Potomac instruments inc. Aa-51 User manual

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I

I .

I

INSTRUCTION MANUAL

AUDIO ANALYZER

Model AA-51

POTOMAC

INSTRUMENTS,

INC.

932 PHILADELPHIA AVE.

PI

SILVER SPRING, MO. 20910

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WARNING

This

instrument

must

be

used

with

a 3

prong

U-

grounded

receptacle

outlet.

Failure

to

use a

grounded

out

l

et

may

result

in

improper

opera-

tion

or

safety

hazard.

I

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AUDIO ANALYZER

MODEL AA-51

INSTRUCTION MANUAL

POTOMAC INSTRUMENTS INC.

932

Philadelphia Av

enue

Silver Spring, Maryland

20910

(301)

589-2662

Revised 1 February 1979

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l

\

WARRANTY

FOR EQUIPMENT

AND

ACCESSORIES

Potomac Instruments Inc. Warrantseach new

equipment

to

be free

of

defects in material

and workmanship,

for

a

period

of

one (1) year

after

date

of

original shipment,

except

for

fuses, transistors, diodes, and integrated circuits,

wh

ich are warranted

for

a period

of

ninety

(90) days.

Any

instrument

which

is

found

within

one year

not

to

meet

the fore-

going standards

after

examination

by

our

factory,

will

be

repaired

or

at

the

option

of

Potomac Instruments replaced

without

charge. This

warranty

does

not

apply

to

equip-

ment

wh

ich

has

been altered,

improperly

handled,

or

damaged

in

any

way.

CHANGES IN SPECIFICATIONS

The

right

is

reserved

to

change the published specifications

of

equipment

at

any

time,

and

to

furnish merchandise

in

accordance

with

current

specifications,

without

incurring

any

liability

to

modify

equipment

previously sold.

RETURNING

EQUIPMENT

TO

THE

FACTORY

Factory

service

is

available

for

repair and

adjustment

of

this

instrument;

however, a des-

criptio

n

of

the

trouble,

plus

the

model

and serial number(s)

of

the

unit

should

be

for-

warded first, whereupon instructions

for

correcting

the

trouble

or

authorization

for

returning

the

equipment

will

be given.

This

applies

to

all units,

whether

under

warranty

or

not.

Shipment

in

an

improper

container

may

result in damaged

equipment

thereby

voiding

warranty.

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TABLE

OF

C

ONT

ENT

S

Paragraph

Title

Page

SECTION

1

GENERAL

DESCRIPTION

1.1

Function

1-1

1.2 Specifications, AA-51 1

-1

SECTION

2

OPERATING

INSTRUCTION

S

2.1 Installation

2-1

2.2 Controls and Indicators

2-1

2.3 Connectors 2-3

2.4

Fuse Location 2-4

2.5

Adjustment

of

Mechanical Zero 2-4

2.6 General Operating Information 2-4

2.7 Operating Procedures 2-5

SECTION

3

THEORY

OF

OPERATION

3

.1

General Description 3-1

3.2

Block Diagram Description 3-1

3.3 Detailed

Theory

of

Operation 3-3

3.3.1 Signal Leveler (BD41), A2 3-3

3.3.2 Phase

and

Ratio Detector (BD

40

),

A3

3-4

3.3.3 IM/WF Demodulator (BD42),

AS

3-5

3

.3.4

Null Amp

(A6)

and Quadrature

Detector

(A4)

3-6

3.3.5 Voltmeter Circuits (BD43, BDS0,

BDS

l)

3-8

3.3.6 Regulated Power Supply,

Al

3

-9

SECTION

4 .

MAINTENANCE

4. I General 4-1

4.2 Periodic Maintenance

4-1

4.3 Alignment Procedure/Performance Test 4-1

4.4 Troubleshooting, General 4-6

4.5 C

ir

cuit Module Troubleshooting 4-7

4.5.1 General 4-7

4.5.2 Power Supply,

Al

4-7

4.5.3 Leveler, A2 4-7

4.5.4

Phase

and

Ratio Detector, A3 4-7

4.5.5 Quadrature Detector, A4,

and

Null Amp, A6 4-8

4.5.6 IM/WF Demodulator,

AS

4-9

4.5.7

Voltmeter

Circuit Assembly, A7 4-9

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Table

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.

