Chapter 5. FUNCTIONING

TM 11-6625-2616-14

CHAPTER 5

FUNCTIONING

age protective circuit prevents damage to U5.

5-1. Introduction

This circuit is formed by diodes CR3, CR4, CR5,

The electrics of the AN/GSM-64A Digital Volt-

CR6, and transistors Q1 and Q2 with associated

meter may be divided into two basic sections: a

bias resistors. Voltage is limited at just over 12

signal conditioning section and a digitizing sec-

volts.

tion. The signal conditioning section processes

b. U5 is connected to form an extremely high

dc voltage inputs into a normalized dc analog

input impedance, low output impedance device

signal. The digitizing section accepts the normal-

which feeds the digitizing circuits on the main

ized dc analog signal from the signal condition-

circuit board.

ing section and converts it to a visual digital

display.

5-4. Digitizing Section-General

The digitizing section functions to accept the

5-2. Signal Conditioning Section-General

nomalized dc analog signal and process it so

a. The signal conditioning circuits are con-

that it may be displayed digitally with the pro-

tained on the signal conditioning circuit board

per decimal and polarity signs, The digitizing

assembly. These circuits provide-

circuits are located on the main circuit board.

(1) Accurate attenuation of large input

In this chapter, paragraphs 5-5 and 5-6 describe

voltages.

the overall operation of the digitizing section

(2) Filtering of the input signal.

and paragraphs 5-7 through 5-12 describe other

main elements of the digitizing circuitry.

(3) Over-voltage protection for solid-state

components.

5-5. Digitizing Section Principle of Operation

(4) A low source impedance to the digitiz-

The ac line frequency is used to trigger the basic

ing section.

timing for the digitizing circuits. A number of ac

b. Three pushbutton switches on this circuit

line pulses are processed, initiating the start of

board allow the instrument to operate as a dc volt-

a flip-flop. This flip-flop is used to gate on a 500

meter, a radiometer, or as an ac voltmeter when

KHz oscillator which drives a frequency count-

an ac converter has been installed.

er. At the 100,000 count the flip-flop is reset,

thus generating a 200-millisecond wide pulse.

c. The volts dc mode has three ranges: 10

This 200-millisecond period is used to allow an

volts, 100 volts, and 1,000 volts full scale. Any

integrating capacitor to charge from the signal

range may be selected by depressing the range

conditioning circuits. At the end of the 200-milli-

change pushbutton or the instrument may be

second period, the frequency counter is reset

operated in the auto-range mode. In the auto-

and the charged integrating capacitor is switch-

range mode, the range attenuator is set to the

ed to a constant current reference supply which

proper range by logic circuits which actuate the

allows a controlled discharge to occur. The fre-

range relays.

quency counter proceeds to count pulses from the

500 KHz oscillator until the capacitor has dis-

5-3. Signal Conditioning Section Circuit

charged to its precharged condition, at which

Description

time a null detector circuit ends the cycle. The

(fig. 5-1 and 9-3)

frequency counter displays the voltage reading

in terms of pulses counted during the discharge

a. Signal high is connected to the attenuator

input, pin 4. Range selection provides the pro-

period. When the 500 KHz oscillator stops, the

ac line pulses again trigger the flip-flop and the

per voltage level into pin 8 of the buffer-amplifier.

cycle is repeated.

Signal filtering is provided by C1 and C2. A volt-

5-1