12/ 11 /1998
Capacitance of PMT and voltage divider
Abstract: The measured capacitance of PMT and voltage divider is 31PF using resonating method. Influence of the capacitance upon the output shape of 3_in_1 card was also investigated.
The schematic set up for measuring the capacitor of PMT and voltage divider is shown below.
The frequency response curve of V2 Vs V1 was measured , using Hspice we can t find the exact inductance of L in test setup , and the total capacitor of PMT and voltage divider C0 . The measured parameter of L was consistent with the value measured in earlier work. The value of in question capacitor C0 = 31PF.
In order to estimate the influence of the capacitance of the PMT and voltage divider upon the shaper output, firstly we demonstrate the output waveform of initially balanced shaper of 3_in_1 card No.115 in figure 1.
Figure 2 is the output wave of shaper while the card was connected with PMT block, due to the additional capacitor of PMT block, the shape has a longer tail and a reduced amplitude 910mv compared with the initial 970mv. Figure 3 shows the shape of output of 3_in_1 card after 31PF capacitance was reduced from shaper, from where we can see that when the capacitance of first stage of shaper was reduced 31PF, the amplitude of the shape increased 8% to 1050mv, and the overshoot increased to about 1%. Figure 4 is the waveform of same card with reduced capacitor after connecting to PMT block. Where the shape and amplitude of output is same as figure 1. It means that if we deduct the capacitance of PMT block from the first capacitor of initial shaper and assemble PMT block and 3_in_1 card , the total capacitor of the shaper will remain the same and the shaper will return to initially normal operating condition.
Figure 1. The initial performance of shaper.
Figure 2. The output of 3_in_1 card after connecting to PMT block
Figure 3. The performance of shaper after reduction of 31PF
Figure 4. The shape of output of same card as Figure 3 connected with PMT block
The capacitance of PMT block, including PMT and voltage divider, is about 31PF. Thus the capacitance of first stage in the shaper should be reduced 31PF from the nominal value. Then the parameters of the shaper should be the values as shown in figure 5 below. The values of C3 and Rl are subject to change if the amplifiers CL501and CL502 change to other types.
Figure 5. schematic layout of 7_pole shaper