15

4.16

4.17

Figure

2.1

3.1

4.1

4.2

4.3

4.4

4.5

4.6

4.7

4.8

4.9

4.10

4.11

4.12

4.13

4.14

4.15

4.16

4.17

4.18

4.19

LIST

OF

TABLES

Title

AA-5

1

Board-level Troubleshooting

Voltage Readings, Leveler,

A2

Voltage Readings, Phase

and

Ratio Detector,

A3

Voltage Readings, Quadrature Detector, A4

Voltage Readings,

IM/WF

Demodulator,

AS

Voltage Readings, Null Amp, A6

Voltage Readings, Voltmeter, A

7

Part

s List AA-51 Chassis

Parts List, Power Supply

PCB

Assembly,

Al

Parts List, Leveler PCB Assembly, A2

Parts List. Phase and Ratio Detector PCB

Assembly, A3

Parts List, Quadrature Detector

PCB

Assembly,

A4

Parts List,

IM/WF

Demodulator PCB Assembly,

AS

Parts List, Null Amplifier PCB Assembly, A6

Parts List, Noise Filter PCB Assembly,

A

7A1

Parts List, Voltmeter Amplifier, A7A2

Part

s List, Voltmeter Buffer Assembly, A7A3

UST

OF

ILLUSTRATIONS

Title

AA-51 Controls and Indicators

AA-51 Block Diagram

IMD

Test

Signal Network

Circuit Module and Test

Point

Location, AA-5I

Component Location, Power Supply

Al

Schematic Diagram, Power Supply

Al

Component Location, Signal Leveler A2

Schematic Diagram, Signal Leveler A2

Component

Location, Phase and Ratio De

tector

A3

SchematicDiagram, Phase and Ratio Detector A3

Component

Location, Quadrature

Detector

A4

Schematic Diagram, Quadrature

Detector

A4

Component Location,

IM

/WF

Demodulator

AS

Schematic

Di

agram, IM/WF Demodulator

AS

Component Location,

Nu

ll Amp A6

Schematic Diagram, Null

Amp

A6

Component

Locabon, Noise Filter A7A1

Component

Location,

Voltmeter

Amplifier A7A2

Component

Location,

Voltmeter

BufferA7A3

Sch

ema

tic

Diagram,

Voltmeter

Assembly A7

Wiring Diagram,

AA-5

l

ii

Page

4-6

4-10

4-11

4-12

4-13

4-14

4-15

4-37

4-39

4-40

4-43

4-45

4-47

4-50

4-53

4-54

4-56

Page

2-2

3-2

4-3

4-16

4-18

4-19

4-20

4-21

4-22

4-23

4-24

4-25

4-26

4-27

4-28

4-29

4-30

4-3 l

4-32

4-33/4-34

4-35/4-36

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1.1 FUNCTION

SECTION 1

GENERAL DESCRIPTION

1.1.l

Th

e

Potomac

Instruments Model AA-51 Audio Analyzer is a solid

state

instrument for

measuring ac voltages,

total

harmonic distortion (THO),

int

ermodulation distortion (IMO), signal

plus noise

to

noise, relative phase, amplitude ratio, and weighted peak flutter (W&F).

1.1.2

The voltmeter contained in

th

e Model

AA-51

can

be used separately for general purpose

voltage

and

gain measurements.

The

voltmeter has a frequency range

of

5 Hz

to

200

kHz and a

voltage range

of

1 mV

to

100 Vrms

full

scale.

1.1.3

Total hannonic distortion levels

of

0.1%

to

I00% full scale are measured in seven ranges

for any fundamental frequency from

20

Hz

to

20

kHz: harmonics

are

indicated

to

100 kHz. InpuI

signals between 0.1 Vrms and

80

Vrms are automatically leveled

and

no

manual adjustment is

required.

Too

high

or

too

low

input

signal levels are indicated

by

front

panel LED

's

. A scope

output

is provided for use with an oscilloscope.

The

instrument

is capable

of

an

isolation voltage

of

:!:200

V

above chassis ground.

1.1.4 Intermodulation distortionlevels

of

0.1%

to

100% full scale are measured in seven ranges

for an SMPTE standard signal

of60

Hz and 7 kHz (4:1 ratio).

Input

signals between 0.1

Vnns

and

80

Vrms are

auto

matically leveled.

Too

high

or

too

low

input

signal levels are indicated by LED's.

