Carrier 590A Manuale di Servizio Scaricare il pdf (Pagina 11)

1.3. POLE-ZERO CANCELLATION

Pole-zero cancellation is a method for eliminating

pulse undershoot after the first differentiating

network. In an amplifier not using pole-zero

cancellation (Fig. 1.1), the exponential tail on the

preamplifier output signal (usually 50 to 500

causes an undershoot whose peak amplitude is

roughly determined from:

undershoot amplitude

differentiated pulse amplitude

differentiation time

preamplifier pulse decay time

For a 1- s differentiation on time and a 50- s

preamplifier pulse decay time, the maximum

undershoot is 2% and this decays with a 50-

s time

constant. Under overload conditions, this

undershoot is often sufficiently large to saturate the

amplifier during a considerable portion of the

undershoot, causing excessive dead time. This

effect can be reduced by increasing the preamplifier

pulse decay time (which generally reduces the

counting rate capabilities of the preamplifier) or

compensating for the undershoot by providing pole-

zero cancellation.

Pole-zero cancellation is accomplished by the

network shown in Fig. 1.2. The pole [s + (1/T

)] due

to the preamplifier pulse decay time is canceled by

the zero of the network [s + (K/R

)]. In effect, the

path across the differentiation capacitor adds an

attenuated replica of the preamplifier pulse to just

cancel the negative undershoot of the

differentiating network.

Total preamplifier-amplifier pole-zero cancellation

requires that the preamplifier output pulse decay

time be a single exponential decay and matched to

the pole-zero-cancellation network. The variable

pole-zero-cancellation network allows accurate

cancellation for all preamplifiers having 30-

s or

greater decay times. Improper matching of the

pole-zero-cancellation network will degrade the

overload performance and cause excessive pileup

distortion at medium counting rates. Improper

matching causes either an under-compensation

(undershoot is not eliminated) or an over-

compensation (output after the main pulse does not

return to the baseline and decays to the baseline

with the preamplifier time constant). The pole-zero

adjust is accessible on the front panel and can

easily be adjusted by observing the baseline with an

oscilloscope with a monoenergetic source or pulser

having the same decay time as the preamplifier

under overload conditions. The adjustment should

be made so that the pulse returns to the baseline in

the minimum time with no undershoot.

1.4. ACTIVE FILTER

When only FET gate current and drain thermal

noise are considered, the best signal-to-noise ratio

occurs when the two noise contributions are equal

for a given input pulse shape. The Gaussian pulse

shape (Fig. 1.3) for this condition requires an

amplifier with a single RC differentiate and n equal

RC integrates where n approaches infinity.

The Laplace transform of this transfer function is

where the first factor is the single differentiate and

the second factor is the n integrates. The active

filter approximates this transfer function.

Figure 1.3 illustrates the results of pulse shaping in

an amplifier. Of the four pulse shapes shown the

cusp would produce minimum noise but is

impractical to achieve with normal electronic

circuitry and would be difficult to measure with an

ADC. The true Gaussian shape deteriorates the

signal-to-noise ratio by only about 12% from that of

the cusp and produces a signal that is easy to

measure, but requires many sections of integration

). With two sections of integration the

waveform identified as a Gaussian approximation

can be obtained, and this deteriorates the signal-to-

noise ratio by about 22%. The ORTEC active filter

network in the 590A provides a fourth waveform

(Fig. 1.3). This waveform has characteristics

superior to the n = 2 Gaussian approximation, yet

obtains them with two complex poles and a real

pole. By this method the output pulse shape has a

good signal-to-noise ratio, is easy to measure, and

yet requires only a practical amount of electronic

circuitry to achieve the desired results. The signal

to noise ratio is degraded by about 17%.

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Carrier 590A Manuale di Servizio Pagina 11

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