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3.7.2

Normal/Complement

The

complement

function

logically

inverts

the

input

signal

to

the

output

amplifier.

The

levels

of

the

two

output

states

are

unchanged,

but

where

the

output

was

in

one

state,

it

goes

to

the

other.

This

function

is

useful

when

pulses

of

duty

cycles

approaching

100%

are

required

or,

in

combination

with

the

polarity

switch

to

obtain

a

greater

range

of

pulse

baseline

offset,

(see

3.7.3).

3.7.3

Amplitude

and

Offset

Polarity

When

the

offset

is

set

to

zero

and

a

positive

pulse

is

selected,

the

output

is

a

pulse

of

positive

amplitude

with

a

baseline

at

earth

potential.

Clockwise

rotation

of

the

offset

control

will

lower

the

baseline

to

-lOV.

If

a

pulse

of

high

duty

cycle

is

required,

select

"Comp".

If

positive

offset

is

required

without

change

of

duty

cycle,

reverse

the

state

of

both

"Norm/

Comp"

and

"+/-

pulse"

buttons.

With

negative

pulse

selected

the

baseline

of

the

pulse

becomes

the

more

positive

level.

This

upper

level

of

the

pulse

can

now

be

varied

between

0

and

+10V.

3.7.4

Summed

Outputs

When

the

"Sum"

button

of

Output

1

is

pressed,

the

amplitude

and

offset

of

Output

1

amplifier

is

added

into

Output

2.

To

prevent

distortion

the

controls

must

be

set

so

that

no

part

of

the

combined

waveform

lies

outside

the

range

-lOV

to

+10V.

Output

2

toggle

switch

must

be

in

the

internally

terminated

position.

It

is

preferable

to

terminate

Output

1

to

prevent

deterioration

of

the

Output

-3TTkiiftlt.tiJFs.

To

obtain

an

amplitude

greater

than

lOV,

select

the

same

signal

source

and

output

polarity

on

each

output

using

one

offset

control

to

keep

the

output

in

the

-lOV

to

+10V

rai^e.

3.7.5

Termination

;

Discussion

(a)

End

Termination

It

is

a

common

convention

to

quote

pulse

generator

amplitudes

into

50S2.

This

pre-supposes

that

a

50fi

terminating

load

to

earth

is

always

used

at

the

far

end

of

the

co-axial

cable

from

the

pulse

generator,

thus

matching

the

cable

(which

should

be

50fi

characteristic

impedance)

and

preventing

reflections.

Output

pulses

photographed

under

these

ideal

conditions

enhance

the

advertising

literature.

In

normal

use,

however,

the

load

to

be

driven

is

rarely

a

true

50fi

resistance,

and

is

often

reactive

and

non-linear.

When

such

a

load

is

connected

across

the

50S2

termination,

reflections

occur,

and

if

the

generator

is

not

carefully

matched

to

the

cable

repeated

reflections

at

each

end

of

the

cable

cause

ringing

of

slowly

dying

amplitude

at

the

load.

At

high

pulse

repetition

frequencies

these

rings

combine

with

the

next

transmitted

pulse,

addding

to

or

subtracting

from

the

leading

edge,

depending

on

phase.

Many

pulse

generators

are,

however,

dependent

on

a

50fi

load

for

their

correct

functioning.

It

is

usually

only

when

they

are

attenuated

to

half

amplitude

or

less

by

the

front

panel

controls

producing

a

fairly

good

match

at

the

generator

that

the

problem

of

multiple

reflections

is

overcome.

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9

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