1.1.S Noise levels as low as -75 dBm

(600

ohms) are measured

ina

passband

of20

Hz

to

20

kHz.

1.1.6 Relative phasebetween two signals is measured in two ranges

of

±54

degrees and ±180

degrees full scale within a frequency range

of

20

Hz

to

20

kHz.

Input

signals between 0.3 Vrms

and

6 Vrms are fully limited;however,

the

amplitude ratio should be within

±6

dB for best accuracy.

1.1.

7 Amplitude ratio

of

two signals is measured over a ±6 dB range within

the

frequency range

of

20

Hz

to

20 kHz.

The

input

signal levels can

be

between 0.3 Vnns

and

6 Vrms.

1.1.8 Weighted peak

flutter

is measured in

two

ranges

of

0.3% and 1% full scale.

The

input

signal

must

be

3.15 kHz ±10% within the amplitude range

of

0.1 Vrmsand

80

rms. A wide band

(0

.2 Hz

to

200

Hz) scope

output

is provided.

1.2 SPECIFICATIONS, AA-51

AC

Voltmeter

Frequ

ency

Range:

Voltmeter

Range:

Voltmeter Accuracy:

Input

Impedance:

Scope

Outpu

t:

20

Hz

to

200

kHz

1 mV

to

100 Vrms (full scale), 10 dB steps

±3%

(full

scale)

330K

ohms

shunted

by

150

pf

capacitor

100 mV p-p

(full

scale)

1-1

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THD Distortion Meter

Fund

amenta

l

Frequency

Range:

Bandwidth

:

Di

s

tortion

Range:

Accuracy:

Dynamic

Rang

e:

Input

Impedance

:

Scope

Output:

Internal

Distortion

& Noise:

IM

Distort

ion

Meter

Input

Signal

Required:

Distortion

Ran

ge:

Accuracy:

Dynamic

Range:

Input

Imp

edance:

Scope

Output:

Internal

Distortion

& Noise:

Signal

Plus

Noise Meter

Frequency

Range (- 3 dB):

Voltmeter

Range:

Signal

Plus

Noise

-

to-Noise

Accuracy:

Residu

al

Hum

&

Noise

:

Input

Imp

edance:

Scope

Output

:

20

Hz

to

20

kHz

in

3

decade

rang

es

100 kHz

0.1%

to

I

00

% full scaJe

±5% (full scale)

0.1

Vrms

to

80

Vrms

automatically

leveled

SOOK

ohms

shunted

by

105

pf

capacitor

100

mV

p-p (full scale)

,;;;;Q

.04

%,

50

Hz - 7.5 kHz;

,;;;;.08%,

20

Hz

-

20

kHz

60

Hz

& 7

kHz

@

4:

1

ratio

(SMPTE

standard)

0.1%

to

100%

±5

%

(full

scale)

0.1

Vrms

to

80

Vrms

automatically

leveled

SOOK

ohms

shunted

by

I

05

pf

capacitor

100

mV

p-p

(full scale)

<.03%

20

Hz

to

20

kHz

I

mV

to

100

Vrms

(full scale)

±5% (full scale)

< - 80 dBm

330K

ohms

shunted

by

150

pf

capacitor

1

00

mV

p-p

(full scale)

1-2

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Phase Meter

Rang

e:

Bandwidth:

Accuracy:

Dynamic Range:

Input

Impedance:

Scope

Output:

Resolution:

Ratio Meter

Range:

Bandwidth:

Accuracy:

Dynamic Range:

Input

Impedance:

Scope

Output:

Resolution:

W&F

Meter

Range:

In

put

Frequency:

Accuracy:

Dynamic Range:

Inpu

t Impedance:

Internal Distortion & Noise:

Scope

Output:

±

54

degrees and ±180 degrees

20

Hz

to

20

kHz

±5% (full scale)

0.3 Vrms

to

6 Vrms

SOOK

ohms

shunted

by

180

pf

capacitor

Rectangular wave

at

input

frequency, 0.

