Block Diagram of FM Transmitter with pre-emphasis
Relationship between FM and PM.
(a) FM scheme by using a phase modulator.
(b) PM scheme by using a frequency modulator.
FM Capture effect
In telecommunication, the capture effect, or FM
capture effect, is a phenomenon, associated
with FM reception, in which only the stronger of
two signals at or near the same frequency will
be demodulated.
The capture effect is defined as the complete
suppression of the weaker signal occurs at the
receiver limiter, if it has one, where it is not
amplified, but attenuated. When both signals are
nearly equal in strength, or are fading
independently, the receiver may switch from one
to the other.
Block Diagram of FM Receiver
FM Receivers
A limiter is a non-linear circuit that compares the input to a certain threshold value.
The output indicates either comparison is true or false (i.e., binary results).
Typically the output is a saturated minimum or maximum value.
Figure 5.19 FM band allocation
PULSE MODULATION
In pulse modulation method the carrier is no longer
a continuous signal but consists of pulse train, some
parameter varied according to the modulating signal.
Starts with Sampling
Volts
time
SAMPLING THEOREM
Analog
signal

Sampling
process
Pulse amplitude
modulated (PAM) signal
Sampling theorem: A bandlimited signal with no spectral
components beyond fm , can be uniquely determined by values
sampled at uniform intervals of
◦ The sampling rate,
.
◦ is called Nyquist rate
10
Sampling
Time domain
x
t)x
t)
x
(t)
s(
(
x(t )
x (t )
xs (t )
Frequency domain
X
(f)
X
f)

X
(f)
s
(
| X(f )|
| X ( f ) |
| Xs ( f ) |
PULSE MODULATION
The process of transmitting signals in the form of pulses
(discontinuous signals) by using special techniques.
•
Pulse Amplitude
Modulation
•
Pulse Width Modulation
•
Pulse Position
Modulation
•
Pulse Code Modulation
Pulse Modulation
Analog Pulse Modulation
Digital Pulse Modulation
Pulse Amplitude (PAM)
Pulse Code (PCM)
Pulse Width (PWM)
Delta (DM)
Pulse Position (PPM)
Pulse Amplitude Modulation (PAM):

The signal is sampled at regular intervals such that each
sample is proportional to the amplitude of the signal at that
sampling instant

Types of PAM: 1. Natural PAM 2. Flat top PAM
Analog Signal
Amplitude Modulated
Pulses
1. Natural PAM:
2. Flat top PAM
Transmission Bandwidth
1
BT 
 fm
2
Advantages :1.
Simplicity of generation and detection.
Disadvantages :1.
Since the amplitude of carrier pulses carries the information ,
the interference of noise is maximum.
1
BT 
 fm
2
2.
BW is too large than max. freq fm.
3.
The peak power required by transmitter varies with modulating
signal.
Pulse Width Modulation (PWM or PLM or PDM):
* In this type, the amplitude is maintained constant but the duration or
length or width of each pulse is varied in accordance with instantaneous
value of the analog signal.
Analog Signal
Width Modulated Pulses
Pulse Width Modulation:
time
max = largest Positive
min = largest Negative
Generation of Pulse Width Modulation
PWM Detector
Advantages :1.
PWM is more immune to noise since information is carried by width
of pulse.
2.
Synchronization is not required as in PPM between Tx and Rx.
Disadvantages :1.
BW is larger than PAM.
2.
In PWM the pulses are varying in width so the peak power at output
is variable
Pulse Position Modulation (PPM):
•In this type, the sampled waveform has fixed amplitude and
width whereas the position of each pulse is varied as per
instantaneous value of the analog signal.
PPM
Generation of PPM
Detection Of PPM
PAM, PWM and PPM at a glance:
Analog Signal
Amplitude Modulated Pulses
Width Modulated Pulses
Position Modulated Pulses