Mitsubishi Electric MUX-25TV - E1 User Manual download pdf (Page 36)

An Audio

Step Generator

This

project

takes

the

tedium

out

of making frequency-

response tests

on

audio

equipment

By Jack Cunkelman

Running a

frequency- response

check

with an audio signal

generator

can

be

quite a

chore.

A

signal

generator

being

the

kind of instrument

it

is, you spend a

lot of time just adjusting level

con-

trols. The greater the

number

of

points to be plotted

in

a test,

the more

time

you

spend on

monitoring

signal

levels

and

making adjustments. The

Audio

Step

Generator described

here

is

an

ideal way

around

this problem.

It automatically

selects a series of fre-

quencies and

maintains

a constant

output level, letting you concentrate

on the frequency-

response test in-

stead of on the test conditions.

Using a combination

of analog

and

digital techniques,

the Audio

Step

Generator

automatically

steps

through 10

frequencies of

your

choice

while

maintaining an output

level that

is constant

within

0.5

dB

over

the entire

20 -Hz to

20 -kHz

range. In addition, sine

-wave signal

distortion

is good enough

to suit

the

needs of

most users

who do

not re-

quire laboratory

precision.

About the Circuit

The Step Generator

is built around

function

generator integrated circuit

ICI in Fig.

1,

selected

for this appli-

cation because

of the ease

with which

the frequency

can be changed.

Ordi-

narily, function

generator chips

do

not produce

low- distortion sine

waves.

However, the

XR2206

chosen

for ICI

here

provides

a 0.5%

distor-

tion figure,

which

is

acceptable

in all

but critical applications.

Potentiom-

eters

RI and

R2

are used

to adjust

distortion

of

ICI

to a

minimum.

While

it is

possible

to generate a

sweep

frequency output

from a func-

tion

generator, the ability

to deter-

mine the

frequency at any

given in-

stant

is lost by doing

so. Hence,

it was

decided

that 10 discrete frequencies

that

could

be stepped

through

would

be

used. Capacitor

CI and

the resis-

tor

connected

to pin

7

of

ICI, which

we will

refer

to as

Rx

but

is actually

identified in

Fig. 1 as

R9

through

R18,

determine

the

frequency of

os-

cillation

from the

formula fo =

1 /(Rx

x Cl),

with fo in Hz,

Rx in

ohms and

Cl

in farads.

Though the

value of either

Rx

or

Cl can

be

varied

to

change

the fre-

quency,

it is easier to

fix the

value

of

the capacitor

and

vary that of the

re-

sistor

in steps.

The Table lists

various

frequency and

resistor

values

needed

to generate

these frequencies.

Trim-

mer

potentiometers or

fixed

resistors

can be

used

for Rx,

depending

on the

frequency accuracy

you

require.

A

Mylar or

polystyrene capacitor

for

CI will provide

maximum

stability.

If you use

a 5% tolerance

capacitor,

the frequencies

will be

more

accurate

when using

fixed

resistors.

When pin 9

of ICI is open,

Rx

de-

termines

the frequency

of oscillation.

Grounding

pin 9

makes R6 the fre-

quency-

determining element.

This

feature

is

used

to provide a

SET LEVEL

position

in the project to permit

set-

ting

initial levels before

starting a

fre-

quency

run, using

SI

to switch

this

function on and

off.

Various

values

for

Rx

are switched

in

with

the

IC3, IC4 and

IC5

CMOS

switches.

The

switches

are turned

off

and

on by divide -by

-10 counter

IC2.

Two NAND

gates in

IC6

are

wired as

an

oscillator

to drive counter

IC2.

This oscillator

sets the

rate at

which

the

frequencies are switched.

The

pulse

output

rates of this oscillator

are

determined

by the

R7 /C5 time

constant.

Using an

audio taper

for

potentiometer

R5 permits the

rate

adjustment

to be spread out over

the

40

/ MODERN

ELECTRONICS

/ April 1986

Say

You Saw

It In Modern

Electronics

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