BRUEL & KJAER 1017 Guida utente

Beat Frequency Oscillator Type 1017

i

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0:

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·

A

beat

frequency

oscillator

which

will

meet

the

numerous

requirements

of

a

signal

source

for

electrical

and

electro-mechanical

measurements

at

low

frequencies

.

fhe

frequency

scale

is

logarithmic

and

covers

the

frequencies

from

2 c/s

to

2ooo

c/s. An

automatic

output

regulator

makes

external

control

of

the

output

voltage

possible.

BRUFJL

&KJJEK

Naarum, Denmark .

eye

80

05

00

.

~

BRUKJA,

Copenhagen

. Telex: 5316

88

1017

Beat Frequency Oscillator

Type 1017

NOVEMBER

1965

Contents

Page

1.

Description

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

General

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Oscillator

and

Mixer

Section

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

100

Hz

Reference

Signal

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Compressor

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Partial

Blocking

of

Frequency

Range

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Output

Amplifier

Section

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Output

Voltmeter

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Overall

Performance

of

Oscillator

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Power

Supply

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

2. Control Knobs,

Terminals

and

Shafts

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

11

3. Operation

........................................................

14

General

..........................................................

14

A.

Initial

Setting

Up

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

B.

Frequency

Calibration

.........................................

14

C.

Use

of

Output

Terminals

marked

LOAD

........................

15

D.

Use

of

ATTENUATOR

OUTPUT

................................

15

E.

Partial

Blocking

of

Frequency

Range

. . . . . . . . . . . . . . . . . . . . . . . . . . . .

15

F.

Remote

Control

Facilities

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

G.

To

Use

the

Compressor

Circuit

................................

17

H.

Use

of

the

100

Hz

REF

.

SIGNAL

..............................

18

I.

Use

of

Frequency

Increment

Scale

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

4.

Combined

Units

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Automatic

Frequency

Recorder

Type

3307

..........................

19

Automatic

Frequency

Recorder

Type

3328

..........................

20

5. Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Electrical

Measurements

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Amplifiers

and

Filters

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Frequency

Response

of

Filter

Circuit

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Low

Frequency

Measurements

on

Amplifiers

........................

25

Frequency

Range

of

High

Quality

Amplifiers

. . . . . . . . . . . . . . . . . . . . . . . . 26

Mechanical

Measurements

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Control

of

Vibration

Exciters

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

6.

Specifications

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Description

General.

The

Beat

Frequency

Oscillator

Type

1017 is

designed

as

a

low

frequency

signal

generator

covering

a

range

from

2-2ooo

Hz.

It

is

an

instrument

suit-

able

for

accurate

measurements

in

the

fields

of

acoustical

and

electro-

mechanical

research

and

design.

The

1017

employs

the

heterodyne

principle

as

the

generating

system

and

consists

of

two

audio

frequency

oscillators,

one

of

which

operates

on

a

fixed

frequency

and

the

other

on

a

frequency

which

can

be

varied.

The

output

of

the

two

oscillators

are

fed

to

a

mixer

and

the

difference

frequency

of

the

two

is

passed

via

a

low

pass

filter

to

an

output

amplifier

stage.

Impedance

Fixed Variable

JJ

Low Pass

Output

Matching

Oscillator

Amplifier

Mixer

Filter

Amplifier

Circuit

Load

Output

Regulating

Amplifier

Attenua.tor

Output

Variable Magnetic Output

Oscillator

Clutch

Attenuator

f59366

Fig. 1.1.

Block

diagram

of

B.F.O. 1017.

A

large

circular

frequency

scale

is

provided

and

is

marked

logarithmically

in

Hz

from

2-2ooo.

The

frequency

control

knob

and

pointer

are

mounted

on

the

shaft

of

a

variable

air-dielectric

condenser

which

tunes

the

variable

oscillator.

The

control

knob

is

fitted

with

a

fine

tuning

arrangement

with

a

reduction

ratio

of

5 : 1.

In

order

to

obtain

extremely

accurate

frequency

selection

an

incremental

adjustment

is

embodied

in

the

fixed

oscillator

which

provides

for

a

frequency

variation

from

+5

to

- 5

Hz

of

the

frequency

to

which

the

instrument

is

set

by

the

main

scale.

