COMMUNICATION TECHNOLOGIES

Archive for the ‘Radio Communication’ Category

Dear friends! Circuit theory which deals with the behavior of electrons within a closed circuit or closed boundary conductors is no doubt a powerful tool for communication engineers, yet it has its own limitations in dealing with radio transmission and microwave problems. An engineer can use the circuit theory in overall design of terminal equipments for radio communication system, but between the outlet terminals of the transmitter and input terminals of the receiver, the circuit theory which was so much successful, remained unsuccessful in explaining the central point of radio, i.e. radiation of electromagnetic waves into space.

Ultimately a more exact theory-electromagnetic field theory or simple field theory comes to rescue, of which circuit theory is a simplified approximation. Electromagnetic field theory directly deals with the field vectors i.e. electric field and magnetic field where as circuit theory deals with voltage V and current I that are the integrated effects of electric and magnetic fields. Undoubtedly, it is the end result voltage and current with which an engineer is interested but the intermediate step, i.e. electromagnetic field is essential. Since large number of variables is involved in field theory so it becomes comparatively more difficult in comparison to circuit theory. Have a nice day!

They separate the required radio signal from the other radio signals which are picked up by the antenna and reject all other signals. A radio receiver is an electronic circuit that receives its input from an antenna, uses electronic filters to separate a wanted radio signal from all other signals picked up by this antenna, amplifies it to a level suitable for further processing, and finally converts through demodulation and decoding the signal into a form usable for the consumer, such as sound, pictures, digital data, measurement values, navigational positions, In consumer electronics, the terms radio and radio receivers are often used specifically for receivers designed for the sound signals transmitted by radio broadcasting services – historically the first mass-market radio application. The modulation method determines the frequency range for a specific transmission. The bandwidth also depends on the bandwidth occupied by the modulating signals themselves.

FM is the process in which the carrier frequency is varied in accordance with the instantaneous value of the modulation signal keeping the amplitude constant. Frequency modulation is widely used in many areas of radio technology including broadcasting and areas of two way radio communication. In these applications its particular advantages can be used to good effect the angle modulation covers both frequency modulation (FM) and phase modulation (PM).FM is much more immune to noise than amplitude modulation. The transmitted amplitude is constant and can be fitted with an efficient amplitude limiter. This characteristic has the advantage of significantly improving immunity to noise and interference. FM has advantage over AM or PM of providing from noise for the lowest modulating frequencies.

FM is a so called angle modulation scheme; it was inspired by phase modulation but has proved to be more useful partly for its ease of generation and decoding. When the audio signal is modulated onto the radio frequency carrier, the new radio frequency signal moves up and down in frequency. The amount by which the signal moves up and down is important. It is known as the deviation and is normally quoted as the number of kilohertz deviation. As an example the signal may have a deviation of 3 kHz. In this case the carrier is made to move up and down by 3 kHz. The main advantages of FM over AM are improved signal to noise ratio to manmade interference, smaller geographical interference between neighboring stations, less radiated power, well defined service areas for given transmitter power

It consists of frequency changer and demodulator or detector and audio amplifier and image frequency. The frequency changer is the combination of mixer and local oscillator. Both of them provide heterodyne function where the incoming signal is converted to a predetermined fixed frequency called intermediate frequency. The circuits are referred to as first detector. The output of the intermediate frequency section is applied to the demodulator which recovers base band or message signal. Detector also supplies voltage to intermediate frequency stages in the form of the AGC circuit. Audio section has several cascaded audio amplifiers and one or more speakers. In image frequency the heterodyne receiver the mixer will develop an intermediate frequency when the input frequency is greater than or less than local oscillator frequency.

So there is two input frequency in the heterodyne receiver. Incoming amplitude modulation wave picked up by the receiving antenna is passed on to the amplifier. It consists of the reselection and an amplifier. It is an adjustable frequency that is lined to desired carrier frequency of the incoming signal. It is used to provide enough initial limiting to prevent unwanted radio frequency called the image frequency. The incoming signal amplifies to requisite level and to determine sensitivity of the receiver. This type of receiver performs the following function other than the demodulating incoming signal. Carrier frequency tuning is to select the desired signal. Filtering is to separate desired signal from other than the modulated signals that may be picked up along the receiver. Amplification is to compensate loss of the signal power during transmission

A more satisfactory approach to efficient us of allowed frequency band is based on pre-emphasis in the transmitter and de-emphasis in the receiver. We artificially emphasis high frequency components of the message prior to modulation in the transmitter low frequency and the high frequency portion of the power spectral density of message are equalized in such a way message fully occupied frequency band allotted. To produced undistorted version of the original message signal at the receiver output. Pre-emphasis filter in the transmitter and de-emphasis filter in the receiver must ideally have frequency responses that are inverse of each other. This choice of frequency response makes average message power at receiver output independent of pre emphasis and de-emphasis procedure.

Because of average message power of receiver output is widely unaffected by combined pre emphasis and emphasis in transmitter and de emphasis procedure. Output SNR is improved by use of pre emphasis in transmitter and de emphasis in receiver. In AM receiver, it combines two assumptions that channel model which assumes communication channel that is distortedness but perturbed by AWGN. Receiver model assumes receiver consisting of ideal band pass filter followed by ideal demodulator received signal is therefore made up of sum of s(t) and w(t) the band pass filter in the model represents combined following action of tuned amplifiers used in actual receiver for the purpose of signal amplification prior to demodulation. The band width of this band pass filter is just wide enough to pass modulated. After modulator its detail depends on the type of modulation used.

