BASIC ELECTRONICS MODULATION DEMODULATION Introduction In radio transmission

BASIC ELECTRONICS MODULATION & DEMODULATION

Introduction • In radio transmission, it is necessary to send the audio signal (20 Hz to 20 KHz) such as music, speech etc. from a broadcasting station over great distances to a receiver. This communication of audio signal does not employ any wire and is sometimes called wireless. The audio signal cannot be sent directly over the air to appreciable distance because radiation of electrical energy is practicable only at high frequencies e. g. above 30 KHz. This difficulty is overcome by superimposing the electrical audio signal on a very high frequency wave called carrier wave

• . The resultant waves are called modulated waves or radio waves and the process is called “modulation”. The modulated or radio waves are sent out from the broadcasting station and carry the audio signal to larger distances. At the radio receiver, the audio signal is extracted from the modulated wave by the process of “demodulation”. The audio signal is then amplified and fed to the speaker for sound reproduction.

Modulation The process of changing some characteristics (e. g. Amplitude, frequency or phase) of a carrier wave in accordance with the intensity of signal is known as ‘modulation’. Modulation means to “change”. In modulation, some characteristic of carrier is changed in accordance with the intensity of the signal. The resultant wave is called the modulated or radio wave and contains the audio signal. Therefore, modulation permits the transmission to occur at high frequency while it simultaneously allows carrying of the audio signal. Depending upon which characteristic of carrier is changed, the modulation can be of three types: (i) Amplitude modulation (ii) Frequency modulation (iii) Phase modulation

In amplitude modulation, the amplitude of the carrier is changed in accordance with the intensity of signal while the frequency of carrier remains unchanged. The audio signal is contained in the amplitude variation of the resultant AM wave. In frequency of the carrier is changed in accordance with the intensity of the signal. The audio signal is contained in the frequency variations of the FM wave. In india, amplitude modulation is used in radio broadcasting. However, in TV transmission, frequency modulation is used for sound amplitude modulation for picture signal.

Modulation Factor • An important consideration in amplitude modulation is to describe the extent to which the amplitude of the carrier wave is changed by the signal. This is described by a factor called modulation factor m i. e. Amplitude change of carrier Modulation factor, m = ––––––––––– Normal carrier amplitude • The value of modulation factor depends upon the amplitude of the carrier and the signal. For example, if both carrier and signal have the same amplitude, say A, then modulation factor is A/A = 1 or 100%. The modulation factor indicates the strength and quality of the transmitted signal. The greater the modulation factor, the stronger and clearer will be the signal. However, modulation should not be more than 100% otherwise distortion will occur.

Demodulation • The process of recovering signal from the modulated wave is known as “demodulation”. It is essentially the reverse of modulation and is, therefore, rightly called demodulation or “detection”. Demodulation is done in the radio receiver by a circuit called “demodulator”. One of such circuits is a diode demodulator. Here the modulated wave is first rectified i. e. negative half of the modulated wave is eliminated. The rectified modulated wave contains the audio signal and the carrier. The carrier is then removed by a filter circuit and the recovered audio signal is amplified and fed to the speaker for sound reproduction.

Superhetrodyne receiver • It was first designed by Major Edwin H. Armstrong during the First World War. At present, all modern receivers utilise the superhetrodyne circuit. In this type of radio receiver, the selected radio frequency is converted to a fixed lower frequency, called intermediate frequency (IF). This is achieved by a special electronic circuit called the mixer circuit. i. There is a local oscillator in the radio receiver itself. This oscillator produces high frequency waves. The selected radio wave is mixed with the high frequency wave in the mixer circuit. In this process, beats are produced and the mixer produces a frequency equal to the difference between local oscillator and radio wave frequency. The circuit is so designed that oscillator always produces a frequency 455 KHz above the selected radio frequency. Therefore, the mixer will always produce an intermediate frequency of 455 KHz regardless of the station to which the receiver is tuned.

ii. The obtaining of fixed IF (i. e. 455 KHz) is known as superhetrodyne principle. As an example, if 600 KHz station is tuned, then local oscillator will produce a frequency of 1055 KHz. Consequently, the output from the mixer will have a frequency of 455 KHz. • It is worthwhile to give a passing reference to the utility of getting fixed intermediate frequency in a superhetrodyne circuit. At this fixed intermediate frequency, the amplifier circuits operate with maximum stability, selectivity and sensitivity. As the conversion of incoming radio frequency to the intermediate frequency is achieved by hetrodyning or beating the local oscillator against radio frequency, therefore, this circuit is called superhetrodyne circuit.

