Electronic Communication Technology - Essay Example

OF THE Electronics and Communications Technology Index Part A –Analogue communication Super-heterodyne receiver 1.1 Introduction 1.2 Principle of operation 1.3 Design and circuit description 1.3.1 RF amplifier 1.3.2 Mixer 1.3.3 Intermediate Frequency (IF) stage 1.3.4 Demodulator 1.3.5 AF Amplifier 1.4 Choice of intermediate frequency 2. AGC and AFC in receivers 2.1 Automatic Gain Control (AGC) in AM receivers 2.1.1 Implementation 2.2 Automatic frequency control (AFC) in FM receivers 2.2.1 Implementation 3. Lab activity on the AM Superhet receiver 3.1 Introduction 3.2 Image frequency 3.3 Mixer 3.4 Intermediate Frequency (IF) 3.5 Detector 3.6 Results Part B – Digital Communication 4. Technical report on Digital Audio Broadcasting (DAB) 4.1 Introduction 4.2 Working 4.2.1 Encoding and compression 4.2.2 Modulation 4.3 DAB Frequencies 4.4 DAB radio systems 4.5 DAB band allocations 4.6 How is DAB different from FM? 4.7 The future of DAB 5. International Telecommunication Union – Radio Regulations 5.1 Introduction 5.2 International Telecommunication Union (ITU) 5.3 ITU radiocommunication sector 5.4 The radio regulation rules 5.5 The need for radio regulation Part A –Analogue communication 1. Super-heterodyne receiver 1.1 Introduction The super-heterodyne receiver is one of the most common forms of receivers in use today. It finds applications in almost all televisions and broadcast radios because of its significant advantages over other types of receivers. Since its initial development, more complicated versions have been developed. However, the basic working principle remains the same. Often abbreviated the superhet, the super-heterodyne radio was designed during the First World War, a time that demanded the need for higher levels of receiver performance in terms of sensitivity and selectivity. However, because of the additional tubes it used, the receiver did not become common until the 1930’s when the levels of performance it provided became an essential requirement and the technology involved became cheaper too. 1.2 Principle of operation The super-heterodyne receiver operates on the principle of heterodyning or frequency mixing in a non-linear fashion. Two different signals are mixed using an RF mixer, to produce an output that is the product of the instantaneous levels of the signals at both the inputs. The resulting output will contain signals at a frequency that is different from the two original signals. If f1 and f2 are the two original signals, the resulting new frequencies are expressed as the sum (f1 + f2) and difference (f1 – f2) of the two. If two signals, one at a frequency of 7.0 MHz and the other at a frequency of 8.0 MHz are heterodyned together, two new frequencies of 15 MHz and 1.0 MHz are produced. Fig. 1. Generation of two different frequencies 1.3 Design and circuit description The basic design of a superhet receiver consists of functional blocks used to convert down the incoming frequency to a fixed intermediate frequency level. Fig. 2. Block diagram of a superhet receiver A suitable antenna is required to receive the radio signals, and they are often built into the receiver itself. The frequencies enter the circuitry from the antenna and then pass through the different stages of the receiver. 1.3.1 RF amplifier The RF amplifier is the first stage of the super-het receiver, and it is used to amplify the signals prior to mixing. The level of amplification has to be carefully chosen. The amplifier must enable the signals to be sufficiently amplified with a good signal to noise ratio and must not overload the mixer. 1.3.2 Mixer The amplified and tuned signals are then fed into a mixer circuit. A local oscillator is connected to the other port of the mixer. This local oscillator consists of a variable frequency oscillator that produces sine waves. The mixer combines the local oscillator signal with the original RF signal, and produces two new frequencies. The local oscillator may also be a frequency synthesiser. The mixer stage is used to enhance the received frequency to an intermediate stage. 1.3.3 Intermediate Frequency (IF) stage The signals leaving the mixer enter the IF stage. This stage consisting of an amplifier and filter, determines the receiver’s selectivity, and renders most of the gain. The filters used at the input and output determine the selectivity of the IF amplifier. Special types of ceramic resonators or crystal filters, and double tuned RF transformers are commonly employed to provide this narrow selectivity. The number of IF amplifiers used in the IF stage is determined by two factors – cost considerations and sensitivity required. In certain cases, selectivity requirements are also taken into account. Filters with high selectivity can produce substantial losses and hence may require additional IF amplifiers for compensation. The bandpass filter (sometimes along with tuned circuits) enables signals on one frequency to be separated from those on adjacent broadcast channels. In order to achieve the desired selectivity and retain the quality of the received signal, the filter must exhibit a flat response across the desired signal spectrum and have a high attenuation to adjacent channels. 