FM GENERATION Direct Method when the frequency of

FM GENERATION üDirect Method: when the frequency of carrier is modulated by information signal üIndirect Method: when the phase of carrier is modulated by information signal 1 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

Generation of FM waves 1. Varactor diode modulator( Direct method) 2. Reactance modulator( Direct method) 3. Crosby FM direct transmitter 4. Armstrong method (Indirect method) 2 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

Modulators – • Carrier frequency can be generated by LC oscillator. • By varying the values of L or C of tank circuit, carrier frequency can be changed. • Properties of BJT, FET and varactor diodes can be varied by changing the voltage across them. • When these components are used with LC tank circuits, we are able to vary frequency of oscillator by changing the reactance of L or C. 3 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

1. Frequency modulation using Varactor Diode – • There exists small junction capacitance in the reverse biased condition of all the diodes. • The varactor diodes are designed to optimise this characteristic. • As the reverse bias across varactor diode is varied, its junction capacitance changes. • These changes are linear and wide (1 to 200 p. F) • All diodes show small junction capacitance in the reverse biased condition. 4 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

- more capacitance. - less capacitance. 5 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

Varactor Diode FM modulator 6 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

CIRCUIT DIAGRAM OF VARACTOR DIODE MODULATOR 7 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

• It is seen that the diode is reverse biased to provide the junction capacitance effect, since this biased is varied by the modulating voltage which is in series with it, the junction capacitance will also vary, causing the oscillator frequency to change accordingly. Advantages of FM using Varactor Diode – • High frequency stability as crystal oscillator is isolated from modulator. 8 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

Disadvantages – 1. To avoid distortion, the amplitude of modulating signal is to be kept small. 2. The varactor diode must have non linear characteristics of capacitance vs. voltage. Use – • This method is used for low index narrow band FM generation. 9 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

2. FET REACTANCE MODULATOR . 10 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

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Modulating signal Career Frequency modulation Phase modulation 12 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

INDIRECT FM GENERATION Integrator 13 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

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+ 15 = RCOE Prof. Vikas R Dubey Engineering Principles of Communication

RECEIVER CHARACTERISTICS: ü The parameters of the AM Receivers are Sensitivity, Selectivity, Fidelity, Image frequency rejection etc. some of which are explained below: ü 1. Selectively ü The selectivity of an AM receiver is defined as its ability to accept or select the desired band of frequency and reject all other unwanted frequencies which can be interfering signals. ü Adjacent channel rejection of the receiver can be obtained from the selectivity parameter. ü Response of IF section, mixer and RF section considerably contribute towards selectivity. 16 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü The signal bandwidth should be narrow for better selectivity. ü Graphically selectivity can be represented as a curve shown in Fig 1. below, which depicts the attenuation offered to the unwanted signals around the tuned frequency. 17 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü 2. Fidelity ü Fidelity of a receiver is its ability to reproduce the exact replica of the transmitted signals at the receiver output. ü For better fidelity, the amplifier must pass high bandwidth signals to amplify the frequencies of the outermost sidebands, while for better selectivity the signal should have narrow bandwidth. Thus a trade off is made between selectivity and fidelity. ü Low frequency response of IF amplifier determines fidelity at the lower modulating frequencies while high frequency response of the IF amplifier determines fidelity at the higher modulating frequencies. 18 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü 3. Sensitivity ü Sensitivity of a receiver is its ability to identify and amplify weak signals at the receiver output. ü It is often defined in terms of voltage that must be applied to the input terminals of the receiver to produce a standard output power which is measured at the output terminals. ü The higher value of receiver gain ensures smaller input signal necessary to produce the desired output power. ü Thus a receiver with good sensitivity will detect minimum RF signal at the input and still produce utilizable demodulated signal. ü Sensitivity is also known as receiver threshold. 19 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü It is expressed in microvolts or decibels. ü Sensitivity of the receiver mostly depends on the gain of IF amplifier. ü It can be improved by reducing the noise level and bandwidth of the receiver. ü Sensitivity can be graphically represented as a curve shown in Fig 2. Below, which depicts that sensitivity varies over the tuning band. 20 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü 4. Double spotting ü Double spotting is a condition where the same desired signal is detected at two nearby points on the receiver tuning dial. ü One point is the desired point while the other is called the spurious or image point. ü It can be used to determine the IF of an unknown receiver. ü Poor front-end selectivity and inadequate image frequency rejection leads to double spotting. ü Double spotting is undesirable since the strong signal might mask and overpower the weak signal at the spurious point in the frequency spectrum. 21 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü Double spotting can be counter acted by improving the selectivity of RF amplifier and increasing the value of IF. ü Consider an incoming strong signal of 1000 k. Hz and local oscillator tuned at 1455 k. Hz. Thus a signal of 455 k. Hz is produced at the output of the mixer which is the IF frequency. ü Now consider the same signal but with 545 k. Hz tuned local oscillator. Again we get 455 k. Hz signal at the output. ü Therefore the same 1000 k. Hz signal will appear at 1455 k. Hz as well as 545 k. Hz on the receiver dial and the image will not get rejected. This is known as Double spotting phenomenon. ü It is also known as Adjacent channel selectivity. 22 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü Image Frequency rejection ratio: ü The image rejection ratio, or image frequency rejection ratio, is the ratio of the intermediate-frequency (IF) signal level produced by the desired input frequency to that produced by the image frequency. ü The image rejection ratio is usually expressed in d. B. ü When the image rejection ratio is measured, the input signal levels of the desired and image frequencies must be equal for the measurement to be meaningful. ü IMRR is measured in d. B, giving the ratio of the wanted to the unwanted signal to yield the same output from the receiver. 23 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü In a good design, ratios of >60 d. B are achievable. ü Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity); the signal yields a perfectly valid IF frequency. ü Rather, it is the measure of the band pass characteristics of the stages preceding the IF amplifier, which will consist of RF band pass filters and usually an RF amplifier stage or two. 24 RCOE Prof. Vikas R Dubey Engineering Principles of Communication

ü In a good design, ratios of >60 d. B are achievable. ü Note that IMRR is not a measurement of the performance of the IF stages or IF filtering (selectivity); the signal yields a perfectly valid IF frequency. ü Rather, it is the measure of the band pass characteristics of the stages preceding the IF amplifier, which will consist of RF band pass filters and usually an RF amplifier stage or two. 25 RCOE Prof. Vikas R Dubey Engineering Principles of Communication