Chopper

TM 11-6625-646-15

transformer T601. Resistors R601 and R602 balance

Depending on the direction of the differential current,

the circuit to ground, simulating the feedback variable

the voltage on the base of transistor Q601 will increase

resistors in the extensible link. The output of the

or decrease, changing the collector current of the

secondary winding of the transformer is connected

transistor accordingly. The voltage at the collector will

through limiting resistors R620 and R621 to a network

change by an amount equal to the input but in the

consisting of diodes CR601, CR602, CR603 and CR604.

opposite direction.

b. During the half the reference voltage cycle when

d. The dc signal output of the phase inverter at the

diodes CR601 through CR604 are conductive, the

junction of resistors R613 and R615 is connected

junction of diodes CR603 and CR604 is at the same

through an external jumper between pins 6 and 7 (which

potential as the junction of diodes CR601 and CR602-

shunts resistor R614) of the card connector to a lag

signal ground potential. During the half-cycle when the

network consisting of resistor R617 and capacitor C601.

diodes are nonconductive, the junction of diodes CR603

This lag network reduces the rate of change in signal

and CR604 is at signal voltage level. Thus the output of

level, simulating the lag in actual link operation. A dual

the phase inverter at the junction of isolating resistor

limiter consisting of diodes CR605 and CR606 limits the

R618 and R624 is interrupted at the power frequency

excursion of the dc signal to approximately 0.5 volt.

rate and converted to a square wave. The phasing of

The dual limiter protects against a high voltage

the interruption and the polarity of the square wave are

condition which might result from a fault in the phase

related to the characteristic of the correction signal in

inverter or a fault in the SAS amplifier. Resistors R618

the SAS amplifier channel.

and R624 isolate the chopper from the phase inverter

and the preamplifier.

1-23. Preamplifier and Filter

e. Twenty-six volts ac from the power supply of the

(fig. 1-2)

test box is applied through pin 19 of the car(d connector

In the preamplifier and filter stages, the chopped output

and is limited, rectified, filtered, and regulated to -20 vdc

of the phase inverter is converted to a waveform

by the network consisting of resistor R625, rectifier

approximating a sine wave of equal amplitude. The

diode CR608, capacitor C602, resistor R619 and zener

two-stage preamplifier has a gain of approximately 1000

diode CR607. Operating voltage is supplied to the

which compensates for the attenuation of approximately

collector of transistor Q601 through limiting resistor

1000 in the filter (integrating network). The threshold

R616. Forward bias is applied to the base through fixed

level of input and output signals is 1 millivolt.

resistor R610 and balance resistor R609.

a. B+ (27 volts dc) from the dc distribution circuits

1-22. Chopper

of the test set is connected to the preamplifier through

pin 16 of the card connector and the decoupling network

(fig. 1-2)

consisting of resistor R653 and capacitor C617 and

The chopper converts the dc output of the phase

resistor R652 and capacitor C621. Operating voltage

inverter to a positive or negative square wave,

(approximately 23 volts dc) is connected to the emitter

depending upon the polarity of the output. Feedback

of transistor Q602 through resistor R632; base bias is

resistor excitation voltage from the SAS amplifier

furnished by resistor R633.

channel is used as a reference voltage to phase-relate

the operation of the chopper to the operation of the

b. The square wave input signal is connected

channel.

through isolation resistor R624 and dc-blocking

a. The 2.5-volt ac excitation voltage from the SAS

capacitor C605 to the base of PNP transistor Q602. The

amplified square wave is developed across collector

amplifier is applied through pins 8 and 9 of the card

load resistor R623 and coupled to the second stage

connector to the primary winding of reference voltage

through dc-blocking