1.3.4 Demodulator Once the signals have been processed in the IF stage, they need to be demodulated. The audio signal is recovered and then amplified further. Different types of transmission require different types of demodulation. Therefore, some receivers may have a number of demodulators that are switched to adapt to any type of transmission the receiver may encounter. While FM signals are detected using ratio detectors or discriminators, AM frequencies need envelope detection to rectify RF signals and a simple low pass RC filter to remove the intermediate frequency remnants. 1.3.5 AF Amplifier The output from the demodulator is finally passed into the audio stage where it is amplified and made to drive loudspeakers or headphones. 1.4 Choice of intermediate frequency Selecting the appropriate intermediate frequency is an important yet compromising factor in the superhet. There are two conflicts associated with the choice of IF frequency. When a low intermediate frequency is employed, it brings the advantages of a higher stage gain and a higher receiver selectivity to attain a narrow bandwidth. When a higher IF is used, it brings the advantage of a lower image response. The choice of intermediate frequency is affected by the selectivity of the RF end of the receiver too. The image frequency problem can be represented using the following diagram (fig. 3). Consider a filter of 10 MHz using an intermediate frequency of 100 KHz with a local oscillator on 10.1 MHz. Because of the way the superhet mixer functions, a 10.2 MHz (called the image frequency) will also be obtained. Fig. 3. Image frequency Fig. 4. Response of RF amplifier tuned circuits Adding more number of tuned circuits set to the required input frequency will help in improving the image rejection of the receiver. However, such circuits are not easy to realise practically. Choosing a high intermediate frequency is an alternate solution. This allows the image frequency and the received frequency to be well separated. 2.1 Automatic Gain Control (AGC) in AM receivers Automatic Gain Control was implemented in radios to address the problem of fading propagation characterised by slow variations in the amplitude of the received signal. At a first glance, using the maximum possible gain to run a receiver seems to be the best option to intercept even weak signals. However, this operating choice is not suitable for causal listening applications. As propagation changes, AM signals can fade up to 30 dB in a few seconds and the strength of the RF input signal varies greatly over time. Automatic Gain Control circuits are used to provide control of gain. These circuits maintain a constant signal level at the output regardless of the signal variations in the input. The AGC provides relatively constant amplitude to enable the system using it to operate over require less dynamic ranges. They are employed in a range of systems and devices where significant amplitude variations in the output signal could lead to poor system performance or data loss. Using an AGC circuit in a receiver not only addresses fading, but it avoids missing weak signals and prevents blasting on strong signals when tuning across a band. 2.1.1 Implementation Automatic gain control is commonly achieved by applying a DC signal from the detector stage to the gain controlled stages so as to decrease gain when signal strength increases. Fig. 5. Automatic Gain Control circuit In most broadcast receivers, the AGC voltage is obtained from the detector. The detected signal is then filtered through a low pass filter to remove all audio variations. The filtered signal will retain only the slow variations in signal strength caused by fading. This voltage is then applied to vary the bias on IF amplifier stages to increase or decrease the gain as required for maintaining a constant signal level at the receiver output. There can be several modifications to this basic circuit. 2.2 Automatic frequency control (AFC) in FM receivers Automatic frequency control is highly desirable in tunable FM receivers to prevent distortion arising from a drift in local oscillator frequency. AFC circuits are employed in receivers that require the oscillator frequency to be accurately controlled by an external signal or in simple words, to keep the system tuned to the frequency of the desired station. 2.2.1 Implementation An AFC circuit performs two functions. It identifies the difference between the oscillator frequency and the actual desired frequency and generates a control voltage relative to the difference. The circuit also utilises the control voltage to modify the oscillator frequency to the desired level. Fig. 6. Block diagram of an AFC receiver A varicap serves to stabilise the IF. The apparent reactance that the varicap generates is integrated with the local oscillator circuitry. For example, we may assume that the IF is 10.5 MHz and the oscillator is tracking below the incoming station. When the oscillator output shows a slight increase in frequency, the intermediate frequency will decrease, causing the discriminator output to lower the varicap’s capacitive reactance. This will decrease the local oscillator frequency to the desired level. If we assume that the oscillator output decreases, the IF and capacitive reactance will increase and as a result, lower then oscillator frequency. 