5V

p-p,

duty

cycle proportional

to

phase angle

3 degrees

±6 dB

20

Hz

to

20

kHz

±5% (full scale)

0.3 Vrms

to

6 Vrms

SOOK

ohms

shunted

by

1

80

pf

capacitor

de

voltage proportional

to

ratio

.:;;;;

±0.25 dB

0.3%and

1%

weighted

peak

flutter

in

two

ranges

3.15 kHz ±10%

±5% (full scale)

0.1 Vfms

to

80 Vrms

SOOK

ohms

shunted

by

150

pf

capacitor

¾.01%

Bandwidth .2

to

200

Hz, sensitivity

500

mV

-p-p

(1

%full scale)

1-3

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External Power:

Weight:

Dimensions

(Wx

HxD)

CM

(IN):

115V

or

23

0V

, 50

or 60

Hz

, I

OW

12

lb -

with cover

38.74

X

13.34

X

20.32

(15¼

X

5¼

X

8)

1-4

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SECTION 2

OPERATING INSTRUCTIONS

2.1 INSTALLATION

The Model AA-51 is ready

for

operation as delivered

and

need

only

be

connected

to

an

AC

power

source.

WARNING

The

AA-5 I

must

be

used

with

a 3-prong

U-grounded receptacle outlet. Failure

to

use a grounded

outlet

may

result in i

m-

proper operation

or

safety hazard.

2.2

CONTROLS

AND

INDICATORS

The following controls and indicators are identified

on

Figure 2-1.

2.2.1

POWER Switch,

SI

Applies AC

power

to

the

unit when

in

the

ON position.

2.2.2

POWER Indicato

r,

DSJ

Illuminates

to

indicate

the

presence

of

main DC,

(+23V)

power.

2.2.3

FREQ

Range Switch,

S2

The setting

of

this switch indicates

the

multiplier

for

the

calibrated

FREQ

dial.

2.

2.4

MODE Switch, S3 (upper)

The

upper MODE switch is used

to

select

the

1%

W&F

, .3% W&F,

RAT

IO, PHXIO,

PHX3 modes

or

, in its

most

counter-clockwise position,

to

transfer

contro

l

of

mode selection

to

the

lowerMODE switch, A7-S

1.

2

.2.5

MODE

Switch

A7-S1 (lower)

The lower MODE switch is used

to

select

VM,

TUNE, THD, lMD

or

NOISE

mode

when

the

upper

MODE switch,S3, is

in

its

most

counter-clockwise position.

2.2.6

INPUT Switch,

S4

The

INPUT switch,

S4,

is used

to

select

either

the

L (J6)

or

R

(J7)

input

in those modes

which require

only

a single input signal.

2.

7 LEVEL Switch,

SS

The LEVEL

pushbutton

is used

to

disable

the

rejection-notchTesponse

of

the

null

amplifier

for

test

purposes.

2-1

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J11

0S2

0S3

J10

Ml

AUOIO

ANAi.

YZEft

AA-51

ON

CHASSIS

OAT

S3

THO

VM

S2

,------F

:

REt>-----

RANGE

~,.

oe-~-oo-,

I (

,--v-1-v--,

1

30

-IO

.3

3 +tO

NOIS€

10

-ao J

c.;

l

(.I

'1,0

Pi

R •

...,..

COM

r

---------•

t

----------

t

----,11

1

11

..

r-i

S1

DS1

S6

...

MECHANICAL

ZERO

J9

A7-S1 A7-S2

FRONT

PANEL

~

• .I. '

A -

.,,.

aa:ESJI

...

,,

.:, I ~ •

'>

\'

• 1 ,

•~•

'1

'•

! "

,;..:

I'~\

I

,1

11

,,- 11

,,

'3\

J8

F1

REAR PANEL

Fi

gure 2.1. AA-

51

Co

nt

rols and Indicators

2-2

GROUND

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2.2.8

SIG

COM

Switch,

S6

The

setting

of

the

SIG COM

switch,

S6,

detennines

whether

the

L

(16)

an

d R

(J7)

signal

input

grounds

(analyzer

circu

it

c

ommon)

are

floating

or

connected

to

chassis

grou

nd

.

2.2.9

RANGE

Switch,

A7-S2

The

RANGE

switch

is used

to

se

lect

the

full scale range

in

Volts,

percent

or

dB

of

the

meter

Ml.

2.2.10

FREQ

Di

al

The

FREQ

dial

is

used

in

conjunction

with

the

FREQ

range

switch,

S2,

to

tune

the

notch

filter frequency

to

the

input

signal

fundamental

frequency.

