Modulation is the process of changing
some characteristic of a carrier wave in
accordance with the intensity of the
message signal. Need for modulation in
communication systems is to reduce the
physical length of the antennas.
Suppose a new antenna is to design in
such a way that it should have minimum
physical length. If the length of the
physical antenna is kept below
500metres. Calculate the range of
carrier frequency if the velocity of light in
free space is 3x108metres.
Answers
Modulation
The information signal can rarely be transmitted as is, it must be processed. In order to use electromagnetic transmission, it must first be converted from audio into an electric signal. The conversion is accomplished by a transducer. After conversion it is used to modulate a carrier signal.
A carrier signal is used for two reasons:
To reduce the wavelength for efficient transmission and reception (the optimum antenna size is ½ or ¼ of a wavelength). A typical audio frequency of 3000 Hz will have a wavelength of 100 km and would need an effective antenna length of 25 km! By comparison, a typical carrier for FM is 100 MHz, with a wavelength of 3 m, and could use an antenna only 80 cm long.
To allow simultaneous use of the same channel, called multiplexing. Each unique signal can be assigned a different carrier frequency (like radio stations) and still share the same channel. The phone company actually invented modulation to allow phone conversations to be transmitted over common lines.
The process of modulation means to systematically use the information signal (what you want to transmit) to vary some parameter of the carrier signal. The carrier signal is usually just a simple, single-frequency sinusoid (varies in time like a sine wave).
The basic sine wave goes like V(t) = Vo sin (2 p f t + f) where the parameters are defined below:
V(t) the voltage of the signal as a function of time.
Vo the amplitude of the signal (represents the maximum value achieved each cycle)
f the frequency of oscillation, the number of cycles per second (also known as Hertz = 1 cycle per second)
f the phase of the signal, representing the starting point of the cycle.
To modulate the signal just means to systematically vary one of the three parameters of the signal: amplitude, frequency or phase. Therefore, the type of modulation may be categorized as either
AM: amplitude modulation
FM: frequency modulation or
PM: phase modulation
Note: PM may be an unfamiliar term but is commonly used. The characteristics of PM are very similar to FM and so the terms are often used interchangeably.
Frequency modulation uses the information signal, Vm(t) to vary the carrier frequency within some small range about its original value. Here are the three signals in mathematical form:
Information: Vm(t)
Carrier: Vc(t) = Vco sin ( 2 p fc t + f )
FM: (t) = Vco sin (2 p [fc + (Df/Vmo) Vm (t) ] t + f)
Summary
In FM signals, the efficiency and bandwidth both depend on both the maximum modulating frequency and the modulation index.
Compared to AM, the FM signal has a higher efficiency, a larger bandwidth and better immunity to noise
FM Performance
Bandwidth
As we have already shown, the bandwidth of a FM signal may be predicted using:
BW = 2 (b + 1 ) fm
where b is the modulation index and
fm is the maximum modulating frequency used.
FM radio has a significantly larger bandwidth than AM radio, but the FM radio band is also larger. The combination keeps the number of available channels about the same.
The bandwidth of an FM signal has a more complicated dependency than in the AM case (recall, the bandwidth of AM signals depend only on the maximum modulation frequency). In FM, both the modulation index and the modulating frequency affect the bandwidth. As the information is made stronger, the bandwidth also grows.