3

Provision

is

made

for

the

calibration

of

the

main

frequency

scale

by

injecting

the

frequency

of

the

power

supply

into

the

meter

circuit

and

comparing

this

on

the

meter

with

the

oscillator

frequency

which

is

set

to

the

power

supply

frequency.

For

calibration,

coarse

and

fine

frequency

adjustment

controls

are

provided

on

the

front

panel

of

the

instrument.

The

injected

power

supply

frequency

used

during

frequency

calibration

does

not

appear

at

the

output

terminals

of

the

instrument.

A

worm

gear

on

the

main

tuning

capacitor

drive

permits

automatic

tuning

of

the

B.F.O.

from

an

external

motor

drive.

The

worm

gear

can

be

engaged

and

released

by

an

electromagnetic

clutch

which

is

operated

from

a

switch

on

the

front

panel.

The

clutch

is a

friction

device,

and

provides

for

manual

tuning

of

the

variable

capacitor

even

when

the

worm

gear

is

engaged.

Provision

for

the

remote

control

of

the

clutch

is

made

at

the

socket

marked

REMOTE

CONTROL

on

the

front

panel.

Facilities

are

provided

for

regulation

of

the

output

voltage

of

the

instrument

by

means

of

a

feedback

loop

(COMPRESSOR

circuit).

Any

suitable

AC

voltage

source

may

be

coupled

to

the

compressor

input,

but

the

compressor

circuit

is

designed

to

be

controlled

by

the

signal

derived

from

the

B.F.O.

output

via

a

transducer

which

is

driven

by

the

B.F.O.

In

this

manner

it

is

possible

to

keep

a

voltage,

current

or

some

other

physical

quantity

constant

during

measurements.

Oscillator

and

Mixer Section. Fig.

1.1

shows

a

block

diagram

of

the

complete

B.F.O.

The

dotted

line

divides

the

diagram

showing

the

separation

between

the

oscillator/mixer/regulator

and

the

output

amplifier

sections

respectively.

The

fixed

oscillator

is

tuned

to

12

kHz

and

the

output

voltage

is

fed

to

the

grid

of

a

pentode,

the

bias

of

which

is

controlled

by

the

rectified

output

voltage

from

the

regulating

amplifier.

The

output

voltage

developed

across

the

grid

circuit

of

the

variable

oscillator

is

fed

to

the

input

of

a

buffer

amplifier

stage.

The

output

voltage

from

this

buffer

amplifier

is

supplied

to

the

mixer

stage.

The

frequency

of

the

variable

oscillator

is

continuously

adjustable

within

12

kHz

to

10

kHz.

The

tuning

is

obtained

by

a

spectaily

designed

variable

capacitor,

which

is

manufactured

to

a

high

degree

of

accuracy.

It

provides

a

frequency

scale

which

deviates

less

than

o.7

degrees

from

a

theoretical

logarithmic

scale.

In

the

mixer

tube,

the

output

voltage

from

the

fixed

oscillator,

derived

via

the

variable-~

amplifier,

is

mixed

with

the

output

voltage

from

the

variable

oscillator.

A

low-pass

filter

with

a

cut-off

frequency

of

5

kHz

is

inserted

in

the

anode

circuit

of

the

mixer

tube

to

allow

only

the

difference

frequencies

of

the

mixer

to

pass

to

the

output

amplifier

section.

100Hz

Reference Signal.

The

main

scale

is

set

on

the

100

HZ

REF.

SIGNAL

and

when

the

pushbutton

marked

100

HZ

REF.

SIGNAL is

depressed

the

4

resonance

circuit

of

the

variable

oscillator

is

paralleled

by

a

fixed

capacitor

and

there

will

be

a 100

Hz

output

signal.

In

this

way

it

is

possible

at

the

same

time

to

align

the

B.F.O.

with

the

fre-

quency

calibrated

paper

when

the

Level

Recorder

Type

2305 is

used,

and

to

check

the

signal

level

at

100 Hz,

because

of

the

distance

from

the

100 Hz Ref.

signal-mark

to

the

beginning

of

the

frequency

scale

(2

Hz) is

the

same

as

the

distance

on

the

paper

between

"10"

and

"20"

when

the

automatic

scanning

is

used.