2.2.11

Meter,

Ml

The

front

panel

meter

, M 1, gives

direct

readings

of

amplitude

ratio

in

dB,

noise

and

signal levels in

Vrms

or

dB, signal phase in degrees,

and

harmonic

distortion,

intermodulation

distortion

and w

eighted

peak

flutt

er

in

percent.

2.

2.12

OS2 HIGH Indicator and OS3

LOW

Indicator

The

HIGH

and

LOW

indicator

LEDs

illuminate

singly

in

THO

,

1MD

and

WF

modes

to

indicate

an

input

signal level which is

too

high

or

too

low

for

proper

operation

of

th

e analyzer.

In

THO

mode

,

both

indicators

will

illumina

te

simultaneously

to

indicate

an

off

-null

condi

ti

on

of

the

null

amplifier.

2.3 CONNECTORS

Front

and

rear

panel

connectors

are

identified

in

Figure

2-1.

2.3.1 L INPUT,

16

and

R INPUT,

J7

The

L

and

R

input

connectors

,

located

on

the

analyz

er

front

panel,

are

used

to

apply

signals

to

the

analyzer

for

measurement.

In

phase

and

ratio

modes

both

inputs

are

used

, whereas

in

other

modes

,

one

of

the

two

inputs

will

be

selected

by

INPUT sw

it

ch

S4.

The

mating

connector

for

J6

and

J7

is

Switchcraft

part

no. O5CL3M, available from

Switchcraft

distributors

or

Potomac

Instruments.

2.3.2

AC

IN

Connector,

J8

Connector

J8 is

located

on

t

he

rear

panel

and receives

the

line

co

rd

connecto

r

to

provide

AC

power

input

to

the

AA-51.

2.3.3

AC OUT

Connector,

19

Connector

J9

is

located

on

the

rear

panel

and

provides

an

unfused

115

/2

30

V

ac

auxiliary

output.

2.3.4 SIG

COM

Output

Connector,

110

and

SCOPE

Output

Connector,

JI

1

Connectors

J

10

and J1

J

are

3-way

binding

post

connectors

which

provide a signal

output

for

an oscilloscope.

2-3

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2.4 FUSE

LOCATION

A I/8 amp,

250V

fuse

(F

1) is accessible

in

a screw-type

holder

on

the

generator

rear panel.

2.5

ADJUSTMENT

OF

MECHANICAL

ZERO

Be

fore

operating

the

AA-51

for

the

first time,

check

the

me

ter

mechanical zero.

To

do

this

,

if

analyzer

is

on

,

tum

off

and

allow 2

to

3

minutes

for

complete

capacitor

discharge.

Turn

the

meter

zero

adjust screw,

shown

in Figure

2-1.

using a small-

bladed

screwdriver

until

an

exact

zero

indication

is achieved.

2.6

GENERAL OPERATING INFORMATION

2.6.1

Input Connections

One

or

two

signal

source

s

may

be

connected

to

the

AA-51 using

the

shielded cables pro-

vided

with

the

analyzer. In these cables

the

alligator clip insulators are

color

coded.

Red

is

the

signal

high lead;

blue

is

the

signal

low

or

common

lead;

and

black

is

the

cable shield

and

chassis

ground.

The

eff

e

ct

of

input

and

test

cable

capacitanc

e

on

frequency

re

sponse

must

be

considered when

the

device

under

test

has

a high ou

tput

imp

e

danc

e.

The

test

cable capacitance is

approximately

400

pf;

this

added

to

the

AA-51

input

capacitance res

ults

in

a capacitive reactance

load

on

the

device

under

test

of

20,000

ohms

at

15 kHz.

For

a

sourc

e

impedance

of

600

ohms

this reactance has negligible

effect,

but

for

a

sourc

e impedance

of

10,000

ohms

the

measured voltage is

down

1.0 dB from

the

no-load value

at

15 kHz. Low-capacitance

test

leads can

reduce

this

effect

to

0.1 -0.2 dB.

In

some

cases

what

is called

"output

impedance"

is

not

the

circuit

generator

impedance

,

but

the

minimum

recommended

load

impedance.

The

true

output

impedance,

which

may

be

lower,

can

be

measured

by

finding

the

resistance

that

drops

the

output

voltage

to

half

its no-load value

at

a signal level low

enough

to

main-

tain

linear

operation.

2.6.2

Grounding

When

the

AA-51

power

cord

is plugged in,

the

chassis is

connected

directly

to

the

AC

power

neutr

al

line

through

the

third

wire

of

the

power

cord.

