7. 1 GENERAL DESCRIPTION

The electronics for the Campbell subsystem[8] consist basically of five components: remote amplifier, local amplifier, inverter, mean square analog, and d-c amplifier.

7. 1.1 Remote Amplifier

The remote amplifier is constructed in such a manner that it can be located as near the reactor as possible while staying outside the biological shielding, the purpose of which is to minimize the distance between the remote amplifier and the detector. It receives its power from the main chassis, and acts as a junction between the detector and the detector polarizing supply located in the main chassis.

The remote amplifier is, in reality, two separate amplifiers housed in the same box, which differ in gain, input impedance, and frequency response. One is referred to as the low-range amplifier and the other as the high-range amplifier. The need for the two different ranges of amplifiers is dictated by the desire to monitor six decades of flux with one chamber.

a. The low-range amplifier consists of a high-gain common emitter stage and two com­plementary doublet stages. The over-all gain is a nominal 1200 before application of frequency breaks.

At the lower flux levels covered by the MMSVM, it is necessary to terminate the signal input cable in the high impedance in order to obtain sufficient usable signal, and to provide adequate pulse overlap so that the fractional standard deviation will fall within acceptable limits while maintaining a minimum averaging time constant. For these reasons, an input impedance of 5, 000 ohms was chosen for the low-range amplifier. Although not an extremely high input impedance, it is a factor of 27 greater than the 185-ohm characteristic impedance of the RQ-114/U cable used to couple the in-core assembly to the amplifier, and it is sufficiently high that the cable appears to be nearly open circuited.

When operating a cable in an unterminated mode, the cable capacity acts to degrade the signal. The cable capacity must therefore be kept to a minimum. It was for this reason that RG-114/U was chosen for the out-of-core signal cable since its capacity is only 6. 5 picofarads per foot.

The frequency breaks on the low-range amplifier were chosen to make best use of the signals from the unterminated cable and still maintain the response necessary for reactor control. The low-frequency break is 8 kc and the high frequency break is at 60 kc. With a detector and cable assembly of 2000 picofarads capacity connected to the input, the high frequency break is lowered to 16 kc.

b. The high-range amplifier consists of a common base input stage and two complementary doublet stages. The gain is adjustable from about 115 to 510 before application of frequency breaks. The adjustable gain makes it possible to align the two scales involved when switching from the low — to high-range amplifier.

At high flux levels, faster system response is necessary. For this reason, the band­width of the high-range amplifier is designed to be from 300 kc to 600 kc, and the averaging time constant is reduced by a factor of 10. It is also necessary to shorten the pulses into the amplifier to a length that will be compatible with the higher fre­quency response. To accomplish this, the input impedance of the high-range amplifier is matched to the characteristic impedance of the signal cable. This reduces the pulse width to a fraction of a microsecond, but pulse pile-up is adequate since the pulse count rate from the detector is high.