3. Lab activity on the AM Superhet receiver 3.1 Introduction The design and construction of an AM superhet receiver involves the principle functions required for frequency conversion by the mixer, amplification and filtering, intermediate frequency amplifier stage, and detection. 3.2 Image frequency The input from the antenna is fed to the RF amplifier. The amplifier offers initial gain and selectivity and reduces local oscillator radiation by isolating the mixer from the antenna. The RF amplifier primarily functions to eliminate image signal which has a frequency greater than the oscillator frequency and give a mixed output at the IF stage. Therefore, the image frequency is eliminated before it enters the mixer, and is calculated using the following formula: f (image) = f (RF) + 2f (IF) To achieve an AM wave with 50% modulation, a 5 MHz modulating signal is required for modulating a 1MHz carrier signal. The bandwidth will therefore be 10 KHz calculated using the formula: 5kHz X 2 = 10 kHz An RF amplifier with -3db bandwidth will pass the carrier and the sidebands. 3.3 Mixer The output from the amplifier is fed to the mixer which is simply a frequency converter device that creates the sum and difference of two frequencies. The local oscillator frequency is higher than the incoming RF signals and is represented as: f (LO) = f (RF) + f (IF) The mixer out is a combination of the LO signal and the received signal represented as the sum and difference in frequencies. 3.4 Intermediate Frequency (IF) The IF is denoted by f (IF) = f (LO) – f (RF) The IF frequency is selected while the other frequencies - f (RF), f (LO), and f (LO) + f (RF) are rejected. f(IF) is fixed at 455 KHz so as to obtain a high selectivity. 3.5 Detector The amplified IF signal is now passed to the demodulator or detector which retrieves the original modulating signal. The detector stage in a superhet converts the incoming IF signal to audio that can be further amplified before being presented to the loudspeaker. The demodulator is a diode detector or envelope detector circuit that rectifies the signals by eliminating the carrier frequency. Thus only the audio portion of the signal is retained. Assuming the low pass filter for the AGC has a low cut off frequency of 1Hz, a 100kOhm resistor will give: f(AGC) = 1/ (2 π RC) = 1 Hz The capacitance C is: C = 1/ (2 π f(AGC)R) = 1.59 nF 3.6 Results An AM superhet receiver with an IF of 470 kHz is tuned between the frequency range 500 kHz to 1500 kHz. The range of frequencies that the local oscillator must cover includes: 1500 kHz + 470 kHz = 1970 kHz 500 kHz + 470 kHz = 970 kHz The acceptable bandwidth for the IF amplifiers are: 550 kHz – 1720 kHz = 1170 kHz 550 kHz – 1720 kHz and covers 107 channels The acceptable bandwidth for the amplifiers is therefore 10 kHz. The AM has a limit of 5 kHz and the audio spectrum range is 20 kHz. Part B – Digital Communication 4. Technical report on Digital Audio Broadcasting (DAB) 4.1 Introduction DAB stands for Digital Audio Broadcasting and it is a next generation system for broadcasting and receiving radio stations. Conventional analogue frequency spectrums such as AM and FM have become too crowded, leaving little or no space for new stations. DAB is a digital technology that has doubled the availability of stations to listen to, as compared with FM radios. 4.2 Working DAB was originally conceived as a means of digitising audio programmes in order to provide CD quality audio and distortion free reception. The capabilities of the technology have transformed further, enabling the DAB system to carry different forms of data and convey pictures, text, and videos as well. The digital radio systems involve two major sub-systems namely the digital audio encoder and compression system, and the modulation system. 4.2.1 Encoding and compression In order to ensure the system is feasible, the data rate has to be substantially reduced from that of standard CDs. The DAB lowers the data rate to one sixth of the bit rate for a linear quality signal of same quality. To accomplish this step, the incoming audio frequency is first assessed. Human ears have a limited hearing threshold and audio signals that are above or below the threshold are not heard. The DAB system analyses the incoming audio and encodes only those signals that fall in the hearing threshold range. Lowering the audio bandwidth enables further reduction in data rate. Fig.7. Human hearing threshold 4.2.2 Modulation The modulation system of the DAB is also termed the Coded Orthogonal Frequency Division Multiplex (COFDM). This system provides a spread spectrum modulation that offers sufficient robustness to effectively avoid reflections and other types of interferences from distorting the reception. CODFM uses 1500 closely spaced individual carriers occupying about1.5 MHz of spectrum. The signals are made orthogonal to each other to prevent interference between