Feedback Sfg611 User manual

~

m

D

w

IL

•

I

I • FEEDBACK INSTRUMENTS

LIMITED

Telephone: Crowborough (089 26) 3322

Manual

611

EdC

0484

12MHz

Sweep

Function Generator SFG611

12MHz

SWEEP

FUNCTION GENERATOR SFG611

Contents

Section 1 Description and Specification

Section 2 Operation

Section 3

Circuit

Description

Section 4 Maintenance

CROWBOROUGH SUSSEX

Cables: Feedback Crowbr Telex ENGLAND

Telex:

95255

Printed in England

by

Fl

Ltd,

Crowborough

3

Feedback

Instruments

Limited

Component replacement

Although

this Feedback manual

was

believed

to

be

correct at

the

time

of

printing,

components supplied may

differ

slightly

from

those described.

We

endeavour

to

improve

our

equipment

continually

by

incorporating

the

latest developments and components,

even

up

to

the

time

of

despatch.

if

it

is

practicabie we

include such new

or

revised

information

in

the

manual.

Whenever possible, replacement components should be

similar

to

those originally supplied. These may

be

ordered

direct

from

Feedback Instruments

Limited

or

its agents

by

quoting

the

following

information:

1.

Equipment

type

2.

Equipment

serial number

3.

Component reference

4. Component value

Standard components

can

often

be

replaced

by

alterna-

tives available locally.

COPYRIGHT

NOTICE

@Feedback

Instruments

Limited

1982

All

rights reserved.

No

part

of

this

publication

may be

reproduced, stored in a retrieval system,

or

transmitted,

in any

form

or

by

any means, electronic, mechanical,

photocopying,

recording

or

otherwise,

without

the

prior

permission

of

Feedback Instruments

Limited.

4

The Health

and

Safety at Work Act

1974

We

are

required under the Health and Safety at

Work

Act

1974,

to

make available

to

users

of

this equipment

certain

information

regarding its

safe

use.

The equipment,

'vvhen

used

in normal

or

prescribed

applications

within

the

parameters set

for

its mechanical

and electrical performance, should

not

cause

any danger

or

hazard

to

health

and

safety

if

normal engineering

practices

are

observed and

they

are

used

in accordance

with

the instructions supplied.

If,

in specific

cases,

circumstances exist in which a

potential hazard may

be

brought about

by

careless

or

improper

use,

these

will

be

pointed

out

and

the

necessary

precautiqns emphasized.

While we

attempt

to

give the fullest possible user

inform-

ation in

our

handbooks,

if

there

is

any

doubt

whatsoever

about any aspect relating

to

the proper

use

of

this equip-

ment

the

user should contact the Product Safety

Officer

at Feedback Instruments

Limited,

Crowborough.

I,

I

II

,f

I

,I

1

I

DESCRIPTION AND SPECIFICATION

Introduction

The 12MHz

Sweep

Function Generator SFG611

is

a

high-performance instrument offering versatile facilities

which simplify many test measurements in the 0.01 Hz

to

12M

Hz

frequency

range.

The push-putton selected waveforms, sine, square

or

triangular,

are

available at levels up

to

1

OV

pk-pk open

circuit from

50ft

The continuous amplitude control,

in conjunction

with

two

fixed attenuators

of

x0.1 each,

give

a suitable

range

of

output

level control. The d.c

offset

facility

gives

a unipolar capability

with

all com-

12MHz

Sweep

Function

Generator SFG611

SECTION 1

binations

of

signal

and

offset free

of

clipping.

Auxiliary

outputs provide a triangular waveform

and

a

TTL-

compatible square waveform capable

of

driving

two

standard

TTL

loads.

The frequency

range

is

selected

by

six push-buttons in

decade

ranges

together

with

a (Hz/1 000) switch,

applicable

to

all

ranges.

The mark/space ratio

of

all the waveforms

can

be

varied

by a continuous control in conjunction

with

three

switch positions.

Fig

1.1

5

Feedback

Instruments

Limited

The comprehensive sweep facilities can

be

used

in either

logarithmic

or

linear mode, covering the need

for

overall

frequency response

or

particular narrow-band measure-

ments

on

465kHz

or

10.7MHz

IF

circuits,

for

example.

