Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Manual de usuario descargar pdf (Pagina 55)

100

servo is represented by

the

width of

a pulse.

Many people

think of that as "digital" radio control, but in truth it's more

analog than it is digital. Here's

why:

The transmitter sends

out a pulse

which is proportional

the

position

a control

stick or

lever. If the stick is

all

the

way

down, for example, the

transmitter

will

send out a 1 millisecond pulse. If

it is

all the

way

up it sends out a 2

millisecond

pulse.

For any position

between

the two

extremes

the transmitter

will

send out a pulse

proportional to the position of the stick. Hence the

term

"proportional" control. Notice,

however, that the number of

discreet positions and the

number of pulse

widths

are not

limited to any number of positions. In a true

digital system

only 32 positions, for example,

would

be allowed. Because

the number of positions are not limited, I've always

consid-

ered this system more analog

than digital.

In digital electronics

today pulse -width modulation

probably interpreted

more strictly than in the examples

above. Basically, every pulse begins at a

very definite time

interval. Information

represented

varying

the

width

the pulse. Some people

refer to this as "duty-cycle

modula-

tion"

since the frequency of the pulses are constant

but the

ratio of "on" to "off' is

varied

the signal.

Taking a look at

Fig.l and use table I as a reference

for

conversion. Notice that

in PWM

each

pulse begins at the start

of a cell. The amount of time (units)

that the signal stays high

is proportional

the

average

voltage

of the modulating

signal

during

that cell.

TABLE 1- PULSE -WIDTH CONVERSION

Average

Voltage Pulse Width

(in units)

Uses

for PWM

By now,

you're

probably

wondering why

anyone

would

want

to use

PWM. Three

examples

come to mind: First the

Class

-D audio amplifier. In class "D" audio amplifiers, the

output

transistors

are

switched on and off using a pulse -width

modulation technique. That keeps the power dissipation

the transistors low and results in a high -power amplifier than

runs fairly cool. Since the

speaker

cannot respond

to the fast

rise and fall times of the signal, it "smooths" out the pulses

and can

reproduce the

audio with amazing accuracy.

Second, there are motor -speed controls.A motor's speed is

proportional to the average voltage applied to it, but it's

torque is proportional to the peak

voltage.

By applying pulses

voltage

to it, the speed can be

varied while keeping the

torque high.

Last, switching power supplies make use of pulse -width

modulation. Newer "switching" power supplies regulate the

voltage

delivered to a load by

varying

the

width

of their

output. An output

filter capacitor acts

much like the speaker

in the example above, and a much smaller, lighter, and cooler

power supply

results.

Pulse- Amplitude Modulation

Pulse-

Amplitude Modulation

or PAM is a technique

which information is represented

by the amplitude

of pulses,

although I don't know

whether to call it analog

or digital. I

haven't seen it used

for many years, but

does

have one

interesting application.

It can be used

to multiplex (or

mux)

many signals on to a

single data path.

In PAM the analog

signal is sampled at regular

intervals.

Each time a sample is taken,

a pulse is sent out

with

amplitude equal to the

analog signal. At the other

end, the

pulse is

put through a low

pass filter and the original analog

signal is recovered. The

sample time interval can

be divided

into as

many intervals as required

and many signals can

sent down the same path. Of

course, some method

of sorting

the samples according

to there "time

slots" is necessary at

the

other

end.

To sum up, PAM

simply samples the signal

in the middle of

the cell and outputs the average

value.

Looking at

Ag. 1, note

that another signal

could be placed (muxed)

in between the

samples and more than

one signal could be sent

via the same

transmission line.

the

receiving end, the

signal is sent

through a low

pass filter and the signal is

recovered.

Pulse-

Position Modulation

Pulse-

Position Modulation or PPM

is almost the opposite

PWM. Remember that

in PWM the pulses occurred

regular intervals. In

PPM they don't. Instead

PPM sends out

pulses

which all have the same

width, but their position

represents the analog information.

For example,

imagine that

each

pulse has a "zero" position.

That is the position

that

they

would

be in

analog

voltage

of zero

volts

would

represented.

Assume that any

voltage

greater

than zero

volts

moves

the pulse forward in time

and any

voltage less than

zero

negative

voltage)

moves

it backward in time,

with

respect to

the zero position. In that

manner, an analog

voltage

is represented by a

shift of position.

A common use of PPM

control the time at

which a

Triac "fires" and thus

the brightness

of a light bulb. For

example,

suppose that the zero position

of the pulse

is at 359

degrees into the 60 Hertz

sinewave.

When

the

Triac fires, it

will

only conduct

for I degree because

the current

will fall to

zero at 360 degrees.

The lamp

will

very,

very

dim. Now

lets modulate the position of the

firing pulse. As the

analog

voltage increases imagine that

the firing angle

moves (or

modulates)

toward zero degrees. As

the firing pulse moves

further and further back

along the sinewave,

the lamp be-

comes brighter and brighter

because the Triac conducts

for a

longer period of time.

You'll notice from Fig. 1,

in PPM the pulses are

all the

same

width, but they do not begin at

the beginning of a frame.

Instead they are "held

off' depending

the

average

value of

the

signal. Use Table 2 to convert the

voltage

to approximate

position,

and back again.

Pulse -Code

Modulation

Yet

another

type of pulse modulation

is Pulse -Code Modu-

lation or PCM. Years

ago PCM

was

a general

term for all

types of pulse modilation

techniques. Today

it usually means

one specific

type. No

wonder it's confusing.

PCM today

refers to a group

of l's and 0's

which are used to represent

information.

The most common example

is the

Analog to

Digital converter

or A/D.

A/D's convert an

analog

voltage

to a digital

"word"

which

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on page

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