carriers. Then, the audio data is spread across the carriers in such as way that each carrier takes only a small portion of the data. This arrangement ensures that sufficient data is obtained to restructure the signal in case of any interference. The combination of several guard bands placed at the starting of each system renders it immune to delays coherent with signals that are sixty kilometres away than the original source. When each carrier is separated by a frequency relative to the data rate, the modulation sideband null values fall at the position of the adjacent carrier. Fig. 8. Spectrum of a digital radio signal This robust design allows the system to operate alongside other digital radio transmitters working on the same frequency. Therefore, it becomes possible to set up a system of transmitters for a network, all operating on the same frequency. When compared with traditional transmitters, DAB systems require lesser power. For instance, systems carrying the main BBC FM networks from major transmitting sites in Europe, require 100 KW, pushing electricity and running costs to be a pivotal factor for bringing savings and environmental benefits. 4.3 DAB Frequencies DAB digital radios can broadcast a range of frequencies from 174.928 MHZ to 239.200 MHZ spanning both terrestrial and satellite locations. Presently, Band 3 frequencies are being widely used. Within the United Kingdom, the DAB multiplexes are being broadcast on seven channels - 11 C, 11 D, 12 A, 12 B, 12 C, and 12 D. 4.4 DAB radio systems The initial launch of the digital radio experienced difficulties in terms of equipment availability. The early implementation required higher levels of current and equipment manufacturers had to invest a large amount of money. The system heavily depended on digital signal processing techniques and complicated development programmes were required for development. In addition to these restrictions, DAB required multi-chip solutions that increased the manufacturing costs and rendered the equipment large and heavy too. The system was not considered suitable for portable receivers. However, all these drawbacks were soon addressed. Manufacturers designed special chips for DAB systems that enabled the manufacturing and associated costs to dramatically reduce and become almost equal with FM receivers. Since then, DAB has been noted for its significant and seamless performance making it the ideal broadcasting medium for the 21st century. 4.5 DAB band allocations Within the UK, the spectrum between 217.5 MHz and 230 MHz has been allocated for digital transmission purposes. The spectrum encompasses seven 1.55 MHz blocks with each carrying a multiplex of services. 4.6 How is DAB different from FM DAB eliminates the multipath interference problem in FM, characterised by distortion in reception caused by radio signals reflecting off hills and buildings. DAB is in fact designed to use this multipath phenomenon to its advantage. The processors in DAB digital radio systems filter out interferences and rectify signal errors. The technology allows receivers to mix delayed signals and on the whole, provide the possibility of a more robust reception. 4.7 The future of DAB DAB has been very successful in the UK and millions of DAB radios have been sold till date. In addition to the UK and Europe, DAB digital radios are becoming increasingly popular in many countries around the world including Canada and Australia. With the radios becoming increasingly cheaper and considering the advantages of the DAB technology as well, it is projected that this number will only continue to rise in the future. 5. International Telecommunication Union – Radio regulations 5.1 Introduction The international radio regulations developed in the framework of International Telecommunication Union (ITU) specify how the satellite orbits and radio frequencies are to be used. Radio has been having an enormous and increasing impact on the society and since the very beginning of radio the ITU Radio Regulations have been developed gradually. 5.2 International Telecommunication Union (ITU) The ITU is the oldest of all international organisations in existence today and it was founded in the year 1875. Today, ITU coordinates and regulates the use of radio frequency spectrum worldwide. The ITU ensures compatibility between different radio systems in different countries based on four principles – Allocation, Allotment, Assignment, and Coordination. The allocation by ITU is based on services and frequency bands. This implies that a designated frequency band must only be used by one or more radio communication applications or services, space or terrestrial. Allotment refers to geographic regions and frequency channels. Specific radio frequency channels are allotted for use over a specific geographical region. Assignment involves radio stations and frequency channels. It implies that a radio station is given authorisation by administration to use a particular radio frequency channel. Finally, coordination between all these aspects is required to improve the utilisation of radio spectrum and prevent interference between different frequency channels across various