|
|
Digital Modulation
|
|
|
|
|
Pulse
Amplitude Modulation |
The PAM wave is obtained in a similar
manner as the AM wave |
|
|
 is
termed as the depth of information (m)
in the same way as AM.
|
Therefore,  |
As,
,
|
 |
Where |
or |
 |
 |
This e quation represents the PAM wave.
As can be seen, the wave contains upper and lower side
band frequencies, besides the modulating and pulse signals.
The frequency spectrum of the PAM wave can be obtained
by remembering that the un-modulated pulse train contains
a set of discrete frequencies. Each of these frequencies
will have a pair of side bands - USB and LSB. The frequency
spectrum so obtained is depicted in below illustration. |
|
Frequency spectrum
of a PAM wave |
PAM wave can be very easily obtained
by the use of a linear amplifier with modulating signal
applied at its input and the un-modulated pulse-train
connected in series with the signal. |
|
(a) PAM modulator
(b) PAM demodulator |
To demodulate PAM waves,
a close look at the frequency spectrum reveals that
it contains modulating frequencies in addition to other
frequency components. Thus, a very simple demodulating
circuit comprises a low pass filter having a cut-off
frequency equal to the highest frequency in the modulating
signal. At the output of the filter is available modulating
signal along with the DC component. A PAM demodulator
function has been illustrated above.
Before sampling a signal, it must be passed through
a low pass filter, so that higher frequencies are eliminated
from the signal and the signal conforms to the requirement
of the sampling circuit. Also, the PAM technique has
the same signal to noise ratio as AM. Thus, it is not
employed in practical circuits but may be employed to
produce other forms of pulse modulation. |
|
| Back to the
top |
|
|
|
|
|
