DC 502

Tektronix DC 502 User Manual

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Theory

of

Operation

—

DC

502

TIME

BASE

AND

CONTROL

CIRCUIT

1

MHz

Clock

A

precise

one

megahertz

clock

provides

the

reference

for

operation

of

the

gate-generating

circuits.

The

output

of

crystal

oscillator

Y200

is

adjustable

by

C201

to

exactly

one

megahertz.

The

four

parts

of

U200

form

a

shaper-buffer

stage

to

produce

square-wave

clock

pulses

and

to

isolate

the

oscillator

from

the

1-MHz

output

line.

Optional

Clock

An

optional

1

MHz

clock

is

available,

using

a

very

stable

5

MHz

crystal

oscillator

and

a

divide-by-five

counter.

This

combination

is

shown

on

the

schematic

as

Y201

and

U201.

Decade

Divider

Units

(DDU's)

The

DDU's

consist

of

seven

cascaded

divided-by-ten

counters,

U209

through

U215.

They

produce

four

gate

times,

0.01

sec,

0.1

sec,

1

sec

and

10

sec,

which

are

made

available

via

the

MEASUREMENT

INTERVAL

switch

to

the

gate

generator

to

establish

the

precise

time

interval

the

GATE

is

open.

The

1-MHz

clock

signal

is

applied

to

pin

14

of

U209,

whose

output

is

connected

to

the

input

of

the

subsequent

decade.

Each

decade

is

clocked

with

a

negative



going

transition.

The

DDU's

are

reset

by

a

CLEAR

pulse,

which

places

a

0

count

in

U209

and

a

9

count

in

each

subsequent

decade.

Gate

Generator

The

gate

generator

pr

oduces

t

he

GATE

control

signal

and

initiates

the

CLEAR,

CLEAR,

and

LATCH

pulses.

The

generating

portion

consists

of

U220A,

U222A,

U220B,

and

U222B.

The

display

time

control

portion

consists

of

Q230,

Q238,

and

Q240.

The

circuit

will

be

described

first

in

the

normal

gate

mode

(MEASUREMENT

INTERVAL

switch

in

one

of

the

four

gate

time

positions).

Assume

that

the

T

o

conditions

are

as

given

in

Fig.

2-1.

The

Q

outputs

of

U220A,

U222A,

U220B,

and

U222B

are

all

LO.

Q230

is

off

and

the

emitter

of

Q238

rises

as

C235

charges.

At

T

v

Q238

reaches

its

firing

potential

and

discharges

the

capacitor.

This

results

in

a

short-duration

LO

pulse

on

the

direct-set

input

(pin

2)

of

U220A,

forcing

its

Q

output

HI

and

its

Q

output

LO.

With

two

HI

inputs

on

NAND

gate

U230A,

its

output

goes

LO

and

the

output

of

®î

)

2-2

REV.

MAY

1974

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Tektronix DC 502 Specifications

General

Brand	Tektronix
Model	DC 502
Category	Cash Counter
Language	English