Figure 1-48.--Frequency discriminator, simplified schematic.

Before the trigger pulse is applied, C13 has been

a decrease in generator field current; but, in the

charged to the collector voltage (Vcc) level through

frequency regulator, an increase infrequency causes the

resistor R25 with the other end clamped to ground

field current to increase.

through diode CR27 and the base-emitter junction of

The discriminator circuit is shown in figure 1-48. It

Q11. When C13 discharges due to the trigger pulse, it

essentially consists of a one-shot multivibrator that puts

turns Q11 off. The collector will rise to the collector

out a constant width pulse whenever a trigger pulse is

voltage level, and resistor R26 will apply base current

applied. The trigger circuitry is designed so a pulse is

to Q10 to hold it saturated after the trigger pulse ends.

applied six times each output cycle to obtain a high

This state (Q10 on, Q11 off) will exist until C13

enough sampling rate to decrease the response time of

charges through R27 to a voltage high enough to allow

the circuit. The multivibrator output is integrated to

Q11 to become forward biased again. At this time the

provide a dc voltage that is proportional and linear with

output pulse ends since Q11 saturates and the base drive

frequency.

of Q10 is removed. The time duration of the output pulse

is controlled by C13 and R27 with CR27 in the circuit

The positive collector voltage input furnishes the

to protect the base junction of Q11 from overvoltage.

circuitry operating biases and 6-phase ac is used to

obtain the trigger pulses. The trigger circuitry consists

The output pulse from the collector of Q11 is fed

through resistor R29 to integrating capacitor C14.

of three single-phase full-wave rectifiers (CR18 and

During the time no output pulse is present, C14 is

CR19, CR20 and CR21, and CR22 and CR21). Each is

discharged through R29 and the collector-emitter

driven from a winding of the T1 star secondary (fig.

junction of Q11.

1-44). The rectified voltages are clipped by the Zeners

(CR24, CR25, and CR26) to obtain a square pulse,

If the frequency of the generator increases, the ratio

which is further shaped by the differentiating circuitry

of charge time to discharge time increases, which in turn

increases the discriminator output voltage proportional

of C10, C11, C12, and R24. The differentiated pulses

to the frequency shift. A decrease in frequency does the

drive the trigger transistor Q9, which saturates when-

opposite. The output is applied to the error voltage

ever a positive pulse is applied.

detector circuit (base of Q1, fig. 1-44) or its equivalent

Transistors Q10 and Q11 form a one-shot multi-

in the frequency regulator.

vibrator with an output that is a 2-millisecond wide pulse

equal in amplitude to the collector voltage. Q11 is

MAINTENANCE

normally held on through R25, CR27, and R28. Thus,

Q10 is held off as its base drive comes from the collector

Under normal conditions, the motor-generator set

of Q11. Since Q9 saturates when a trigger pulse is

and control equipment require inspection and cleaning

applied, the collector voltage of Q10 is at ground

as designated by the PMS maintenance requirement

cards. When you inspect the motor generator, observe

potential whenever a pulse is applied.

Figure 1-48.--Frequency discriminator, simplified schematic.

1-44