Compressor

(Signal

Regulator).

The

object

of

this

circuit

is

to

control

the

output

of

the

B.F.O.

by

a

control

signal

derived

from,

for

example,

the

output

voltage

of

a

vibration

pick-up

mounted

on

a

vibration

table

which

is

excited

by

the

B.F.O.

By

this

method

the

acceleration,

velocity

or

displacement

of

the

vibration

table

can

be

kept

constant.

To

obtain

a

high

degree

of

regulation

the

working

point

of

the

variable-,u

pentode

is

chosen

on

the

non-linear

portion

of

its

Ia/Vg

characteristic

near

cut-off.

The

control

voltage

must

be

within

the

frequencies

given

by

LOWER

LIMITING

FREQ.

HZ, (2, 6, 20

or

60

Hz

and

2ooo

Hz).

The

lower

limits

are

indicated

by

the

COMPRESSOR

SPEED

position.

The

voltage,

which

should

be

of

approximately

1 V

or

higher,

is

fed

in

via

the

screened

socket

marked

COMPRESSOR

INPUT,

on

the

front

panel.

A

variable

potentiometer

marked

COMPRESSOR

LEVEL,

connected

in

the

input

circuit

of

the

regulating

amplifier

is

used

to

control

the

output

voltage

instead

of

the

OUTPUT

LEVEL

potentiometer,

which

should

be

in

its

maximum

position

when

the

compressor

circuit

is

in

use.

Under

these

conditions

the

COMPRESSOR

LEVEL

control

is

adjusted

to

a

position dependent

on

the

amount

of

com-

pression

or

voltage

output

required.

Turning

this

control

in

a

counter-clock-

wise

direction

increases

the

output

voltage

and

decreases

the

degree

of

com-

pression.

When

the

control

is

turned

fully

clockwise

(on "10")

the

signal

applied

to

COMPRESSOR

INPUT

is

fed

to

the

compressor

amplifier

without

any

attenuation.

The

amplified

control

voltage

is

rectified

in

a

full

wave

rectifier,

designed

to

give a DC

output

voltage

approximately

proportional

to

the

average

value

of

the

control

voltage,

which

is

an

advantage

when

the

control

voltage

is

not

a

pure-sine-wave.

The

speed

of

regulation

can

be

varied

by

means

of

the

switch

marked

COMPRESSOR

SPEED

which

changes

the

time

constant

of

an

R-C

filter

network

in

the

rectifier

circuit.

Compressor

speeds

of

3, 10, 30

and

100 dB/sec.

can

be

chosen.

A

special

circuit

has

been

included

which

precharges

the

capacitors

in

the

compressor

rectifier

circuit

before

they

are

switched

in.

In

this

way

no

un-

desired

change

in

the

B.F.O.

output

voltage

takes

place

when

the

COM-

PRESSOR

SPEED

switch

is

operated.

However,

the

circuit

enables

only

the

capacitor

in

the

compression

speed

sequence,

to

which

the

COMPRESSOR

SPEED

switch

is set,

to

be

charged

to

a

voltage

level

equal

to

that

of

the

capacitor

actually

in

use

.

It

is

therefore

necessary,

where

the

5

+30

Full

Indicating

Meter

deflecting~

~20

Graduation of

dB-·L

·1:

~

al

.....

0

Q.l

01

0

.....

0

>

.....

::3

a.

.....

-10

-20

-30

-40

-50

g

-60

L

-70

--

-

1--

--

.....

\

!--"

Distorted Signal

'·

\

l..4-

\

\\

1\

" I\

\\

Maximum

\ dynamic -

1--

range of

" Compressor

"OUTPUT

LEVEl:'\

-

control

turned

j

~

down 0

dB

~-

-1-

-10

dB

-==-

-30

dB

----

~

-50

dB

""-

----

~

\II

i'-

1-'-

Noise-

1\~

~

1--

>Level

of B.

