BRUEL & KJAER 1623 - Using the Tracking Frequency Multiplier; Using the Special Noise Filter

BRUEL & KJAER 1623

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4.6.

USING

THE

TRACKING

FREQUENCY

MULTIPLIER

Once

the

1

623

has accepted a

tuning

signal (indicated by an

illuminated

Track

window

and a stable

frequency

display)

the

filter

can be

tuned

to

any

harmonic

or

subharmonic

of

this

frequency

within

the

range

99/1

to

1/99

and

within

the

frequency

range

2Hz

to

20kHz.

The desired

ratio

is set on

the

TRACKING FREQUENCY MULTIPLIER

Numerator

and

Denominator

knobs.

For

example,

if

a

photoelectric

or

magnetic

pickup is used

to

provide

a

tuning

signal

from

the

blades

of

a

five-bladed

fan,

the

filter

is

tuned

to

the

fundamental

rotation

frequency

by

setting

the

TRACKING FREQUENCY MULTIPLIER

to

1/5.

If

it

is

further

required

to

exam-

ine

the

5th

harmonic

of

the

fundamental

rotation

frequency

one

should

set

the

tracking

frequency

ratio

to

5/5

rather

than

1/1

because

any

slight

unevenness

in

the

angular

spacing

of

the

blades

will

be averaged

out

. This is

due

to

the

tuning

circuitry

in

the

1623

only

seeing one

of

the

five pulses

from

the

pickup

per

revolution.

It

should

be noted

that

the

frequency

range

2Hz

to

20kHz

applies

to

the

filter

tuned

fre-

quency.

It

is,

to

a

certain

extent,

possible to

tune

the

filter

from

signals

which

have

fre-

quencies

outside

these

limits

if

the tracking

frequency

multiplier

is used

to

bring

the

tuned

frequency

inside

the

limits

and

assuming

a

well

defined

waveform

of

high

level.

4.7.

USING

THE

SPECIAL

NOISE

FILTER

The

right

hand

pushbutton

of

the

TRACKING

MODE

pushbuttons

marked

Noise

Filter

couples a

low

pass

filter

into

the

tuning

signal

input

which

can

be used

where

it

is

su-

spected

that

high

frequency

ripple

may

be

disturbing

a

low

frequency

tuning

signal.

The

rejection

characteristic

of

the

noise

filter

is

shown

in Fig

.4.

7.

0

10

20

40

50

60

20 30 40 50

~

~

\

\

\

100

200 300 500

\

(

~

I

1000 2000

Frequency Hz

~'--..

""'

3000

770311

Fig. 4.

7.

Frequency

response

of

the

special

low-pass

noise

filter

in

1623

In

normal

circumstances

with

the

Noise

Filter

pushbutton

in,

the

noise

filter

automati-

cally

becomes

inoperative

when

the

tuning

signal

frequency

exceeds approx.

200Hz

and

couples

in

again

when

the

tuning

signal

frequency

falls

below

approx.

200Hz.

23

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