamp to provide an amplified negative input signal to a

LeCroy MQT300A charge to time converter IC. The output of this 

module goes to a LeCroy 1877S TDC which is used in ' Common Start '

mode. A gating signal must be provided for the MQT300A as well as 

a ' Time ' signal for the TDC Common Start and Timeout. A fast 

clear signal may be provided after the gate to reduce the deadtime 

to about one microsecond or after the TDC conversion starts to 

reset the MQT 300A quickly for the next reading. 



	Estimate of maximum output signal from 'pulse height  VTX '

generated by muons near the target at chamber voltages of 1.8 kV 

and 2.0 kV.

TN-34 states that the VTX gain is 1 mv per fC, the chamber gain is

 10^4 at 1.8 kV and 3.5*10^4 at 2 kV and also that a muon near the

 target yields a maximum amplitude of 530 initial electons.

 @ 1.8 kV     530e * 10^4 / 6*10^3 e per fC = 883 fC

           This is reduced to 88.3 fC by the 10 % chamber splitter.

    	          88.3fC * 1 mv per fC = 88.3 mv

 @ 2 kV       530e * 3.5*10^4 / 6*10^3 e per fC = 2,930 fC

           This is reduced to 293 fC by the 10 % chamber splitter.

                 293 fC * 1 mv per fC = 293 mv

 - For comparison, a positron would yield 2.7 mv @ 1.8 kV 

                                      and 8.9 mv @ 2 kV.



Characteristics of LeCroy MQT300A Charge to Time Converter 

                and total deadtime estimates



Maximum full scale negative charge input is 2,620 pC. This charge 

is captured simultaneously in three ranges during the gate.

At the maximum sensitivity ramp current, ( Iramp = 320 microamp ) 

the conversion factors for the ranges look like this:

 HIGH 2620   pC full scale   0.78 ns per pC  or  1,282 fC per ns

                                      yielding  2.04 microsec max

 MID   326.5 pC full scale   6.25 ns per pC  or    160 fC per ns 

                                      yielding  2.04 microsec max

 LOW    40.8 pC full scale  50    ns per pC  or     20 fC per ns 

                                      yielding  2.04 microsec max

  To this must be added the residual pedestals for each range:

         HIGH 500 ns, MID 650 ns, LOW 800 ns 

      The TDC timeout to start TDC conversion must occur after

this total of approx. 2.84 microsec. 

    The ramp current can be increased by up to a factor of 4.375 

( 1,400 microamp ) to reduce the conversion and pedestal time at

the  expense of time resolution.  [ signal range of 466 ns plus 

pedestal of 183 ns = 649 ns ] The fast clear of the MQT300A would

also be  reduced from 900 ns to about 440 ns.

       The TDC conversion time is 1.75 microsec minimum and fast

clear settling time is 250 nsec.



Deadtime - LOW range

 Iramp = 320 microamp   

       normal operation  2.04  +  0.8  +  1.75 = 4.59 microsec.

       Fast clear after gate                   = 0.90 microsec.

 Iramp = 1,400 microamp  

       normal operation  0. 466 + 0.183 + 1.75 = 2.4  microsec.

       Fast clear after gate                   = 0.44 microsec. 



Opamp stage - estimate of Qin to MQT300A using previously tested

inverting opamp circuit. 

    The Burr Brown OPA689 opamp circuit set for an inverting gain

of six should work quite well with these fast pulses. A rough

estimate of the  input charge provided to the MQT300A with  input

resistance set to 200 ohms can be made by applying the maximum VTX

output values over the pulse width of 20 nsec.

         Qin = Av * VTXmax * dt / Rin

@ 1.8 kV      Qin = 6 *  88 mv * 20 ns / 200 ohm =  52 pC

@ 2.0 kV      Qin = 6 * 300 mv * 20 ns / 200 ohm = 150 pC 

	This puts the input signals into the MID range of the

MQT300A indicating that this is a reasonable setup.



TDC counts and actual deadtimes at maximum signal.

  Counts = Qin * MID range conversion factor / TDC time per count 

  Actual deadtime = Qin / Qin max mid * conversion time max 

                            + pedestal MID + TDC conversion time

                                                               

@ 1.8 kV and Iramp =   320 microamp 

   counts = 52 pC * 6.25 ns per pC / 0.5 ns per count = 650 counts

   actual deadtime =    2.7 microsec

             Iramp = 1,400 microamp            

   counts = 52 pC * 1.43 ns per pC / 0.5 ns per count = 148 counts

   actual deadtime   =  2.0 microsec

@ 2.0 kV and Iramp =   320 microamp

  counts = 150 pC * 6.25 ns per pC /0.5 ns per count = 1,875 counts

  actual deadtime =     3.3 microsec     

             Iramp = 1.400 microamp

  counts = 150 pC * 1.43 ns per pC / 0.5 ns per count = 429 counts

  actual deadtime =     2.1 microsec 

	

	The tradeoff between TDC output resolution and deadtime can

be made, if necessary, after the amount of crosstalk in the  pulse 

height channel and the actual operating voltage is known. 

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