F.a

.

7 6 5 4 3 2 1 - 0 +1 2 3 4 5

dB

/622&0

Approx. 1V

L +

Relative variation of "COMPRESSOR

INPUT"voltage

Fig. 1.2.

Typical

compressor

characteristics

at

four

different

settings

of

OUTPUT

LEVEL

potentiometer.

COMPRESSOR

SPEED

is

decreased

during

operation

of

the

B.F.O.,

to

leave

the

COMPRESSOR

SPEED

switch

for

a

certain

period

of

time

(10-15

seconds)

in

each

position

before

it

is

switched

to

its

When

the

compressor

is

fed

with

the

signal

from

the

B.F.O.

via,

for

example,

a

transducer,

i.e. a

direct

loop

system

is

used,

it

should

be

noted

that

at

low

frequencies

the

choice

of

too

high

a

re

g

ulation

speed

will

result

in

distortion

of

the

B.F.O.

output

signal

because

the

regulating

amplifier

in

this

case

tries

to

control

the

signal

wave

shape

itself

instead

of

the

output

level.

The

6

lowest

frequency

at

which

a

particular

compressor

speed

should

be

used

is

indicated

around

the

COMPRESSOR

SPEED

switch

(distortion

limit

approx.

2

%).

High

pass

filters

in

the

compressor

input

circuit

make

the

regulation

arrange-

ment

insentive

to

undesired

,

very

low

frequency

variations

in

the

drive

signal,

such

as

those

produced

by

large

surges

in

the

power

supply

voltage

or

mechanical

interference

in

vibration

test

systems.

The

limiting

frequency

of

these

filters

for

frequency

independent

operation

of

the

cnmpressor

coincides

with

the

above

mentioned

LOWER

LIMITING

FREQUENCIES

as

marked

around

the

COMPRESSOR

SPEED

switch.

A

level

change

of

2 dB

in

the

input

signal

to

the

regulating

amplifier

results

in

a

change

of

approximately

45 dB

in

the

B.F.O.

output

voltage.

In

other

words,

the

output

level

from

a

transducer,

fed

by

the

B.F.O.,

can

be

kept

constant

to

within

2 dB

when

the

variation

in

level

transfer

through

the

transducer

is

within

45 dB.

See

Fig.

1.2

where

the

regulation

characteristic

is

illustrated.

The

compressor

circuit

can

be

switched

off

by

means

of

a

separate

"On-Off"

switch.

To

ensure

good

regulation

at

all

output

levels

the

voltage

from

the

fixed

oscillator

is

raised

approximately

10 dB

when

the

automatic

regulation

is

switched

on.

A

band-pass

filter

in

the

form

of

two

inductively

coupled

circuits

is

inserted

in

the

anode

circuit

of

the

variable-,u

pentode

amplifier

to

prevent

any

unwanted

harmonics

generated

by

the

non-linearity

of

the

variable-,u

circuit

from

reaching

the

mixer

stage.

Partial

Blocking

of

Frequency

Range.

As

previously

mentioned,

the

frequency

scale

is

logarithmic

and

calibrated

2-2ooo

Hz.

When

the

capacitor

is

set

to

frequencies

above

2ooo

Hz

or

below

2

Hz

the

fixed

Oscillator

can

be

blocked,

and

consequently

no

output

voltage

will

be

obtained.

For

automatic

recording

of

frequency

characteristics,

i.e.

when

using

the

Level

Recorder

Type

2305,

this

is a

great

advantage

as

no

unwanted

curves

will

then

appear

on

the

corresponding

section

of

the

fre-

quency

calibrated

paper

.

The

cut-off

section

can

be

made

wider

by

adjusting

the

cam

discs,

connected

to

the

rear

end

of

the

capacitor

spindle.

However,

if

the

REMOTE

CONTROL

plug

is

removed

there

will

be

no

blocking

at

any

part

of

the

scale.

In

application

where

the

B.F.O. is

employed

in

conjunction

with

the

B & K

Level

Recorder,

and

where

automatic

recording

is

required,

the

blocking

arrangement

can

also

be

used

for

remote

lifting

of

the

Level

Recorder's

writing

pen.

