Advanced Measurement Technology 142AH Kullanım ve Servis Kılavuzu - Sayfa 12

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Fig. 4.1. Simplified Block Diagram of the ORTEC 142AH Preamplifier.

The corona blower that is furnished with the

142AH can be used to flush the interior connector

surfaces with air when corona effects are

observed. It can be used on both the Input

connector on the preamplifier and on the

connectors of the cables that are to be attached.

It can also be used to clean the connectors at the

detector end of the input cable. Simply insert the

glass tube into the connector and squeeze the

bulb to blow air into the connector.

With the protection circuit in, the input cable can

be removed and reconnected without catastrophic

damage to the preamplifier, but the user must be

cautious to prevent touching the interior of the

connector with anything other than a good

insulator, such as the glass tube of the blower;

lethal high voltages are present on the center pin

of the Input connector under these conditions.

4.3. ENERGY OUTPUT

The charge-sensitive loop is essentially an

operational amplifier with a 1-pF capacitive

feedback. The conversion gain is nominally 45

mV/MeV, and can be increased by decreasing the

value of the feedback capacity but a subsequent

increase in rise time will result. The upper limit

on the conversion gain is the stray capacity in the

circuit with C4, the 1-pF capacitor, is removed

completely. The stray capacity is about 0.1 to 0.2

pF. If less conversion gain is desired, the value of

the feedback capacity can be increased, but this

may affect the stability of the preamplifier. The

maximum recommended additional capacity is

1.5 pF.

The energy output signal from the preamplifier is

a fast-rise-time voltage step with an exponential

return to the baseline is about 500 µs. The

polarity of the E output signals is inverted from

the signal polarity at the detector output. When

the (normal) positive bias polarity is used for the

detector, the detector output pulses are negative

and the E output of the preamplifier is positive, as

shown in Fig. 4.2.

4.4. TIMING OUTPUT

As indicated in Fig. 4.1, the T output from the

preamplifier is a transformer differentiated and

inverted version of the E output. This

differentiation removes low-frequency noise for

better timing results. Due to the differentiation of

the charge loop output, any overshoot present will

appear to increase the rise time of the timing

output as shown in Fig. 4.3. This, however, does

not affect typical timing experiment results since

it is the initial slope of the waveform that carries

the information of importance in timing. Similarly,