Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Manual do Utilizador descarregar pdf (Página 74)

100

zero pulse -generator output is

equivalent to a short across the

generator

input.

Referring back to Fig. 9, you can see that that

effectively places the voltage

directly across the resistor. The

positive plate of the

capacitor

brought to ground so the

output

goes negative

with

respect to ground. Since we let the

capacitor

charge

fully

+ 10 volts,

the

output

conversely

goes to -10

volts.

The capacitor then

begins

to discharge through the resistor.

The

discharge curve

looks

just like that shown earlier in Fig.

In one time constant, the output voltage

drops to 36.8% of

the

initial capacitor

voltage

or .368

x -10

= -3.68

volts.

After

5 time

constants

or 1 mS,

the capacitor fully discharges to

zero. You can see the effect in Fig. 10. As you can see, the

differentiator produces

narrow

positive

and negative pulses at

the switching points of the input pulse. The width of those

pulses depends upon the time constant. If the time constant is

very

short compared to the input pulse width,

the capacitor

charges

and discharges, fast creating

very-

narrow output

pulses.

See

Fig. 10.

On the other hand, if the time

constant is long compared to

the

pulse duration of the input pulse, then the capacitor may

not fully

charge

discharge before the input pulse switches.

That condition is illustrated in Fig. 11.

Assume that the time

constant is long compared to the pulse width. During the

input pulse, the capacitor charges up to

+6 volts. The

resistor

voltage will

drop from

10 to

volts as the figure

shows. Then, the input

switches

to zero. The capacitor is

connected

across

the output resistor as

described before.

But

it only has 6 volts on it so

the output switches to -6

volts.

Then the capacitor discharges through the resistor

to about

-2.4

volts

before the input pulse switches

again.

With

the pulse on again, the +10 input adds to the -2.4

volts on

the capacitor

giving

10-2.4 = 7.6 volt

output pulse.

The capacitor begins to charge to this

value

and again the

output

decreases.

With

larger time constants,

the capacitor never fully

charges or

discharges during the

input

pulse. As a result, the

output is not what

you could really

call

pulses.

It is instead

more of a distorted square wave. If you

make the time

constant

very

long compared

to the pulse duration, the capac-

itor

will

charge

very

little

and the output

will

very

nearly

the same

as the input as Fig. 11 shows.

To create short, narrow

pulses

as desired, the differentiator

must have

very short

time constant compared

to the pulse

duration.

call the pulse duration td and the

time constant

then for a good differentiator:

T4td

td>T

where >means

much greater

than and

<means

much less

than. For a good differentiator,

the pulse

width

ought

to be at

least

ten times the time

constant.

Now, let's look at an integrator

circuit. It too is a simple

series

RC circuit except the output is

taken from across the

capacitor.

See

Fig.

12.

Let's again assume the input

pulses of Fig. 8A.

When

the

input switches

from

0 to +

volts,

the capacitor acts initially

as a short so the output

is zero. But then, the capacitor begins

to charge toward

volts.

The output rises with the shape

you saw back in Fig. 6.

If the pulse duration is long

enough.

the capacitor

will

fully charge before the pulse switches back

+10

INPUT PULSE

DURATION

VERY

LONG

TIME

CONSTANT

COMPARED TO

THE PULSE

DURATION

V e = 4V

-6v

+7.6V

Fig. 11 -Pulse

distortion is

easily created in

differentiator with a long

time constant. Notice how

the

voltage

does not

stabilize, but drifts

all over the place.

PULSEINPUT

R 10K

ANN-

C = .02µF

Fig. 12 -An integrator setup

similar to

that

of a

differentiator, except

the

output

is taken off the capacitor.

Its action

is therefore the

opposite

of a differentiator.

.10V

0 -

í -

Fig. 13 -The output of an

integrator is

a distortion of

the

input

pulse.

you

make the time

constant shorter

comparison

to the

pulse

duration, the output will begin to

resemble the input

and

closely.

zero.

In other words,

the time

constant is short compared

the

pulse duration.

When

the input

pulse switches

back

to zero,

the generator

acts

as a short. Therefore, the

capacitor

will

begin to dis-

charge. If

the time constant is short,

the capacitor

will

fully

discharge before the pulse

changes positive again.

Using the RC and pulse-

duration

values given

earlier, the

input

and output waveforms will

be as shown in Fig. 13. That

is not

a particularly useful

waveform.

It is just

distorted

form

of the input pulse. Making the time

constant even

shorter will

cause the capacitor to charge and discharge more

quickly so

that the output looks very

much like the input as

shown

by the dashed lines in Fig. 13.

(Continued

on page

106)

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