This

is a

great

asset

in

for

example

measurements

where

the

compressor

circuit

of

the

B.F.O. is

used.

In

this

instance

the

pen-lifting

arrangement

of

the

Level

Recorder

can

be

controlled

from

the

frequency

blocking

circuit

by

making

the

appropriate

connections

to

the

REMOTE

CONTROL

jack

of

the

B.F.O.

In

cases

where

the

entire

frequency

range

(

2-2ooo

Hz)

of

the

B.F.O. is

utilized,

the

normal

frequency

blocking,

which

functions

outside

the

scale

graduation,

should

be

set

out

of

operation.

The

7

writing

pen

of

the

Level

Recorder

can

now

be

lifted

fro~

the

paper

outside

the

freque~cy

range

of

interest

and

a

proper

working

of

the

compressor

also

at

the

initial

frequency

(2

Hz)

is

ensured

during

the

automatic

scan.

If

the

described

method

is

not

utilized,

the

following

would

take

place:

No

signal

will

be

present

in

the

range

2ooo

Hz

to

2

Hz

(outside

the

scale

graduation).

The

compressor

of

the

B.F.O.

will

be

in

such

a

condition

that

when

the

scale

pointer

goes

inside

the

scale

graduation

(2

Hz)

full

output

level

will

not

be

transmitted

at

2

Hz,

and

after

the

chosen

time

delay

(Compressor

Speed}

the

signal

level

will

rise

to

the

proper

(preset)

value.

A

deflection

on

the

recording

paper

which

is

not

a

response

of

the

measured

object

would

thus

be

recorded.

Output Amplifier Section.

The

voltage

from

the

low

pass

filter

is

fed

to

the

control

grid

of

the

first

tube

in

the

two

-

stage

output

amplifier

via

a

variable

potentiometer,

which

is

used

to

control

the

output

level

of

the

instrument,

when

the

compressor

circuit

is

not

in

use.

This

potentiometer

is

marked

OUTPUT

LEVEL

on

the

front

panel.

The

gain

of

the

amplifier

is

stabilized

to

a

suitable

degree

by

means

of

a

voltage

negative

feedback

circuit.

In

order

to

overcome

the

use

of

heavy

and

large

sized

components

(un-

fortunately

necessary

at

low

frequencies),

the

output

stage

has

been

designed

as

a

single-ended

push-pull

circuit

using

two

EL86

tubes

or

their

equivalents.

Output Circuit.

The

output

circuit

is

coupled

to

an

impedance

matching

circuit,

employing

an

auto-transformer

as

matching

device.

This

provides

four

different

output

impedances

that

can

be

selected

with

the

switch

on

the

front

panel

marked

MATCHING

IMPEDANCE.

The

different

settings

of

the

switch

indicates

"6

0",

"60

lJ",

"600

lJ"

and

"6000

lJ"

respectively.

It

should

be

noted

that

the

actual

output

impedance

of

the

B.F.O.

is

10.-20%

only

of

the

indicated

values,

but

with

a

loading

corresponding

to

the

indicated

value,

maximum

power

output

is

obtained

with

a

minimum

of

harmonic

distortion.

The

output

voltage

from

the

auto

-

transformer

is

available

at

the

output

terminals

marked

LOAD,

the

right

hand

terminal

of

which

is

grounded.

A

fifth

position

of

the

MATCHING

IMPEDANCE

switch

is

marked

"Att.".

In

this

position

it

connects

the

output

of

the

amplifier

to

an

attenuator

with

10

steps

of

10

dB

providing

output

voltage

values

from

120 p,V

to

12 V.

This

variable

attenuator

switch

is

marked

ATTENUATOR,

and

the

output

is

con-

nected

to

the

screened

socket

at

the

top

right

hand

side

of

the

front

panel.

The

output

impedance

of

the

attenuator

circuit

is

approximately

50 Q

and

is

constant

over

the

entire

frequency

range.

Output Voltmeter.

The

voltage

at

the

output

terminals

of

the

instrument

is

indicated

by

a

vacuum

tube

voltmeter

which

measures

the

average

value

of

the

output

8

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