Internal

or

external sweep

(VCF)

can

be

selected

by

means

of

a slide switch. The internal sweep generator

is

continuously

adjustable

from

50Hz

to

0.001

Hz

by

means

of

a single logarithmic

control.

A triangular

frequency sweep enables

both

directions

of

the sweep

to

be displayed, and

an

associated auxiliary

output

is

provided

for

driving the oscilloscope timebase. Also

incorporated

is

a 'single-shot' sweep

facility,

enabling the

SFG611

to

be

used

with

X-Y

recorders, a

TTL

square

wave

output

then operates

as

a pen

lift

control.

An

effective bright-trace marker gives high-resolution

calibration

of

response features

for

use

with

an

external

frequency meter.

6

Mechanical

The SFG611

is

housed in

an

ABS plastic

case

made in

two

halves

each

secured by

two

screws on

each

side.

The

case

provides the main structural strength

of

the

instrument. Removal

of

the

case

gives

access

to

all

components.

Without

the cover the SFG611 consists

of

a horizontal

PWB

fixed

by

small plastic brackets

to

the

front

and rear panels, providing a structure strong

enough

for

normal handling and maintenance. The

low

power dissipation

of

the SFG611 obviates the need

for

ventilation

holes.

All

controls

are

situated on the

front

panel, fig 1.1.

I

~

I

Specification

Frequency

Range

0.01

Hz

to

10M

Hz.

Selected

by

six pushbuttons in

decade

ranges

from

10

to

10M, plus (Hz/1000) switch

position applicable

to

all

ranges.

Range

Overlap

Frequency dial calibrated nominally 1

to

10,

with

over-

range from 0.2

to

12

Main

Output

Waveforms

Sine, Square, Triangular,

with

pushbutton selection

Source Impedance

son

Amplitude

1

OV

pk-pk maximum open circuit

Amplitude

Control

Single continuous control calibrated 0

to

1

OV

pk-pk (at

nominal 1V pk-pk intervals)

Stepped

Attenuator

Two

fixed attenuators selectable x0.1

each

12MHz

Sweep

Function

Generator SFG611

Offset

Adjustable fro,n

-5V

through 0

to

+5V giving unipolar

signal capability.

All

combinations

of

signal and offset

are free

of

~lipping.

Offset

not

subject

to

continuous

output

amplitude control

Termination

BNC socket

Mark/Space

Control

Affects all waveforms from main and auxiliary outputs.

Continuous control operational in threeswitch positions:

a)

fixed mark, variable

space,

continuously variable

from

1

:1

to

1:19

b)

Continuously adjustable mark

space

ratio

from

19:1

to

1:19, at a fixed frequency

of

approximately x0.1

main frequency setting.

c)

variable mark, fixed

space,

continuously variable

from

1

:1

to

19:1

7

Feedback Instruments

Limited

Sweep Facility

Control

Slide switch enables internal

or

external sweep (VCF)

to

be selected

External

Gives normal unswept operation,

but

a voltage applied

to

the

'VCF'

socket modifies the frequency

of

operation

as

follows:

Linear mode: Voltage applied

to

the VCF

input

in-

creases

frequency at

the

rate

of

about

+0.3V

per 1 major

scaledivison (negative voltages decrease frequency).

Logarithmic mode: Voltage applied

to

the VCF

input

decreases

frequency at

the

rate

of

about 5V per factor

of

10.

Internal

Provides a triangular waveform

from

the socket

'TB

out

/''

and a corresponding square waveform

from

the

'TB

out

U ' socket. Sweep frequency rate adjustable

by

sinQie

logarithmic

control

from

50Hz

to

0.001 Hz.

Triangle amplitude:

5V

pk-pk

(OV

to

+5V)

from

1

k.O.

Square waveform

TTL:

(OV

to

+5V)

to

drive

two

standard loads available

from

'TB

out

U

'.