This

may

or

may

not

provide

minimum

pickup

of

unwanted

hum

and

noise

in

the

measuring circuits.

Often

the

AA-51

must

be

grounded

solidly

to

the

equipment

under

test

to

obtain

minimum

hum

pickup.

This

connection

may

be

made

using

the

test

cable shield,

which

is

connected

to

the

chassis,

or

by

using a

separate

wire from

the

ground

binding

post

on

the

rear

of

the

unit. In

such

cases

it

may

also

be

necessary

to

isolate chassis

ground

from

power

line

ground

, using a three-prong-to-two-prong

adapter

to

obtain

minimum

noise

pickup.

If

this

is

done

be

certain

to

observe

th

e following warning:

WARNING

Be ce

rt

ain

that

the

chassis

of

the

AA-51 is solidly

grounded

to

an

earth-grounded

point

before using

an

adapter

or

other

means

to

br

eak

the

power

cord

ground

connection.

Otherwise a dangerous

voltage

may

be

present

on

th

e AA-51 chassis.

2.6.3 Use

of

Scope T

erminal

The

scope

te

rmin

als provide a l

00

mV p-p

output

for

full scale

meter

deflection.

These

terminals

can

be

used

to

monitor

the

output

signal

with

an

oscilloscope.

The

combination

of

2-4

www.SteamPoweredRadio.Com

the

analyzer and oscilloscope provides more significant information

about

the

device under test

tban the expression

of

magnitude alone. The impedance

at

the scopeterminals is 2200 ohms

and excessive loading should be avoided to maintain

the

accuracy

of

meter reading. Watch for

a

drop

in the reading when connectinga load; for high-impedance oscilloscope inputs the

effect

is

negligible.

2.6

.4 Signal Common Chassis-Float Switch

For

mostmeasurements this switch should

be

in the CHASSIS position.

The

FLOAT

position

is

for

use when neither

of

the pair

of

signal terminals under test is

at

ground potential.

In

this mode

the

signal common may be

at

up

to

±200 volts

de

or

peak ac with respect to

chassis ground.

If

the terminals under test are a balanced pair so

that

signal voltage exists

on

both

AA-51

input

leads, the

VM

and NOISE functions operate normally,

but

distortion measure-

ments will

not

be

correct.

For

distortion measurements

on

a balanced line a balanced-to-unbalanced

transformer between the device under test and

the

AA-Sl

is

recommended.

2.6.5 Balanced Line Measurements

The AA-51 measuring circuits are unbalanced,

that

is, one

of

its

input

terminals (designated

Signal Common)

is

directly connected

to

the measuring circuitground. Signal Common may be

floated with respect

to

chassis ground,

as

explained in 2.

6.4,

but

the measurements possible

on

balanced lines arelimited. Voltage measurements are possible,

but

the THD measurement circuit

does

not

function well in the FLOAT mode,

and

a transformer is required.

2.6.6 Equipment Performance Measurements

Equipment Performance Measurements (EPM)

to

determine a broadcast facility's audio

performance are required

by

the

FCC and described in various sections

ofthe

FCC Rules, Part 73.

These tests are commonly referred to

as

an "Audio Proof" although

the

FCC perfers to use

the

term

"Proof"

only

for

measurements

to

verify the performance

of

AM

directional antenna systems.

Useful guides

to

these EPMs for AM and FM stations have been prepared by Broadcast Engineering

Magazine and are available from Broadcast Engineering, P.O. Box 12901, Overland Park, Kansas 66212.

2.7

OPERATING

PROCEDURES

2.7.1

GENERAL

The AA-51 audio analyzer should

be

operated

from

a 115 volt

or

230

volt ac power

sourceaccording to

the

rating on the nameplate

on

the

rear

of

the

unit.

CAUTION

Switch (SIG COM) from (CHASSIS) to (FLOAT) when measur-

ing signals between

two

points which are

both

above ground

potential.

2.7.2

Voltmeter

Mode

The AA-51 becomes a general purpose ac voltmeter when the MODE selectoris set

to

VM

position (Upper MODE Switch max.

CCW).

Perform

the

following steps when ma.king

voltage measurements:

a.

Position RANGE selector to a range exceeding

tl1e

value

of

the

signal

to

be

measured.

2-5

www.SteamPoweredRadio.Com

b.

