NARROW-BAND FREQUENCY MODULATION For small values of , Thus the expression for FM signal

can be expanded as since Then since

Amplitude spectrum (single-sided plot)

WIDE-BAND FREQUENCY MODULATION Wide-band FM signal. In order to compute the spectrum of an angle-modulated signal with a sinusoidal message signal, let Where q(t) = Phase deviation

The corresponding FM signal is given by It may alternatively be written as where Re(x) denotes the real part of x.

The parameter  is known as the modulation index and is the maximum value of phase deviation of the FM signal. Consider the function z(t) given by It is periodic with frequency fm and can therefore be expanded in a Fourier series as follows.

The Fourier coefficient are given by

Hence we can rewrite cn as The integral on the right hand side is a function of “n” and  and is known as the Bessel function of the first kind of order n and argument .

It is conventionally denoted by Jn(  ). That is, Thus we get

x(t) is accordingly given by The discrete spectrum of x(t) is, therefore, given by

PROPERTIES OF BESSEL FUNCTIONS Property - 1: For n even, we have Jn () = J-n () For n odd, we have Jn () = (-1) J-n () Thus, Jn () = (-1)n J-n ()

Property - 2: For small values of the modulation index  we have J0 ()  1 J1 ()  /2 J3 ()  0 for n > 2 Property - 3:

TABLE OF BESSEL FUNCTIONS

AMPLITUDE SPECTRUM

With the increase in the modulation index, the carrier amplitude decreases while the amplitude of the various sidebands increases. With some values of modulation index, the carrier can disappear completely.

POWER IN ANGLE-MODULATED SIGNAL The power in an angle-modulated signal is easily computed

Thus the power contained in the FM signal is independent of the message signal. This is an important difference between FM and AM. The time-average power of an FM signal may also be obtained from

where

TRANSMISSION BANDWIDTH OF FM SIGNALS Theoretically, a FM signal contains an infinite number of side frequencies so that the bandwidth required to transmit such signal is infinite. However, since the values of Jn() become negligible for sufficiently large n, the bandwidth of an angle-modulated signal can be defined by considering only those terms that contain significant power.

In practice, the bandwidth of a FM signal can be determined by knowing the modulation index and using the Bessel function table.

Example: Determine bandwidth with table of bessel functions Calculate the bandwidth occupied by a FM signal with a modulation index of 2 and a highest modulating frequency of 2.5 kHz.

Referring to the table, we can see that this produces six significant pairs of sidebands. The bandwidth can then be determined with the simple formula where N is the number of significant sidebands.

Using the example above and assuming a highest modulating frequency of 2.5 kHz, the bandwidth of the FM signal is

DETERMINE BW WITH CARSON'S RULE An alternative way to calculate the bandwidth of a FM signal is to use Carson's rule. This rule takes into consideration only the power in the most significant sidebands whose amplitudes are greater than 2 percent of the carrier. These are the sidebands whose values are 0.02 or more.

Carson's rule is given by the expression In this expression, f is the maximum frequency deviation, and fm is the maximum modulating frequency.

We may thus define an approximate rule for the transmission bandwidth of an FM signal generated by a single of frequency fm as follows:

Example: Assuming a maximum frequency deviation of 5 kHz and a maximum modulating frequency of 2.5 kHz, the bandwidth would be

Comparing the bandwidth with that computed in the preceding example, you can see that Carson's rule gives a smaller bandwidth.

FM SIGNAL GENERATION They are two basic methods of generating frequency-Modulated signals Direct Method Indirect Method

DIRECT FM In a direct FM system the instantaneous frequency is directly varied with the information signal. To vary the frequency of the carrier is to use an Oscillator whose resonant frequency is determined by components that can be varied. The oscillator frequency is thus changed by the modulating signal amplitude.

For example, an electronic Oscillator has an output frequency that depends on energy-storage devices. There are a wide variety of oscillators whose frequencies depend on a particular capacitor value. By varying the capacitor value, the frequency of oscillation varies. If the capacitor variations are controlled by m(t), the result is an FM waveform

INDIRECT FM

Angle modulation includes frequency modulation FM and phase modulation PM. FM and PM are interrelated; one cannot change without the other changing. The information signal frequency also deviates the carrier frequency in PM. Phase modulation produces frequency modulation. Since the amount of phase shift is varying, the effect is that, as if the frequency is changed.

Since FM is produced by PM , the later is referred to as indirect FM. The information signal is first integrated and then used to phase modulate a crystal-controlled oscillator, which provides frequency stability. In order to minimize the distortion in the phase modulator, the modulation index is kept small, thereby is resulting in a narrow-band FM-signal

The narrow-band FM signal is multiplied in frequency by means of frequency multiplier so as to produce the desired wide-band FM signal. The frequency multiplier is used to perform narrow band to wideband conversion. The frequency deviation of this new waveform is “M” times that of the old, while the rate at which the instantaneous frequency varies has not changed