Operating Modes

Lever switch

'run/S-S/trig'

gives

continuous operation

in

'run'

position. When moved

from

'run'

to

'S-S'

(single shot) the triangle

output

goes

to

zero

and

then

executes a half cycle

(OV

to

+5V) when the switch

is

moved

to

the

'trig'

position. When operated in the

'S-S'

8

condition

the

'TB

out

lf

· may

be

used

as

a 'pen

lift'

control

for

X-Y

recorders. The 'set

max-min'

spring

return lever switch over-rides the sweep triangle generator

to

hold the waveform at its maximum (5V)

or

minimum

(OV) excursion. In the run condition this maximum

or

minimum

is

held

only

during the operation

of

the

'set

max-min'

switch.

In

'single-shot' the conditon

of

maximum

or

minimum

is

held until some other con-

dition

is

initiated.

Frequency Marker

An effective bright-up marker

for

use

with

oscilloscope

displays

of

frequency

responses

is

available

as

ahesitation

in

the

sweep triangle voltage. The position

of

this

is

controlled

by

a 'marker' control. The triangular voltage

and frequency at which this occurs

can

be

held

by

an

associated 'marker' pushbutton.

Sweep Width

This control

gives

the

proportion

of

the sweep triangle

voltage

used

to

control the frequency

of

the internal

sweep generator. The

scale

is

calibrated

from

0%

to

80%

for

linear sweep,

and

from

0

to

3

decades

for

logarithmic

sweep. In both conditions

('lin'

and

'log') the maximum

frequency

is

set

by

the main dial

and

occurs when the

triangle

output

voltage from

'TB'

is

maximum (+5V).

The calibration on the

sweep

width

control indicates the

sweep

width

as

a

proportion

of

the main frequency

setting.

e.g.

linear mode, main dial 50kHz, sweep-width

80%,

gives

sweep

from

10kHz

to

50kHz, sweep-width

two

decades,

gives

sweep from 500Hz

to

50kHz.

I

Auxiliary

Outputs

TTL

Square

wave

Amplitude

TTL

compatible (nominal 0

to

+5V)

Rise time

<50ns

unloaded

or

(50+4.4C)

ns

with

CpF added capacitance

Sink capacity

Two

standard

TTL

loads

Termination BNC

Triangular

wave

Amplitude 1V pk-pk

Impedance

50S1

Termination BNC

Power

Requirements

Line voltage

110/220/240V,

50

or

60Hz, selected

by

external slide.

Consumption

25VA

Fuse

31-5mA slow

blow

(20mm x 5mm)

12MHz

Sweep

Function

Generator

SFG611

Dimensions

and

Weight

Width

Height

(with

feet)

Depth

Weight

300mm (12in)

115mm (4.5in)

226mm (9in)

2.7kg

(61b)

9

..,

0 11)

11)

a.

a-

Q)

n

"

~

c

3

11)

~

r

3"

;::;·

CD

a.

--- - ------

I

OPERATION

Installation

of

SFG611

The

function

generator

is

packed

in inserts

of

expanded

polystyrene

to

prevent

damage

in

transit.

On

opening

the

end

of

the

corrugated

cardboard

container

the

inserts

together

with

the

SFG611

should

be

withdrawn

smoothly

from

the

container.

Take

care

that

the

inserts

and

SFG611

are held

together

during

this

time.

Inspect

the

SFG611

and

if

any

damage

is

evident

immediately

notify

the

carriers.

Voltage selection

Ensure

that

the

instrument

is

set for

the

appropriate

supply

voltage

by

inspecting

the

voltage

selector

on

the

back

panel. Reset if necessary

before

connecting

to

the

supply.

Wire

connections

The

colour

code

of

the

power

supply

cable is:

Brown

Blue

Green/Yellow

Live

Neutral

Ground

The

ground

wire

is

connected

to

the

front

and

back

panels

of

the

SFG611

and

to

the

terminal

marked

'

on

the

front

panel.

Preliminary settings

Users

unfamiliar

with

the

instrument

are

recommended

to

set

the

controls

as follows

before

reading

further,

if

they

wish

to

try

out

the

controls

as

they

are

described.

12MHz

Sweep

Function

Generator SFG611

SECTION 2

This

procedure

may

also be helpful

at

any

time

that

a

fresh

activity

is

to

be

started.