Connect

signal

to

either

the

Lor

R

connector

with

INPUT

sw

itch

in

proper

position

to

read

signal.

c.

Select

a

meter

RANGE

to

give a reading as

close

to

full scale as possible.

d.

Observe

meter

indication

and

meter

RANGE

setting

to

obtain

signal level.

2.7.3 Total Harmonic Distortion Mode

a.

Position

MODE

selector

to

TUNE

(Upper

MODE

Switch

max.

CCW).

Th

e

SIG

COM

switch

must

be

at

CHASSIS.

b.

Connect

test

leads

from

device

under

test

to

either

the

Lor

R

connector

with

INPUT

switch

in

proper

pos

i

tion

to

read

signal.

c.

See

that

the

HIGH

and

LOW lights are

both

out.

If

the

LOW

light

is

on,

switch

the

FREQ

switch

one

step

above

or

below

the

correct

position

(or

press

LEVEL

switch

S5)

and

read

the

meter:

if

the

pointer

is

at

full scale,

indicating

that

signal leveling is

effective,

the

THO

reading

will

be

accurate.

Normally

this

occurs

for

input

levels

as

low

as

50

mV

RMS.

At

such

low

levels,

however

,

the

AA

-5

I's

residual

noise

may

cont

ribut

e

to

the

reading.

Accurate

readings

are

also

pos

s

ible

for

input

levels

above

that

which

turns

on

the

HIGH

light,

as

high

as

100

volts

,

if

leveling

is

effective

and

i

nstru

-

ment

distortion

is

low

.

d.

Position

FREQ

rang

e

selector

and

frequency

dial

to

fundamental

frequency

of

input

signal.

Adjust

frequency

dial

to

obtain

a

minimum

indication

on

the

meter.

e.

Position

MODE

se

l

ector

to

THD.

HIGH

and

LOW lights will

light

together

until

automatic

nulling

is

nearly

completed.

If

the

lights

do

not

go

out,

recheck

the

dial

setting

in

TUNE

~steps a-c).

f.

Observe

meter

indication

and

RANGE

switch

set

tin

g.

For

example,

if

meter

indicates

.8

and

the

RANGE

selector

is

in

1

%

position,

distortion

measured

is

0.8

%.

If

the

THD

reading

seems

abnormally

high,

the

problem

is

often

power

frequency

(hum)

or

other

noise

pickup,

which

can

be

identified

by

using

an

oscilloscope

at

the

SCOPE

terminal.

g.

If

a series

of

THO

measurements

is

being

made

at

various

known

frequencies,

in

most

cases

the

MODE

switch

can

be

left

at

THO

and

the

dial

reset

by

eye

to

each

new

frequency.

The

AA-51

has

enough

pull-in

rang

e

to

null

auto-

matically

without

using

the

TUNE

step

if

the

dial

is

set

carefully,

except

that

it

may

be

ne

cessary

to

swit

ch

to

TUNE

and

back

to

THD

for

nulling

near

20

kHz.

2.

7.4

Intermodulation Distortion

a.

Position

MODE

selector

to

IMO

(Upper

MODE

Switch

max.

CCW).

b.

Connect

test

leads

from

device

under

test

to

either

the

L

or

R

connector

with

input

switch

in

proper

position

to

read

signa

l.

2-6

www.SteamPoweredRadio.Com

c. See

that

the

HIGH

and

LOW lights are

both

out.

lf

the

LOW light lights,

switch

the

MODE

switch

to

TUNE

and

FREQ

to

xl0,

and

read

the

meter.

If

the

pointer

is

at

full scale,

the

IMD

reading will

be

accurate (see 2.7.3c).

d.

Observe

meter

indication

and

RA

NGE switch setting

to

determine

the IMD

percentage. A dual-trace oscilloscope used

to

compare

the

SCOPE

output

voltage

with

the

input

test

signal will indicate

at

what

point

on

the

test

signal waveform the distortion occurs.

2.7.5 Noise Mode

a. Position MODE selector

to

NOISE.

(Upper

MODE

Switch

max

. CCW).

b.

Connect

test

leads from device

under

test

to

either

the L

or

R

connector

with

INPUT switch

in

proper

position

to

read signal.

c. Observe

meter

indication and RANGE sw

itch

setting

to

determine

the

noise voltage.

The

noise voltage

may

be

observed

for

analysis

with

an

oscilloscope

at

the SCOPE terminal.

d.

