Bradley 233 - Page 15

40 pages

Save Page as PDF

To Next Page

To Next Page

To Previous Page

To Previous Page

Loading...

(b)

Source

Termination

One

end

of

a

cable

must

be

matched

to

prevent

multiple

reflections.

Most

pulse

generators

are

designed

to

use

only

one

termination

at

full

amplitude,

otherwise

the

voltage

or

current

requirements

are

double.

Because

the

load

to

be

driven

by

a

general

purpose

pulse

generator

should

preferably

be

unspecified,

the

more

satisfactory

solution

is

to

match

the

source.

With

source

termination

the

amplitude

of

the

wavefront

launched

into

the

cable

is

half

the

open

circuit

output

voltage.

This

is

because

the

cable

itself

presents

a

SOP

load

to

the

pulse

generator

until

the

wavefront

is

reflected.

If

the

end

of

the

cable

is

open-

circuit

100%

reflection

occurs,

but

the

reflected

wavefront

is

absorbed

in

the

source

termination.

The

waveform

at

the

end

of

the

cable

is

a

single

step

at

the

full

open

cir

cuit

amplitude.

The

overall

effect

is

that

at

the

end

of

the

cable

the

pulse

generator

presents

a

clean

pulse

from

a

SOP

source

impedance.

A

clear

demonstration

of

the

superiority

of

source

termination

is

provided

by

exam

ination

of

the

case

of

a

load

consisting

of

a

4.7/nF

capacitor

with

one

inch

leads.

With

end

termination,

rings

of

S%

full

amplitude

are

still

present

after

2S0nsec(4ft

cable

length).

This

ringing

is

superimposed

on

the

CR

response.

Fig.

A.

Using

source

termination

the

waveform

is

a

single

2V

Snsec

spike

followed

by

a

clean

CR

response.

The

spike

is

caused

by

the

lead

inductance

(of

about

50nH)

differentiat

ing

the

output

pulse.

There

is

no

visible

reflection

and

the

cable

length

does

not

affect

the

waveform.

Fig.

B.

Fig.

A.

Fig.

B.

Deflection

Factor

2V

50ns

-

10

-