Set:

'marker

position'

off

(anti-clockwise

to

operate

switch)

'run-s/s-trig'

switch

to

'run'

sweep

switch

to

ext

'log-lin'

to

'lin'*

attenuator

buttons

out

'offset'

to

central

position

(zero

offset)

'output

volts'

to

a

convenient

value

for

observation

*

Note

It

is

recommended

that

the

SFG611

is

always

operated

in

the

'lin'

mode,

unless log sweeping

is

required,

as

this

is

the

mode

that

gives

minimum

drift

and

the

most

accurate

frequency

calibration.

Frequency selection

If a

frequency

below

1OHz

is

required,

move

the

'log-lin'

switch

to

the

'Hz/1

000'

position.

For

frequencies

above

10Hz

select

'lin'

or

'log'.

(Other

possibilities will

become

apparent

in

later

paragraphs).

Press

the

frequency

range

selector

pushbutton

whose

adjacent

markings

span

the

desired

frequency

(in Hz

or

mHz

as

the

case

may

be). Finally

set

the

'Frequency'

dial

to

obtain

that

particular

frequency

in

the

range.

11

Note

that

each actuai range

extends

somewhat

above

and

below

the

nominal

range indicated

by

the

switch

markings.

Thus

for a

frequency

of

5Hz

the

'log-lin'

switch

would

be

set

to

'Hz/1

000',

the

required

range

button

would

be

'1

K-10K'

and

the

'Frequency'

dial

would

be

set

to

5.

Note

that

the

frequency

calibration

is

altered

when

the

'mark/space'

facility

is

in

use

(bracketed

positions

of

the

log-

I

in

switch;

see

'Mark/Space

ratio

control',

Outputs

Main

output

One

of

the

three

waveform

selector

buttons

should

be

depressed

to

select a triangle,

sine

or

square

wave

as

required.

The

selected

waveform

will

then

appear

at

the

output

socket

with

an

amplitude

which

may

be

set

by

the

'output

volts

pk-pk'

dial,

whose

calibration

should

be

correct

when

neither

of

the

attenuator

buttons

is

depressed,

and

no

load

is

connected

to

the

output

socket.

This

output

waveform

may

be raised

or

lowered

5V

in

d.c level

by

turning

the

'offset'

control.

Thus

if

the

amplitude

is

set

to

say 6V pk-pk (±3V pk), and +4V

of

offset

are

applied,

the

resulting signal will oscillate

between

+1

V

and

+7V. It

is

always possible

to

set

every

part

of

a

waveform

to

be

of

the

same

polarity

in

this

way,

and no

combination

of

adjustments

will cause

clipping

of

the

combined

(alternating

plus

offset)

output.

The

combined

waveform

with

offset

is

connected

to

the

output

socket

effectively

in

series

with

a

50-ohm

source

impedance,

either

directly,

or

through

either

or

both

of

12

the

attenuators

introduced

when

the

'X0.1'

buttons

ara

depressed.

Each

button

introduces

20d8

(X0.1

in

voltage)

attenuation,

leaving

the

output

impedance

unchanged.

Auxiliary

outputs

Two

auxiliary

outputs

are provided which are

synchron-

ous

with

the

main

outputs.

Their

relative phases are

indicated

in

fig

2.1.

Their

amplitudes

and levels are

unaffected

by

the

main

output

controls.

The

auxiliary

square

wave

output

can

be

used

to

drive

two

TTL

loads.

Alternatively,

if it

is

loaded

with

50

ohms,

it

produces

a fast

square

wave,

OV

to

0.6V

approximately.

QUX

(

Vpk-pk

outputs +

sv

I I

ov

_

__

_ I

ma1n

output

square

triangle

s·me

I I L

A 8 A

Fig 2.1

I

Mark/space ratio control

This

control

is

brought

into

operation

when

the

'log-lin'

switch

is

in

any

of

the

three

bracketed

positions

adjacent

to

the

control.

With

the

switch

turned

to

U,

i.e clockwise

one

step

from

'lin',

the

mark/space

control

affects

only

the

half-cycle

indicated

by

A in

fig

2.1,

slowing it

down

by

a

factor

which

may

be

adjusted

between

1 and

19

times

compared

·with

the

other

half-cycle,

whose

duration

is

unaffected.

Similarly

with

the

switch

two

positions

further

on

atn,

the

control

affects half-cycle

B.

With

the

switch

in

the

intermediate

position,

marked

Hz/1

0,

both

half-cycles are slowed

down.