countries. The ITU operates as three separate sectors: 1. Radiocommunication Sector 2. Telecommunication Standardisation Sector 3. Telecommunication Development Sector 5.3 ITU radiocommunication sector To keep pace with the economic, technological, and political changes, the ITU regularly reviews the spectrum management rules. This is required to ensure economical, equitable, and rational use of the radio frequency spectrum by all radio communication services in the world including those utilising the geostationary-satellite orbits. The Radio Communications sector consists of the following organs: 1. Radiocommunication Bureau 2. Radiocommunication Advisory Group 3. Radiocommunication Assemblies & Study Groups 4. Radio Regulations Board 5. World and Regional Radiocommunication Conferences 5.4 The Radio Regulation Rules Regulations define the allocation of radio frequencies to specific uses and they determine operating procedures for stations, impose restrictions, and coordinate, examine, notify, and record procedures. The aim of setting these regulations is to ensure that orbital positions and radio frequencies are used efficiently, and the national frequency assignments are internationally recognised. It is to be noted that short-range devices and low-power applications do not have to register their frequency assignments with the ITU. Table 1 ITU frequency bands Band No. Symbol Frequency Wavelength 4 VLF Very Low Frequency 3 to 30 kHz Myriametric waves 100 to 10 km 5 LF Low Frequency 30 to 300 kHz Kilometric waves 10 to 1 km 6 MF Medium Frequency 300 to 3000 kHz Hectometric waves 1000 to 100 m 7 HF High Frequency 3 to 30 MHz Decametric waves 100 to 10 m 8 VHF Very High Frequency 30 to 300 MHz Metric waves 10 to 1 m 9 UHF Ultra High Frequency 300 to 3000 MHz Decimetric waves 100 to 10 cm 10 SHF Super High Frequency 3 to 30 GHz Centimetric waves 10 to 1 cm 11 EHF Extremely High Frequency 30 to 300 GHz Millimetric waves 10 to 1 mm 12 THF Tremendously High Frequency 300 to 3000 GHz Decimillimetric waves 1 to 0,1 mm Source: International Telecommunication Union, International Telecommunication Union and General Secretariat, ITU. Radio Regulations, Volume 1. Table 2 Frequency bands in which all emissions are prohibited Lower Frequency Upper frequency Unit RR Footnote 1400 1427 MHz 5.340 2690* 2700 MHz 5.340 10.68* 10.7 GHz 5.340 15.35* 15.4 GHz 5.340 23.6 24 GHz 5.340 31.3 31.5 GHz 5.340 31.5* 31.8 GHz 5.340 48.94* 49.04 GHz 5.340 50.2* 50.4 GHz 5.340 52.6 54.25 GHz 5.340 86 92 GHz 5.340 100 102 GHz 5.340 109.5 111.8 GHz 5.340 114.25 116 GHz 5.340 148.5 151.5 GHz 5.340 164 167 GHz 5.340 182* 185 GHz 5.340 190 191.8 GHz 5.340 200 209 GHz 5.340 226 231.5 GHz 5.340 250 252 GHz 5.340 *) with some exceptions Source: International Telecommunication Union, International Telecommunication Union and General Secretariat, ITU. Radio Regulations, Volume 1. 5.5 The need for radio regulation After several years of continuous evolvement, the radio is entering a fresh era. The communication technologies have combined with computers to enhance the intelligence of conventional applications and create new ones. The impact that radio has had on the society is enormous, and it continues to grow. At such a level of progress, it is important to understand the consequences. Radio frequency is pivotal in numerous areas such as weather forecasts, national defense, air-traffic control, disaster warning, air navigation, public safety, and much more. Radio and television broadcasting continue to be the primary sources of everyday information for a number of people in different corners of the world. They also play an important role in fulfilling the needs of illiterate individuals who constitute a large part of the world population. The number of space and terrestrial radio stations continues to be on the rise and the frequency demands follow the same increasing trend too. The ITU is recording an increasing number of frequency assignments in the recent past. The introduction of new trends and technologies is being encouraged by the trends in competition, privatisation, liberalisation, and deregulation. In this global scenario which widely accepts the uses of RF spectrum, the need for radio regulation cannot be understated. It will clear the path to economic growth and prove to be the main engine responsible for driving the living standards. Works Cited Witts, Alfred Thomas. The superheterodyne receiver: its development, theory and modern practice. University of Michigan. Sir I. Pitman & sons, ltd., 1944. Print. Hagen, Jon B. Radio-Frequency Electronics: Circuits and Applications. Cambridge University Press, 1996. Print. Hoeg W, and Lauterbach, Thomas. Digital Audio Broadcasting: Principles and Applications of Digital Radio. John Wiley, 2003. Print. International Telecommunication Union, International Telecommunication Union and General Secretariat, ITU. Radio Regulations, Volume 1, International Telecommunication Union, 2001. Print Gandy, C. DAB: an introduction to the Eureka DAB System and a guide to how it works. Technical Report WHP-061, British Broadcasting Corp, June 2003. 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