Analog modulation is the simplest form of the modulation. In analog modulation techniques, the modulation is applied continuously in response to the analog information signal. The process of the analog modulation has been shown in the Figure 1, below. Here the original signal at the baseband frequency has been shifted to the broadband frequency (Fc).
Common analog modulation techniques
1. Amplitude Modulation (AM): Here the amplitude of the carrier signal is varied in accordance to the instantaneous amplitude of the modulating signal.
2. Angle Modulation: Here the frequency or phase of the carrier signal is varied in accordance with the strength of the modulating signal. Consequently, the Analog Modulation has two forms:
i) Frequency Modulation (FM): In this case, the frequency of the carrier signal is varied in accordance to the instantaneous frequency of the modulating signal)
ii) Phase Modulation (PM): In this case, the phase of the carrier signal is varied in accordance to the instantaneous phase of the modulating signal)
Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a high frequency carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in signal strength may be used to specify the sounds to be reproduced by a load speaker, or the light intensity of television pixels. “Undulatory currents” are the initial implementations of the Amplitude modulation. These were the first method to successfully produce quality audio over telephone lines in 1870’s. The Figure 2 illustrates the process of modulation, by showing the modulating, carrier and modulated signals. Figure 3, further illustrates the Amplitude modulation process by varying the amplitudes of the modulating (input signal) and plotting the corresponding modulated signal.
Advantages of Amplitude Modulation
- Coverage area of AM Receiver is wider than FM because atmospheric propagation.
- AM is long distance propagation because of its high power.
- Circuit is the cheapest and least complex.
- Can be easily demodulated using a Diode Detector.
Disadvantages of Amplitude Modulation
- Amplitude modulation is very much sensitive to noise and hence the performance is very weak.
- Signal of AM is not stronger than FM when it propagates through and obstacle.
- Only one sideband of AM transmits Information Signal, so it loses power on other sideband and Carrier. Hence the power efficiency of the Amplitude Modulation is very poor.
- Noise mixes AM Signal in amplitude when it propagates in free space that it makes it difficulty to recover the original signal at receiver in a highly noisy environment
Frequency modulation, FM is widely used for a variety of radio communications applications. FM broadcasts on the VHF bands still provide exceptionally high quality audio, and FM is also used for a variety of forms of two way radio communications, and it is especially useful for mobile radio communications, being used in taxis, and many other forms of vehicle. in view of its widespread use, frequency modulation, FM, is an important form of modulation, despite many forms of digital transmission being used these days. Since its first introduction the use of frequency modulation, FM has grown enormously. Now wideband FM is still regarded as a very high quality transmission medium for high quality broadcasting. FM, is also widely used for communications where it is resilient to variations in signal strength.
Frequency Modulation is the technique in which, the frequency of the carrier wave is changed in accordance with the Amplitude of the modulating signal. The process is shown in the Figure 4 below.
Advantages of Frequency Modulation
There are many advantages of using frequency modulation. These have been widely used for many years, and will remain in use for many years.
- Resilient to noise: One of the main advantages of frequency modulation is that it has been utilized by the broadcasting industry to take care of 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: Signal variations are reduced since noise effect is eliminated. This means that one of the advantages of frequency modulation is that it does not suffer 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
There are a number of dis-advantages to the use of frequency modulation. Some are can be overcome quite easily, but others may mean that another modulation format is more suitable.
- 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 uses the digital transmission techniques such as 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.
Comparison of FM and AM Transmission
Both the Amplitude and Frequency Modulation have their own advantages and disadvantages. However a comparison of the general performance is shown in the table below:
|AM Broadcasting||FM Broadcasting|
|It requires smaller transmission bandwidth.||It requires larger bandwidth.|
|It can be operated in low, medium and high frequency bands.||It needs to be operated in very high and high frequency bands.|
|It has wider coverage.||Its range is restricted to 50 km.|
|The demodulation is simple.||The process of demodulation is complex.|
|The stereophonic transmission is not possible.||In this, stereophonic transmission is possible.|
|The system has poor noise performance.||It has an improved noise performance.|
Frequency Modulation and the Phase Modulation are the two forms of the angle modulation. The main characteristic of the angle modulation is that the amplitude of the carrier frequency is maintained constant, whereas the frequency or phase is changed. In the phase modulation, the phase of the carrier wave is shifted in accordance with the amplitude of the modulating frequency. Phase modulation is a form of modulation that can be used for radio signals used for a variety of radio communications applications.
As it will be seen later that phase modulation and frequency modulation are closely linked together and it is often used in many transmitters and receivers used for a variety of radio communication applications from two way radio communications links, mobile radio communications and even maritime mobile radio communications. Phase modulation is also the basis for many forms of digital modulation based around phase shift keying, PSK which is a form of phase modulation. As various forms of phase shift keying are the favored form of modulation for digital or data transmissions, this makes phase modulation particularly important.
Before looking at phase modulation it is first necessary to look at phase itself. A radio frequency signal consists of an oscillating carrier in the form of a sine wave is the basis of the signal. The instantaneous amplitude follows this curve moving positive and then negative, returning to the start point after one complete cycle – it follows the curve of the sine wave. This can also be represented by the movement of a point around a circle, the phase at any given point being the angle between the start point and the point on the wave.
Phase modulation works by modulating the phase of the signal, i.e. changing the rate at which the point moves around the circle. This changes the phase of the signal from what it would have been if no modulation was applied. In other words the speed of rotation around the circle is modulated about the mean value. To achieve this, it is necessary to change the frequency of the signal for a short time. In other words when phase modulation is applied to a signal there are frequency changes and vice versa. Phase and frequency are inseparably linked as phase is the integral of frequency. Frequency modulation can be changed to phase modulation. The information regarding sidebands, bandwidth and the like also hold true for phase modulation as they do for frequency modulation, bearing in mind their relationship.
Unlike its more popular counterpart, i.e. frequency modulation (FM), PM is not very widely used for radio transmissions. This is because it tends to require more complex receiving hardware and there can be ambiguity problems in determining whether, for example, the signal has changed phase by +180° or -180°. PM is used, however, in digital music synthesizers such as the Casio CZ synthesizers, or to implement FM Synthesis in digital synthesizers such as the Yamaha DX7. The Phase modulation signals have been illustrated in the Figure 5 and Figure 6 below.