The

relative

durations

can be

adjusted

by

the

rotary

control,

and

the

time

of

the

whole

cycle

is

increased

to

approximately

ten

times

that

indicated

by

the

frequency

calibration

(i.e

frequency

is

reduced

tenfold).

The

frequency

of

the

main

output

waveforms

can

be

altered from

the

calibrated value

by

the

application

of

a

control

voltage. This voltage

may

be derived

from

an

external

source

or

from an internal

sweep

oscillator,

selected

by

the

'ext-int'

switch

(below

the

sweep

rate

control).

If

'ext'

is

selected,

the

sweep

width,

sweep rate and

marker

controls

will have

no

effect.

A signal voltage

applied

to

the

VCF

socket

alters

the

frequency

as

follows:

12MHz

Sweep

Function

Generator SFG611

In all linear

modes

(lin,n,

Hz/10,U,

Hz/1

000)

a positive signal increases

the

frequency,

and

neg-

ative signal decreases it,

at

a rate

of

about

one

major

scale division

(one-tenth

the

nominal fre-

quency

range)

per

0.3V.

In

logarithmic

mode

a positive VCF signal decreases

the

frequency

and

a negative signal increases it

by

about

one

decade

(i.e

factor

of

10) per

5V.

In

neither

case

should

the

VCF

signal

take

a value

which

would

drive

the

frequency

outside

the

limits zero

and

20% above

the

nominal range set by

the

frequency

selector

switch.

13

Swept-frequency

operation

In

order

to

use

the

internal

sweep

oscillator,

set:

'log-lin'

to

either

'log'

or

'lin'

as

desired

'ext-int'

to

'int'

'run-S/S-trig'

to

'run'

With

these settings

the

sweep

oscillator

produces at

the

VCF(fB

socket a

triangular

waveform

which

always

sweeps between

OV

(for

minimum

frequency) and

+5V

(maximum

and

nominal

frequency),

irrespective

of

the

sweep

width

control

setting.

The

rates

of

rise and fall

are equal, and

the

sweep

frequency

is

adjustable over a

wide

range

by

the

'sweep rate'

control.

For

oscilloscope

(~

()~·

O

SfG

611

®o

V\

"-JIV\T

"\.1"

onn

on

o o

DOD

o o

DO

+5Vpk

manitor

displays

of

frequency

response

it

is

recommended

that

an

X-

Y oscilloscope

use

this

voltage

for

X-deflection,

fig

2.2.

The

'sweep

width'

control

selects a

portion

of

that

waveform

for

VCF,

thus altering the

extent

of

the

decrease in

frequency.

Meanwhile

the

adjacent socket

provides a square wave suitable

for

pen

lift

control

of

a recorder,

or

for

triggering

an

oscilloscope

if it

has

no

X-Y

facility.

The

'run-SIS-trig'

control

stops and starts

the

sweep

oscillator

as

follows.

In

the

'run'

condition

the sweep

oscillator

runs con-

tinuously.

XY

oscilloscope

RIR

'----.~.._._.._..,)

X

0

if

required

14

Fig

2.2

Signal connections

for

swept-frequency

operation

(

-!-

not

shown)

I

In

the

'S/S'

condition

the

sweep

signal goes

to

zero and

will remain

there

until a

new

sweep

is

started

by

moving

the

switch

against

the

spring bias

to

'trig'.

Each

time

a

sweep

is

triggered its

TTL

output

goes low while

the

frequency

increases;

during

the

return

half-cycle

of

the

sweep

oscillator

(frequency

decreasing)

the

TTL

output

goes high.

The

TTL

output

can

therefore

be

used

as

a

'pen

lift'

control

when

recording

frequency

responses.

12MHz

Sweep

Function

Generator SFG611

Frequency

setting

for

internally-swept

operation

using

the

calibrations

of

the

SFG611.

The

maximum

frequency

of

a

sweep

should

be

set

first.

It

is

simply

that

which

is

set

on

the

normal

frequency

controls.

The

minimum

frequency

is

that

frequency

multiplied

by

a

factor

(less

than

one)

which

is

determined

by

the

sweep

width

control.

This

may

be

shown

by

~imple

examples.

Example

1.

