The comparatively low cost of equipment for a frequency modulation broadcasting station, resulted in rapid growth in the years following World War II. Within three years after the close of the war, 600 licensed FM stations were broadcasting in the United States and by the end of the 1980s there were over 4000. Similar trends have occurred in Britain and other countries. This is because of crowding in the amplitude modulation (AM) broadcast band and the inability of standard AM receivers to eliminate noise, the tonal fidelity of standards stations is purposely limited.
FM does not have these drawbacks and therefore can be used to transmit music reproducing the original performance with a degree of fidelity that cannot be reached on AM bands. FM stereophonic broadcasting has drawn increasing numbers of listeners to popular as well as classical music, so that commercial FM stations draw higher audience ratings than AM stations. FM is the method of varying a carrier waves frequency proportionally to the frequency of another signal, in our case the human voice. This compares to the other most common transmission method, AM. AM broadcasts vary the amplitude of the carrier wave according to an input signal. Standard FM broadcasts are based in the 88-108 MHz range; otherwise known as the RF or Radio Frequency range. However, they can be in any range, as long as a receiver has been tuned to demodulate them.
2.2 Communications theory
In a broad sense, the term communication refers to sending, receiving and processing of information by electronic means. Communications started with wire telegraphy in the eighteen forties, developing with telephony some decades later and radio at the beginning of last century. A modern communications system in first concerned with the sorting, processing and sometimes storing of information before its transmission. The actual transmission then follows, with further processing and the filtering of noise. Finally for the reception, this may include processing steps such as decoding, storage and interpretation. In order to become familiar with this system, it is necessary first to know about amplifier and oscillators, the building blocks of all electronic processes and equipment. With these as a background, the everyday communications concepts of noise, modulation and information theory, as well as the various systems themselves, may be approached. The communications system exists to convey a message. This message comes from the information source.
Figure : Block diagram of communication system
Transmitter is required to process and possible encode the incoming information so as to make it suitable for transmission and subsequent reception. Eventually, in a transmitter, the information modulates the carrier is superimposed on a high-frequency wave. The actual method of modulation varies from one system to another. Modulation may be high level, and the system itself may be amplitude modulation, frequency modulation, pulse modulation or any variation or combination of these, depending on the requirements.
There are great varieties of receivers in communication system, since the exact form of a particular receiver is influenced by a great many requirements. Among the more important requirements are the modulation system used. As stated initially, the purpose of a receiver and the form of its output influence its construction as much as the type of modulation system used. The output of a receiver may be fed to a loudspeaker, video display unit, various radar displays, television or computer. In each instance different arrangements must be made, each affecting the receiver design. Note that the transmitter and receiver must be in agreement with the modulation and coding methods used and also timing or synchronization in some system.
Small signal Amplifier
Figure : Block diagram for voice communication using RF
Noise may be defined in electrical terms as any unwanted introduction of energy tending to interfere with the proper reception and reproduction of transmitted signals. In radio receivers, noise may produce unwanted pulse/signal or perhaps cancel out the wanted ones. Noise can limit the range of systems, for a given transmitted power. It affects the sensitivity of receivers by placing a limit in the weakest signals that can be amplified. It may sometimes even force a reduction in the bandwidth of a system itself.
There are numerous ways of classifying noise. It may be subdivided according to type, source, effect, or relation to the receiver, depending on circumstances. It is most convenient here to divide noise into two broad groups, noise whose sources are external to the receiver and noise created within the receiver itself. External noise is difficult to treat quantitatively and there is often little that can be done about it, short of moving the system to another location. Internal noise is both more quantifiable and capable of being reduced by appropriate receiver design. Because noise has such a limiting effect and also because it is often possible to reduce its effects through intelligent circuit use and design, it is more important for all those connected with communications to be well informed about noise and its effects.
2.3.1 Description of Modulation
The point to modulation is to take a message-bearing signal and superimpose it upon a carrier signal for transmission. Modulation is the process of shifting the frequency of a signal so that the resulting signal is in a desired frequency band. This is done by using a high frequency carrier signal to transmit a lower frequency information signal. In other words, the information signal modulates the carrier signal to the desired frequency. Two common types of modulation are Amplitude Modulation (AM) and Frequency Modulation (FM). A third type, Phase Modulation (FM or PM) is very similar to FM and is often used to mean the same as FM. For case of transmission carrier signal are generally high frequency for severable reasons:
For easy (low loss, low dispersion) propagation as electromagnetic waves
So that they may be simultaneously transmitted without interference from other signals
So as to enable the construction of small antennas ( a fraction, usually a quarter of the wavelength)
So as to be able to multiplex that is to combine multiple signals for transmission at the same time.
2.3.2 Need for Modulation
There is an even more important argument against transmitting signal frequencies directly. All sound is concentrated within the range from 20Hz to 20 kHz, so that all signals from different sources would be hopelessly and inseparably mixed up.
In order to separate the various signals, it is necessary to convert them all to different portions of the electromagnetic spectrum. Each must be given its own frequency location. This also overcomes the difficulties of poor radiation at low frequencies and reduces interference. Once signals have been translated, a tuned circuit is employed in the front end of the receiver to make sure that desired section of the spectrum is admitted and all unwanted signals are rejected. The tuning of such a circuit is normally made variable and connect to the tuning control, so that the receiver can select any desired transmission within predetermined range such as the very high frequency (VHF) broadcast band used for frequency modulation.