Characteristics of a radio receiver • The ultimate goal of a radio receiver is to extract the signal from the desired radio wave and build up this signal to such a level so that it can operate the load (e. g. Speaker, earphones). In checking the specifications of a radio receiver, four characteristics are of general importance viz sensitivity, selectivity, fidelity and noise figure. (i) Sensitivity. It is a measure of the level of the input signal to produce “the standard output”. Clearly, sensitivity is a function of the amount of amplification or number of stages used in the receiver. At first thought it might seem that a receiver could be designed for any degree of sensitivity merely by increasing the number of stages. Although the gain would be increased, it may not be usable. When the noise level generated within the receiver is stronger than the signal to be received, increased sensitivity is wasted. The receiver output will contain more noise than signal.

(ii) Selectivity. It is measure of the ability of a radio receiver to select the desired radio wave while rejecting all others. This function is performed by the tuned circuits ahead of the detector stage. The selectivity of a radio receiver depends upon the number of tuned circuits and the Q of these circuits. (iii) Fidelity. It is a measure of how faithfully the receiver reproduces the original signal. A receiver is said have high fidelity if it amplifies all the frequencies equally well. (iv) Noise figure. Although a receiver can be designed for any gain, the usable gain is limited by the noise output from the receiver. In a radio receiver is tuned between stations. An ideal receiver-one that generates no noise— would have a noise figure of unity or 0 db.

BASIC ELECTRONICS REGULATED D. C POWER SUPPLY

INTRODUCTION In general, electronic circuits using tubes or transistor require a source of d. c power. Batteries are rarely used for the purpose as they are costly and require frequent replacement. In practice, d. c power for electronic circuits is conveniently obtained from commercial a. c lines by using rectifier-filter system, called d. c power supply. The rectifierfilter combination constitutes an ordinary d. c power supply.

An ordinary d. c power supply has two drawbacks. First, the d. c output voltage changes directly with input a. c voltage. Second, d. c output voltage decreases tsa the load current increases because of increases voltage drop in transformer windings, rectifier and filter circuit. These variations in d. c output voltage may cause inaccurate or erratic operation or even malfunctioning of many electronic circuits.

A regulated d. c power is that in which the d. c output voltage remains constant irrespective of the a. c mains fluctuations or changes in load. A regulated power supply consist of an ordinary power supply and voltage regulating devices(e. g zener diode, zener in conjunction with transistor ). The output of an ordinary power supply is fed to the voltage regulator which produces the final output. The output voltage remains constant whether the load current changes or there are fluctuations in the input a. c voltage.

Voltage Regulations. Voltage regulation is an important consideration in the selection of a power supply. The variations of d. c output voltage w. r. i the amount of load current drawn from the supply is known as “voltage regulation”. % where Voltage regulation =Vnl-Vfl VFL V NL =d. c output voltage at no load V FL= d. c output voltage at full load

ZENER VOLTAGE REGULATOR. When a zener diode is operated in the breakdown or zener region, the voltage acrossit remains substantially constant for a large change through it. This characteristic permits it to be used as a voltage regulator. The zener diode of zener voltage Vz is reverse connected across the load across which constant output desired. A series resistance is connected in the circuit which absorb the output voltage fluctuation so as to maintain constant voltage across the load. The following points may be noted regarding the zener diode as a voltage regulator: (i) The zener will maintain a constant voltage (=Vz) across the load so long as the input voltage does not fall below Vz.

(ii) The current through the series resistance is always constant. If the load current increases, the current through the zener must decrease to such a value so as to keep the same current through the series resistance. (iii) The zener will cease to operate sa a regulator if current through it is zero. (iv) There is a safe value of current which a zener can carry. If this value is exceeded, the zener may be destroyed due to overheating.

(v) A zener should always be reverse connected because it utilises the reverse characteristic for voltage regulation. (vi) When the desired regulated output voltage is higher than the rated voltage of a zener diode. Two or more zeners are connected in series. However, in such circuit care must be taken to select the zener diodes that have the same current rating. (vii) For zener diodes with breakdown voltage lower than 5 V, zener mechanism predominates; between 5 V and 8 V both zener and avalanche mechanism are involved whereas above 8 V the avalanche mechanism alone takes over.

(viii) The zener voltage of a zener diodes depends upon temperature. In zener breakdown, the value of breakdown voltage decreases as the junction temperature increases. On the other hand, in a avalanche breakdown, the value of breakdown voltage increases as the junction temperature increases. (ix) When zener is in the breakdown region, current is limited only by the external series resistance.