For

signal-to-noise ratio measurements (actually signal-plus-noise-to-noise

ratio)

,

a referencelevel may

be

established

by

applying

the

signal

in

the

NOISE

mode.

An example is, applying

modulation

to

a transmitter

to

obtain

a referencelevel

and

then

removing

it

to

measure noise.

In

the

NOISE

mode,

for

sine waves,

the

reference voltage applied

must

not

exceed 8.0

volts RMS

up

to

7.5 KHz

or

4.0

volts

at

15

kHz

for

accurate results.

For

higher voltages use

the

VM

mode

to

estab

lish

the

referencelevel.

The

meter

sensitivity

for

VMis

the

same as

for

NOISE.

If

in

doubt,

look

for

distortion due

to

too-high levels

in

the NOISE circuits using

an

oscilloscope

at

the

SCOPE terminal.

2.7.6 PhaseAngle Mode

a.

Position MODE switch

(upper)

to

PHASE x 10.

b.

Connect

test

leads from the

Land

R

input

connectors

to

the

points

on

the

device

under

test

between

which

the

phase angle is

to

be

measured.

c.

With signals applied, see

that

the

HIGH

and

LOW lights are

both

out.

If

not

or

if

in

doubt,

switch

to

the

VM

mode

and

measure

the

input

levels

to

see

that

they are within

the

specified range

of

0.3

to

6.0

volts.

d. Read

the

meter;

if

the

indication

is

less

than

5

on

the

PHx l0 range, switch

to

the

PHx3 range. Multiply

th

e reading

by

10

or

3

to

obtain

the

phase

angle.

Note

the

sign

of

the

reading. A positive angle means

that

the

R

input

lags

the

L

input,

or

the

R

input

is delayed

in

time

with

respect

to

the

L input.

e.

Fast

phase changes

may

be

seen

by

observing

the

output

signal

at

the

SCOPE terminal using

an

oscilloscope. This signal is a square wave

with

duty

cycle

proportional

to phase angle;

it

is symmetrical

(50

percent

duty

cycle)

for

an

angle

of

zero.

2-7

www.SteamPoweredRadio.Com

2.7.7 Ratio Mode

a. Positi

on

MODE switch (upper)

to

RATIO.

b. Connect

test

leads from

the

Land

R

input

connectors

to

the

points

on

the

device

under

test between which the amplitude ratio is

to

be

measured.

The

L

input

is

the

reference signal in

the

RATIO circuit, and the

meter

response

time

for

R voltage changes is faster than

for

L

voltage changes.

c. With signals applied, see

that

the

HIGH

and

LOW

lights are both

out.

If

not

or

if

in

doubt,

switch

to

the

VM

mode

and measure

the

input

levels

to

see

that

they are within

the

specified range

of

0.3

to

6.0

volts.

d. Read

the

indicated ratio directly in dB. A positive indication means

that

the

R voltage is greater than

the

L voltage.

e. A de voltage proportional

to

the

ratio

is available at the SCOPE terminal.

This voltage is

zero

for

a

ratio

of

zero dB, and changes

+3

.0, - 1.5

volts

for

ratios

of

±6

dB.

2.7.8

Wow

and

Flutter

Mode

a.

Position MODE switch (upper)

to

1% W&F.

b.

Connect

test

leads from

either

the L

or

R

connector

to

the

device

unde

r

test

with

input

switch in

the

proper

position

to

read

the

signal. See

that

the

HIGH and

the

LOW

lights are

both

out.

c. Observe percentage

of

W&F

indicated

on

meter.

If

meter

indication is

small, switch MODE selector

to

.3%

W&F

for

increased sensitivity.

d.

The

instantaneous

output

of

the

wow and flutter

demodulator

may be

observed

on

an oscilloscope

at

the

SCOPE terminal.

The

sensitivity is

constant

for

flutter frequencies from

0.2

to

200

Hz. There is

no

weighting

filter

at

this point.

2-8

www.SteamPoweredRadio.Com

SECTION

3

THEORY

OF

OPERATION

3.1 GENERAL DESCRIPTION

ThL'

Modd

AA-5 1

Audio

Analyzer

as

shown

in

the

overall

block

diagram.

Figure

3.

I.

consists

basirally

or

a

POTOMAC INSTRUMENTS INC. AA-51 User manual

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