It

is

required to sweep from

300kHz

to

600kHz

to examine the response

of

a

465kHz

IF

transformer.

First

the

highest

frequency

is

set

by

pressing

the

range

selector

between

1

OOk

and 1M

and

setting

the

frequency

dial

to

6.

Assuming a linear

sweep

is

required,

the

lowest

frequency

is

500/o

of

the

highest.

So

'lin'

is

selected,

and

the

sweep

width

is

set

to

50%

(the

inner

scale).

In

this

case

the

maximum

sweep

rate

might

be

usable,

but

note

the

later

paragraph

on

distortion.

15

Feedback

Instruments

Limited

Example

2.

An

audio

circuit

is

to be tested between

20Hz

and 20kHz. A logarithmic display

of

response

is

required.

'Log'

condition

is

selected.

The

high-end

frequency

is

set

by

selecting range 1

Ok-1

OOk

and setting 2

on

the

frequency

dial.

The

lowest

frequency

is

3 decades lower

than

the

highest; sweep

width

is

therefore

set

to

3.

(More generally

the

number

of

decades

is

the

common

logarithm

of

frequency

ratio).

In

this

example

a

much

lower sweep rate will

be

re-

quired,

certainly well below

the

20Hz

bottom

frequency

of

the

sweep range.

In

setting

up

this display

the

oscilloscope's horizontal

sensitivity

may

conveniently

be adjusted so

that

the

trace

is

just

9cm

long.

The

display scale

is

then

3cm

per

decade,

the

graticule lines corresponding

to

frequencies

of

20,

43,

93,

200,

400Hz

etc.

For

many

practical

purposes

these

can be

taken

as

20,

40,

100,

200,

400Hz

etc,

see fig

2.3.

Frequency

checking

It will be

found

when a

swept-frequency

characteristic

is

being displayed

that

the

main

frequency

control

acts

in

a

manner

similar

to

an 'X

shift':

that

is

to

say

the

display can be moved laterally.

If

for

example it

is

required

to

find

out

the

centre

frequency

of

a

resonant

circuit-

fig 2.4(a)

-all

that

need be

done

is

to

turn

the

16

-

....

-

Oo#

~

(a)

-

....

-

...

I

... ... ...

...

....

a -

-

....

...

-

....

a

pprox

frequency,

Hz

Fig2.3

(b)

Fig 2.4

I

,I

I

main frequency

control

until

the

point

to

be

measured

is

at the right-hand

end

of

the

trace-

fig 2.4(b)

-and

then

to

read

off

the frequency

from

the frequency dial.

If

alternatively

it

is

desired

to

leave

the controls un-

disturbed the frequency

of

interest

can

be scaled

directly

if

the

sweep

is

linear. When

logarithmic

sweep

is

in

use

and the frequency ratio

is

at

all large the desired

frequency f may

be

calculated in terms

of

the end

frequencies f1, f2

by

applying the

following

formula

to

the

lengths I

and

m measured

from

the

oscilloscope face

(fig 2.5).

f =f1 e

mIn

(f

2

/fl)

f.1

f

fl

~

l

_J

~I

Fig2.5

12MHz

Sweep

Function

Generator SFG611

Using external frequency meter

Several controls

are

provided which greatly facilitate

frequency-response

work

when

used

in

conjunction

with

an

external frequency meter. The latter may con-

veniently

be

driven

from

the

auxiliary

TTL

output

near

the main

output

socket.

The end-frequencies

of

the sweep may be accurately set

using the 'set'

control.

If

the

'set'

control

is

moved

to

'max'

the

sweep oscillator

is

held in the

condition

giving

maximum

frequency;

similarly

'min'

gives the

minimum

frequency reached

during

a sweep.

To

adjust the end frequencies

of

a

sweep,

first

switch

to

'max'

and set the main frequency

controls; then switch

to

'min'

and adjust the sweep

width

until

the

desired

minimum

frequency

is

obtained.

While swept operation

is

in progress the frequency

meter

may

be

expected

to

read the average frequency.

Note

that

this

will

be

half the sum

of

the end frequencies

only

if

the sweep

is

linear. During

logarithmic

swept

operation

the

average frequency

is

f (av) =

fmax

-

f

In(~)

f .

mm

17

Feedback

Instruments

limited

Frequency marker

A

marker

is

provided

for

use

with

the

arrangement

of

fig

2.2.

