Advanced Measurement Technology ORTEC A-576 A-PAD, Operating And Service Manual

6

5.  THEORY OF OPERATION

The complete schematic for the A-576 A-PAD (No.

amplitude for 5, 6, and 7 MeV to the input to the

752500)  is included  at the back  of this manual.   

preamplifier. A screwdriver calibration, included on the

Fig. 5 is a block diagram of the electronics.

PWB for factory adjustment of the test pulser circuit,

The detector voltage (TEST/BIAS/OFF) switch on the

front-panel toggle switch is set at BIAS, the test pulser

front panel turns power ON and OFF for the variable

is turned off and does not appear in the output

100-V bias power supply. The power is on to the BIAS

spectrum.

supply in both the TEST and BIAS positions.

The polarity is set by Jumper J1, which is actually

inverted by the preamplifier and are furnished as the

three separate jumpers (see schematic). The indicated

input to the amplifier that includes the first integration

polarity is negative, which is appropriate when an

and differentiation shaping circuits. The amplifier can

ORTEC Ruggedized detector is used. When the

be calibrated by a factory-adjusted trim potentiometer

detector voltage is negative, its output pulses are

on the PWB. The output is a shaped negative pulse

positive. When they pass through the inverting charge-

that can be observed at test point TP3. 

sensitive preamplifier, they are negative, and this is
the polarity that is then selected by the pair of jumpers

The pulse at TP3 is furnished into a fixed-level

(J3) on the PWB. If a conventional surface barrier, or

discriminator (U3), and through a diode dc-restorer

ion-implanted, detector is used, all five of these

and integration network. If the amplitude represents

jumpers must be changed to "+" because these

~2.5 MeV or more, the discriminator fires and

detectors, positively biased, generate negative output

generates a positive-shaped pulse that is used to

pulses, which are then inverted, resulting in positive

enable the stretcher. This pulse is also provided as the

pulses at the J3 location in the circuit.

COUNT output signal. The pulse from the second

The 100-V power supply output on the PWB can be

If the stretcher is enabled, the peak amplitude is

tested for both polarity and amplitude at test point

stretched to improve the measurement accuracy of the

TP1, which is identified on the PWB. The variable 0 to

multichannel analyzer, which uses the ENERGY

100 V bias voltage can be measured at the front-panel

output for measurement. If the stretcher is not enabled

test point jack labeled HV. 

(because the pulse represents <2.5 MeV), the output

The detector leakage current can be measured across

stretched. The output of the stretcher is a negative

the 1.1-M

6

 resistor between front-panel test points HV

pulse that can be monitored at test point TP4 on 

and DET, using a 10-M

6

 impedance meter. The

the PWB.

voltage (in volts) converts to µA of detector current.
The output of the Bias supply is applied  through a

The biased amplifier accepts the stretcher output and

high impedance (totaling 11 M

6

) to the dc connection

the bias level selected by the front-panel ENERGY

between the detector and the preamplifier.

RANGE switch (low end of range). The bias level, or

The output from the charge-sensitive preamplifier can

at 3, 4, 5, or 6 MeV by the ENERGY RANGE switch,

be checked at TP2.  The nominal conversion gain is

which can be adjusted by ±10% by the front-panel

45 mV/MeV. The preamplifier gain from the External

screwdriver control, E BIAS. 

Pulser input is nominally "

1" at TP2. 

The pulser switch (TEST/BIAS/OFF) on the front

bias level from the input pulse amplitude and to

panel turns power on and off for the test pulser. The

amplify the excess amplitude by a factor of 10 on the

test pulser can be set to furnish the equivalent pulse

3

&

5, 4

&

6, 5

&

7, and 6

&

8 MeV ranges, and by a factor 

should not be changed by the customer. When the

Pulses from the detector and/or the test pulser are

integration shaping circuit is furnished to the stretcher.

is passed to the biased amplifier without being

lower level, accepted into the bias amplifier can be set

The function of the biased amplifier is to subtract the