5. TYPES OF LINK/CHANNEL
A channel or link is either low-pass or band-pass.
Low-Pass Channel
A low-pass channel has a width with frequencies
between 0 and f.
Band-Pass Channel
A band-pass channel has a bandwidth with
frequencies between f1 and f2.
6. ANALOG TO ANALOG
MODULATION
Modulation of an analog signal or analog-to-
analog conversion is the representation of analog
information by an analog signal.
Modulation is needed if the medium has a band
nature or if only band-pass bandwidth is
available to us.
E.g. radio station
8. TYPES OF ANALOG TO ANALOGTYPES OF ANALOG TO ANALOG
MODULATIONMODULATION
AM
FM
PM
9. AM(AMPLITUDE MODULATION)
In AM transmission, the carrier signal is
modulated so that its amplitude varies with the
changing amplitudes of the modulating signal.
The frequency and phase of the carrier remains
the same.
Only the amplitude changes to follow variations
in the information.
The modulating signal becomes an envelope to
the carrier.
11. AM BANDWIDTH
The band width of an AM Signal is equal to twice
the bandwidth of the modulating signal and
covers a range centered on the carrier frequency.
BWt = 2 x BWm.
13. BAND WIDTH NEEDS OF A RADIO
STATION USING AM
The bandwidth of an audio signal is usually 5
KHz.
Each AM radio station needs a minimum
bandwidth of 10KHz.
AM stations are allowed carrier frequencies
anywhere between 530 and 1700 KHz(1.7 MHz).
Each station’s carrier frequency must be
separated by at least 10 KHz(one AM bandwidth)
to avoid interference.
15. NOTE
FCC Federal Communications Commission(FCC)
allows 10KHz for each AM station.
16. Example 1Example 1
We have an audio signal with a bandwidth of 4 KHz.
What is the bandwidth needed if we modulate the signal
using AM? Ignore FCC regulations.
SolutionSolution
An AM signal requires twice the bandwidth of the
original signal:
BW = 2 x 4 KHz = 8 KHz
17. FREQUENCY MODULATION (FM)
In FM transmission, the frequency of the carrier
signal is modulated to follow the changing
voltage level(amplitude) of the modulating signal.
The peak amplitude and the phase of the carrier
signal remains constant.
As the amplitude of the information signal
changes, the frequency of the carrier changes
correspondingly.
19. BANDWIDTH REQUIREMENTS
FOR FM
The bandwidth of an FM signal is equal to 10
times the bandwidth of the modulating signal.
Like AM bandwidth, FM also covers a range
centered on the carrier frequency.
21. BAND WIDTH NEEDS OF A RADIO
STATION USING FM
The bandwidth of an audio signal(speech and
music ) broadcast in stereo is almost 15 KHz.
Each FM radio station therefore needs a
minimum bandwidth of 150 KHz.
The FCC allows 200 KHz(0.2 MHz) for each
station to provide some room for guard bands.
FM stations are allowed carrier frequencies
anywhere between 88 and 108 MHz.
FCC requires that in a given area only alternate
bandwidth allocations may be used.
23. PHASE MODULATION
In PM transmission, the phase of the carrier
signal is modulated to follow the changing
voltage level(amplitude) of the modulating signal.
The peak amplitude and frequency of the carrier
signal remain constant.
As the amplitude of the information signal
changes, the phase of the carrier changes
correspondingly.