The

marker

controls

are

inoperative

if

the

rotary

marker

control

is

set

to

'off'.

However as

this

control

is

rotated

away

from

the

off

position

the

marker

appears

as a

brightening

of

the

oscilloscope

trace.

It

may

be

posii:ioned

anywhere

across

the

screen.

(It

is

produced

by

causing

the

sweep

process

to

pause

momentarily,

and

therefore

alters

the

sweep

period slightly).

If a

feature

(e.g a resonance)

is

to

have its

frequency

measured,

the

marker

should

be

set

to

mark

the

feature

on

the

display. Pressing

the

marker

pushbutton

will

then

hold

the

frequency

of

the

oscillator

at

the

value

marked;

the

marked

frequency

can

then

be

read

directly

on

the

frequency

meter.

Display Distortion

\Nhen a

swept

frequency

is

used

to

examine

the

charac-

teristic

of

a

circuit

it

is

important

that

the

circuit

under

test

is

given long

enough

to

respond

properly

to

each

frequency.

This

means

that

the

frequency

must

be

swept

sufficiently

slowly. This

is

especially

true

of

resonant

circuits

where

there

is

an

exponential

build

up

of

amplitude

in response

to

the

sudden

application

of

a

resonant

signal as

shown

in

fig

2.6.

18

input

output

I

Fig2.6

I

From

this

familiar response it can be seen

that

the

average response

of

the

resonant

circuit

is

effectively

delayed

in

time

with

respect

to

the

stimulus.

This

sort

of

effect

occurs

when

the

stimulus

is

a

swept-frequency

signal;

the

main response

of

the

circuit

tends

to

occur

after

the

stimulus

is

past

so

that

the

response

is

displaced

(fig

2.7).

(a)

(b)

Fig 2.7

With a sweep

generator

employing

a

'blanked'

and/or

rapid

return

it

is

not

easy

to

tell

when

this

sort

of

distortion

is

occurring. With

the

SFG611

where

12MHz

Sweep

Function

Generator

SFG611

sweeping

occurs

in

both

directions,

the

response

when

sweeping

too

fast

appears

as

in

fig

2.8.

On

reducing

the

sweep

speed

the

two

traces

come

together

to

give a

true

display.

The

SFG611,

by

always providing

sweep

up

and

sweep

down

of

frequency,

gives a

continual

check

on

the

suitability

of

the

sweep

speed

chosen

and also gives

optimum

display

brilliance

through

the

absence

of

trace

blanking.

Further,

the

continuously

adjust-

able

sweep

rate

control

allows

optimum

repetitive

rate

consistent

with

minimal

distortion.

Fig 2.8

191

Feedback Instruments Limited

20

FEEDBACK

/~

OSFG611

0

0-t:;x

()

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@ 0

0 t6

ODDDDD

¥

DOD

00

DD

monitor

r

f

a

-J....--

I

a

j

'-

...

~·:·

~

:·.~.:.:.:.:·~

·

..

:.:.:

·=

·:

·.:

·:

.....

. . . .

...

XY

oscilloscope

tifJ

X

Fig2.9

......

.:.

positive X

f

/:.

-1display for

m

~oo~::t

spot

J

Fig 2.10

I

2.10

Phase

Measurement

of

the

phase

shift

of

a signal in a

system

under

test

may

be

made

as in fig

2.9.

Usually a +ve

voltage

on

the

Y

and

X

inputs

of

the

oscilloscope

moves

the

spot

upwards

and

to

the

right

respectively.

This

may

I

:·:·:·1·:·:·:·

I

-

12MHz

Sweep

Function

Generator

SFG611

be

verified

by

short-circuiting

the

system

to

obtain

zero

phase

shift.

The

display

can

be arranged

symmetri-

cally

about

the

X axis as in fig

2.10,

i.e

a=a'.

To

simplify

calculations

it

is

convenient

to

adjust

the

X

deflection

to

be

nine

scale divisions as in fig

2.11.

/

..

.. . . . . .

Fig2.11

display

for

positive

X

moves

spot

to

left

21

feedback SFG611 User manual

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