ORTEC 420 - Circuitry Overview

5-1
5. CIRCUIT DESCRIPTION (See Drawings 420-0001-SI and 420-0002A-B1.)
Resistors R1 and R5 form a resistive voltage divider v/hich divides the input signal from
a maximum of 10 volts to a maximum of 5 volts at the base of Q1 and Q9. Q1 and Q2
form a long-tail pair which is the lower level discriminator. Transistors Q3 through Q8
are necessary to obtain the control desired on the hysteresis of the circuit. Q3 and Q5
are normally on in the absence of an input signal and hold the base of Q2 at zero volts.
When on input signal exceeds the threshold level set by the "E" discriminator, R2, the
trigger pair, Q1 and Q2, regenerates and triggers another trigger pair formed of Q7 and
Q8. Q7 turns on and Q8 turns off. When Q8 turns off, this allows the emitter of Q4 to
fall toward —12 volts and it is caught at —E. When the emitter of Q4 and base of Q6 fall
negative, Q6 turns on and Q5 turns off, which sets the base of Q2 at —E. Since the base
of Q1 has a baseline voltage equal to —E and the base of Q2 has a baseline voltage of
-E in the presence of the signal, the trigger pair will reset when the signal makes the
transition from the positive condition to the negative condition, i.e., zero crossing point.
The signal at the collector of Q7 is differentiated by means of L3 and is fed to the base
of Q15. The negative portion of the signal will cause Q15 to conduct if the upper level
discriminator has not been triggered. A signal from the collector of Q6 is used to reset
the upper discriminator when it has been triggered, and this signal is fed to the base of
Q11. R14, the zero adjust trimpot, is used to zero the "E" pulse height dial. The adjust
ment of the walk adjust potentiometer, R6, will be explained in Section 6.1. Switch SI is
used to obtain operation in either bipolar or unipolar mode. In the bipolar position, tran
sistor Q4 has the value of —E imposed on its base at all times; however, in the unipolar
position this base sees a constant —75 mV dc voltage. This dc voltage is the amount of
hysteresis on the lower level discriminator in the unipolar mode.
The upper level discriminator is formed by Q9 and QIO. The output from QIO is
differentiated and fed to the base of Q12. When switch S2 is in the differential position,
this signal is sufficiently large to trigger the multivibrator composed of Q12 and Q13.
If, however, 52 is in the integral mode, this pulse cannot trigger the multivibrator com
posed of Q12 and Q13. Once this multivibrator is triggered, it could recover with an RC
recovery time of approximately 100 microseconds; however, it is reset at essentially zero
crossing time by means of the reset pulse which is present by way of Q11. Due to the
long time constant involved with this multivibrator, this circuit con handle pulse widths
which are very wide, e.g., 20 microseconds.
An output from Q13 will cause Q14 to inhibit any signal that appears at the base
of Q15; thus, the anticoincidence function is performed between the "AE" discriminator
and the "E" discriminator. This anticoincidence function assures that the input pulsie must
be within the selected window to obtain an output. Q16, Q17, Q18, and Q19 form a
delay generator. The delay control, R46, will vary the delay of the output logic signal