Although this separation of signals has removed a number of difficulties encountered in the absence of modulation, the fact still remain that unmodulated carriers of various frequencies cannot by themselves be used to transmit intelligence. An unmodulated carrier has constant amplitude, a constant frequency and a constant phase relationship with respect to some reference. A message consists of varying quantities. Speech for instance, is made up of rapid and predictable variations in amplitude (volume) and frequency (pitch). Since it is impossible to represent these two variables by a set of three constant parameters, an unmodulated carrier cannot be used to convey information. In a continuous-wave modulation system (amplitude or frequency modulation) one of the parameters of the carrier is varied by the message. Therefore at any instant its deviation from the unmodulated value (resting frequency) is proportional to the instantaneous amplitude of the modulating voltage and the rate at which this deviation takes place is equal to the frequency of this signal. In this fashion, enough information about the instantaneous amplitude and frequency is transmitted to enable the receiver to recreate the original message.
2.3.3 Frequency Modulation
There are two fundamental types of communication systems: baseband systems and passband systems. In baseband systems, signals are transmitted without any changes to their frequencies. Passband communication systems, on the other hand shifted the frequency spectrum of signal to a new frequency location, which is called the carrier frequency. The human voice has strong components with frequencies of the order of 1 kHz and less. Transmitting such a signal using electromagnetic waves with a baseband system would lead to a number of problems. These problems include:
1) Antenna length.
In order for an antenna to efficiently radiate energy, it must be longer than x/10, where x is the wavelength of the radio waves. From the relation between frequency and wavelength, it can be seen that
Î» = c / f
Where f is the frequency of the signal and c is the speed of light. For a 1 kHz signal,
Î» = 3 x 108 m/s / 1 x 103 Hz
= 300 Km
Thus, for efficient radiation of energy, and antenna for this system must be 30 km in
length. Such an antenna would be very difficult and expensive to implement.
Speed of light = frequency x wavelength
= 3 x 108 m/s
Wavelength = 3 x 108 m/s / 108 MHz
= 2.78 m
Antenna Length = 0.25 x wavelength
= 0.69 m
The design and positioning of the antenna is as crucial as the module performance itself in achieving a good wireless system range. The following will assist the designer in maximizing system performance. The antenna should be kept as far away from sources of electrical interference as physically possible. If necessary, additional power line decoupling capacitors should be placed close to the module. The antenna plate should be kept clear of any objects, especially any metal as this can severely restrict the efficiency of the antenna to receive power. Any earth planes restricting the radiation path to the antenna will also have the same effect. Best range is achieved with either a straight piece of wire, rod or PCB track with a wavelength of 7cm. Further range may be achieved if the wave antenna is placed perpendicular in the middle of a solid earth plane measuring at least 10cm radius. In this case, the antenna should be connected to the module via some 50 ohm characteristic impedance coax.
If two signals were to be transmitted over baseband in the same geographic region, they would interfere with each other, and both signals would be distorted.
3. Signal Efficiency
Signals transmitted in baseband have a lot of noise and interference associated with them, which results in lower signal efficiency and poor signal propagation. In order for radio communication to be feasible these problems need to be dealt, and the solution is to shift the frequency of the transmitted signals to a higher frequency.
Figure : Frequency Modulation (FM) Spectrum
An important component in modulation and radio frequency devices is the mixer or multiplier. In most circuits designers strive for linearity, but mixers and multipliers work on non-linearity. As the name implies, a multiplier uses non-linear circuit components, usually diodes, to multiply two signals together. The result is a number of harmonics and other frequencies arithmetically related to the frequencies of the original waveforms. A more specific type of multiplier is called a mixer. A mixer is made especially to produce only the sum and difference of the root frequencies. Mixers are most usually operating on sinusoidal signals so an example of the output of a mixer fed with two frequencies, f c & f o would look like:
cos ( 2Ï€Æ’ct ) x cos ( 2Ï€Æ’ot ) = cos ( 2Ï€[Æ’ c + Æ’ o ] t ) + cos (2Ï€[Æ’ c - Æ’ o ] t)
Example frequency wave:
Figure : Example waveform of frequency modulation
2.3.4 Derivation of FM Equation
As was done with AM, a mathematical analysis of a high-frequency sine wave, modulated by a single tone or frequency, will be used to yield information about the frequency components in an FM wave, FM power relations, and the bandwidth of an FM signal. From the definition of frequency deviation, an equation can be written for the signal frequency of an FM wave as a function of time
Æ’signal = Æ’c + kÆ’e M (t) = Æ’c + kÆ’E M sinÆ’Ï‰ M t
And substitution of Î´ = kÆ’ x E M
Æ’signal = Æ’c + Î´ sin Ï‰ M t
It seems to be saying that the frequency of the transmitter is varying with time. This brings up the same type of problem that was observed when we looked at a time display of AM and then performed a mathematical analysis in an attempt to determine its frequency content. With AM, the signal appeared to be a sine wave that amplitude was changing with time. At the time, it was pointed out that a sine wave, by definition, has constant peak amplitude, and thus cannot have peak amplitude that varies with time. What about the sine wave frequency? It also must be a constant and cannot be varying with time. As was the case with AM, where it turned out that our modulated wave was actually the vector sum of three sine waves, a similar situation is true for FM. An FM wave will consist of three or more frequency components vector ally added together to give the appearance of a sine wave that frequency is varying with time when displayed in the time domain. A somewhat complex mathematical analysis will yield an equation for the instantaneous voltage of an FM wave of the form shown here:
E FM (t) = E c sin (Ï‰ c t + mÆ’sin Ï‰ Mt)