Chapter 3 ANGLE MODULATION

Introduction Angle modulation is the process by which the angle (frequency or phase) of the carrier signal is changed in accordance with the instantaneous amplitude of modulating or message signal. Besides using the amplitude of a carrier to carrier information, one can also use the angle of a carrier to carrier information. This approach is called angle modulation, and includes frequency modulation (FM) and phase modulation (PM). The amplitude of the carrier is maintained constant. The major advantage of this approach is that it allows the trade-off between bandwidth and noise performance.

Cont’d… ANGLE MODULATION is classified into two types such as Frequency modulation (FM) Phase modulation (PM) Used for : Commercial radio broadcasting Television sound transmission Two way mobile radio Cellular radio Microwave and satellite communication system

PHASE MODULATION(PM) The process by which changing the phase of carrier signal in accordance with the instantaneous of message signal. The amplitude remains constant after the modulation process.

Phase Modulation PM (phase modulation) signal

Phase Modulation

Forms of phase modulation Although phase modulation is used for some analogue transmissions, it is far more widely used as a digital form of modulation where it switches between different phases. This is known as phase shift keying, PSK, and there are many flavours of this. It is even possible to combine phase shift keying and amplitude keying in a form of modulation known as quadrature amplitude modulation, QAM.

FREQUENCY MODULATION PRINCIPLES In FM the carrier amplitude remains constant, the carrier frequency varies with the amplitude of modulating signal.

Frequency Modulation FM signal

FM Increasing fc Modulating signal Decreasing fc Increasing fc Resting fc Increasing fc Decreasing fc Increasing fc Resting fc FM Modulating signal Carrier

Frequency Deviation The amount of change in carrier frequency produced by the modulating signal is known as frequency deviation Δf The frequency deviation of a radio is of particular importance in relation to bandwidth, because less deviation means that more channels can fit into the same amount of frequency spectrum. Calculate the maximum frequency deviation for the FM signal v(t)= 10 cos(6000t+ 5sin2200t) Explanation: A standard FM signal is represented by v(t)= Ac cos(2πfct+ kfsin2πfmt) Ac = carrier amplitude fc= carrier frequency kf= modulation index fm = modulating frequency = 2200/2π = 350 Hz kf = frequency deviation/modulating frequency 5= freq deviation/ 350 Therefore, deviation= 5 *350 = 1750Hz

Modulation Index In FM, modulation index is the ratio of frequency deviation(Δf) to the modulating frequency(Fm). It is expressed as follows. m = Example: Max. frequency deviation of carrier = +/- 25KHz Max. modulating frequency = 10 KHz modulation index = 25/10 = 2.5 It is known as deviation ratio when both and are used with maximum value in the calculation.

Compare Angle Modulation and Amplitude Modulation properties of amplitude modulation Amplitude modulation is linear just move to new frequency band, spectrum shape does not change. No new frequencies generated. Spectrum: S(f) is a translated version of M(f) Bandwidth ≤ 2W Properties of angle modulation They are nonlinear spectrum shape does change, new frequencies generated. S(f) is not just a translated version of M(f) Bandwidth is usually much larger than 2W

Angle Modulation Pro/Con Application Why need angle modulation? Better noise reduction Improved system fidelity Disadvantages Low bandwidth efficiency Complex implementations Applications FM radio broadcast TV sound signal Two-way mobile radio Cellular radio Microwave and satellite communications

Advantages of frequency modulation Resilient to noise:   One of the main advantages of frequency modulation that has been utilised by the broadcasting industry is the reduction in noise. As most noise is amplitude based, this can be removed by running the signal through a limiter so that only frequency variations appear. This is provided that the signal level is sufficiently high to allow the signal to be limited. Resilient to signal strength variations:   In the same way that amplitude noise can be removed, so too can any signal variations. This means that one of the advantages of frequency modulation is that it does not suffer audio amplitude variations as the signal level varies, and it makes FM ideal for use in mobile applications where signal levels constantly vary. This is provided that the signal level is sufficiently high to allow the signal to be limited. Does not require linear amplifiers in the transmitter:   As only frequency changes are required to be carried, any amplifiers in the transmitter do not need to be linear. Enables greater efficiency than many other modes:   The use of non-linear amplifiers, e.g. class C, etc means that transmitter efficiency levels will be higher - linear amplifiers are inherently inefficient.

Disadvantages of frequency modulation Requires more complicated demodulator: One of the minor dis-advantages of frequency modulation is that the demodulator is a little more complicated, and hence slightly more expensive than the very simple diode detectors used for AM. Also requiring a tuned circuit adds cost. However this is only an issue for the very low cost broadcast receiver market. Some other modes have higher data spectral efficiency: Some phase modulation and quadrature amplitude modulation formats have a higher spectral efficiency for data transmission that frequency shift keying, a form of frequency modulation. As a result, most data transmission system use PSK and QAM. Sidebands extend to infinity either side: The sidebands for an FM transmission theoretically extend out to infinity. To limit the bandwidth of the transmission, filters are used, and these introduce some distortion of the signal.