As the carrier to noise ratio is reduced, clicks are heard in the receiver output. As the carrier to noise ratio reduces further, crackling, or sputtering sound appears at the receiver output. Near the breaking point the theoretically calculated output signal to noise ratio became large, but its actual value is very small. This method is said to be fm threshold effect. It is the minimum carrier to noise ratio yielding an fm improvement which is not significantly deteriorated from the value predicted by the usual signal to noise formula. The amplitude and the phase of the change randomly with the time. The angle varies approximately to multiples of the noise. As the carrier to noise ratio is decreased further, the clicks per unit time is increased.

The threshold is said to be occurred when these clicks are very large. The threshold reduction can be achieved in fm demodulated with the negative feedback. The local oscillator is replaced by voltage controlled oscillator. The output frequency of the voltage controlled oscillator is controlled by the modulated signal. Therefore the output frequency of the voltage controlled oscillator changes as per low frequency variations of the demodulated signal. It does not depend upon the high frequency variation of the narrowband noise. It acts as the tracking filter. It tracks only the slowly varying frequency of wide-band fm waves. It respond only to narrowband of the noise.

Frequency modulation stereo multiplexing is used for stereo transmissions. It is basically division multiplexing, it is used for frequency modulation radio broadcasting. The essential requirements fro stereo multiplexing is that it should be compatible with monophonic receivers. There are two signals of left and right message channels. The sum of signal and the difference signal are generated. The sum signal is directly given to output without any processing. It is used by monophonic receivers. The oscillator frequency of 19 KHz is amplified by k times and given to output. It is used as a reference at the receiver for coherent reception. The oscillator frequency is doubled and used by product modulator. The product modulator produces DSB-SC signal.

The final multiplexed signal output thus consists of three signals. The base band low pass filter separates sum signal. The band pass filter centered on twice the frequency separates the DSB-SC signal and a narrow band fleeter tuned to the critical frequency separates reference carrier. The reference carrier is doubled in frequency to obtain. It is multiplied with received DSB-SC signal to be obtained. The abs band low pass filter then removes the high frequency carrier of 4 times of that of the carrier frequency and gives difference signal. The left and right channels of message signal are then obtained from the sum and difference. The three multiplexed signals are separated by three appropriate filters.

The automatic frequency correction is incorporated in FM transmitter to keep carrier frequency stable. The discriminator reacts only to small changes in the carrier frequency but not to the frequency deviations in the carrier. This higher frequency is fed to the mixer for which the other input frequency is from the stable crystal oscillator. This frequency will be fed to the discriminator. Since the discriminator is tuned to the correct frequency difference which should exists between the oscillator and its input frequency is somewhat higher. This voltage is applied to the reactance modulator whose Tran conductance is increased by the positive voltage developed by the discriminator. This increases the equivalent capacitance of the reactance modulator thereby decreasing the oscillator frequency.

The frequency is increased in the carrier frequency is thus lowered and brought to the correct value. The voltage is developed by the discriminator may be fed to the varactor diode connected across the tank circuit of the oscillator and be used for the automatic frequency correction purpose. In directly modulated fm transmitters many times the frequency modulation is carried out to a lower frequency and with a smaller frequency deviation. Then passing this frequency modulated wave through frequency multiplier circuit. The desired carrier frequency and the desired frequency deviation is achieved. The frequency multiplication required for getting the desired carrier frequency from a smaller carrier frequency may not turn out to be exactly the same as desired frequency deviation.

In addition to basic signals like speech, music or codes the visual nature should also be transmitted. In facsimile transmission an exact reproduction of a document or picture or still photograph is provided at the receiving end. In television system movement is present. So television requires more BW than facsimile system. It usestransmission of photographs, transmission of documents, whether maps and so on and transmission of language texts. Since the available spot is small a sequence scanning spot is used and this produces triangular waveform and now maximum output is produced. Since the triangular waveform is not the best, a rectangular spot is used in which the height is about 0.8 times the width.

The basic signal obtained from information scanned is not suitable for direct line transmission because low frequencies are difficult to amplify. So amplitude modulation is employed. Time division multiplexing is used. Two channels are given 300 to 3400 HZ and carrier frequencies are 1300 HZ and 1900 HZ. In radio transmission, fading is the main difficulty and it destroys the picture information. So, FM is used for some part of transmission path because FM systems are not affected by amplitude fading. This system is called sub carries frequency modulation.

Radio Transceivers:

Radio transceivers are also part of the base station subsystem. The radio transceivers used with cellular telephone systems voice channels can be either narrowband FM for analog systems or either PSK or QAM for digital systems with an effective audio-frequency band comparable to a standard telephone circuit. The maximum output power of a cellular transmitter depends on the type of cellular system. The radio transceiver that detects the strongest signal is selected. This arrangement is called receiver diversity. Modern cellular base station antennas are more aesthetically appealing than most antennas and can resemble anything from a widow shutter to a palm tree to an architectural feature on a building.

Communications Protocol and Cell-site Controllers:

The last constituent of a cellular telephone system is the communications protocols, which governs the way telephone calls, are established and disconnected. The protocol implemented depends on whether the voice control channels are analog or digital and what method subscribers use to access the network. Each cell contains one cell-site controller that operates under the direction of the switching center. A cell-site controller manage each of the radio channels at each site, supervises calls, turns the ratio transmitter and receiver on and off, injects data onto the control and voice channels, and performs diagnostic tests on the cell-site equipment.