Anode Preamp-Shaper and Discriminator

The primary function of the anode electronics is to determine the timing of the track hit with an accuracy of better than 1 bunch crossing (25ns). Therefore the anode preamp-shaper requires fast shaping. Good timing resolution can be obtained by sensing the first few electrons from the initial ionization in the CSC. However, shorter shaping time results in higher intrinsic electronics noise. For shaping time of 30 ns and for nominal chamber gas gain, the CSC signal arise from the avalanche produced by a single electron is comparable to the equivalent input noise of the amplifier. To stay safely above this noise level, discriminator threshold needs to be set at 7 to 10 initial electrons, which can result in large time jitter. These considerations lead us to adopt the scheme of a two-threshold discriminator which is triggered above the noise level while at the same time achieve precision timing.

In this scheme, a high-threshold discriminator is driven by the initial signal from the amplifier. The threshold level is adjustable from 10% to 70% of a nominal MIP signal. The resulting pulse serves as the enable for the precision-time discriminator. The precision time discriminator consists of a constant-fraction shaper and a low-level discriminator. The constant-fraction shaping is done by adding the differentiated amplifier signal and the corresponding delayed and inverted signal. The resulting pulse is further amplified and delivered to the input of a low level zero-crossing discriminator. The zero crossing point corresponds to approximately half the rise time of the input signal. The threshold of the low-level discriminator is used for adjusting the start time of the output pulse. The design specifications for the anode preamp-shaper-discriminator circuits are listed below.

Equivalent Input Noise (rms):     0.5 fC at 0 pF;  (1.7 fC at 200 pF)
Shaper Peaking Time:              30 ns
Shaped Waveform:                  Semi-gaussian with two exponent tail cancellation
Nominal input charge (i)          142 fC (8.8 x 10**5 e's)
Transfer Function (gain):         5 mv/fC (differential output)
Non-Linearity:                    <10% from 0-1.5 volt
Two-threshold Discriminator:      High threshold disc used as ENABLE;
                                  Low threshold zero crossing disc driven by const-frac 
                                  shaped pulse.
High-level Threshold:             Adjustable from 20 - 500 mv
Discriminator Slewing Time:       2 ns

Footnote:

i) Corresponding to the average ionization deposition (Landau
   peak)  for a normal incident minimum ionization particle. For a gas
   gain of 10**5, this input charge is equal to
   (N_ion)(f_att)(f_aval)(f_sh) x 10**5, where f_aval is the
   fraction of the avalanche charge collected by the anode and all other
   factors are defined the same way as in footnotes i and ii of Table 1.
   Simulation (CMS TN/95-194) gives N_ion = 180 (for 10 mm gas gap); F_att = 0.5;
   f_aval = 0.82; f_sh (30ns shaper peaking time) = 0.12.