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FM TRANSMISSION

FM TRANSMISSION. P.J. PARDESHI Asst. Professor MITCOE. What is frequency modulation?. When the frequency of carrier wave is changed in accordance with the message signal , The process is called frequency modulation. In FM the carrier amplitude remain constant the carrier frequency varies

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FM TRANSMISSION

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  1. FM TRANSMISSION P.J. PARDESHI Asst. Professor MITCOE

  2. What is frequency modulation? • When the frequency of carrier wave is changed in accordance with the message signal, The process is called frequency modulation. • In FM the carrier amplitude remain constant the carrier frequency varies • It is a type of Angle modulation • Why Frequency modulation is called nonlinear-modulation?

  3. Frequency Modulation Waveform

  4. In fact, someone needed to carry part of the walkie talkie on their back because it was so large. • Around the same time Al Gross was working on his model, Donald Hings was working on his version of the walkie talkie which he called a “packset”. Hings invention was used during War World Two starting in 1942 and was very important in the war effort.

  5. After World War IIthe use of the hand held radio spread throughout different public sectors. With more compact designs police and fireman began to rely more on the devices for communication. • Later walkie talkie use moved from public to private sectors and to everyday use for the average person or even toys for children.

  6. All India Radio

  7. Officially known since 1956 as Ākāshvāṇī, is the national public radio broadcaster of India. • All India Radio is one of the largest radio networks in the world and is headquartered at the AkashvaniBhavan in New Delhi. • AIR’s home service comprises 414 stations today located across the country, reaching nearly 92% of the country’s area and 99.19 % of the total population. • AIR originates programming in 23 languages and 146 dialects.

  8. FM Broadcasting in India

  9. The Regulations Regulates AIR Regulates Private Radio Broadcasting Both regulators are overseen by the Ministry of Information and Broadcasting, Government of India, which is in charge of all media regulation.

  10. Applications: FM Radio FM radio uses a modulation index, m > 1, and this is called wideband FM. As its name suggests the bandwidth is much larger than AM.

  11. Television Sound: In terrestrial TV broadcasts, the video information is transmitted using AM . However the sound information is transmitted using FM, in order to reduce possible interference between the video and sound signals. In this case, the maximum deviation of the carrier,  fc , is chosen to be 50kHz, and the information baseband is again the high fidelity range 20Hz to 15kHz. Therefore the bandwidth required for TV Sound is: BW Of TV Sound=2(50k+15k) =130khz Satellite TV. Some satellite TV transmissions broadcast an analogue video signal using FM. This helps to obtain an acceptable signal at the receiving station In this case, the maximum deviation of the carrier  fc , is chosen to be about 10 MHz, with a video baseband of around 5MHz. Therefore the bandwidth required for Satellite TV is: BW of satellite TV =2(10+5) =30Mhz

  12. Phase Modulation(PM) • PM is the modulation technique in which carrier phase varies based on analog baseband information signal to be transmitted using wireless device. • If a constant amplitude as well as constant frequency sine wave carrier is given to the phase shifter the output is phase modulated signal. • Phase modulation is referred as indirect frequency modulation due to the fact that phase modulation produces frequency modulation. • The effect of variation in amount of phase shift is proportional to change in the carrier frequency.

  13. PM Phase Modulation (PM) In phase modulation the angle is varied linearly with the message signal m(t) as :

  14. FM & PM • In FM, modulation index is inversely proportional to the modulating frequency but In PM that is independent of the modulating frequency. • Phase and frequency are inseparably linked as phase is the integral of frequency. • Frequency modulation can be changed to phase modulation by simply adding a CR network to the modulating signal that integrates the modulating signal.

  15. Waveform

  16. Relation between FM & PM

  17. ADVANTAGES OF FM • Resilient to the noise : AM waves do not have constant envelopes and therefore more affected by static or noise than FM. Unwanted electromagnetic waves do not cause the frequency of FM carrier wave to change. • Resilient to signal strength variations • Does not require linear amplifiers in the transmitter • Enables greater efficiency than many other modes

  18. ADVANTAGES OF AM • It is simple to implement • it can be demodulated using a circuit consisting of very few components • AM receivers are very cheap as no specialized components are needed. • .

  19. Comparison of FM with PM

  20. Comparison of FM and AM

  21. Frequency Spectrum and Eigen Values • The Equation of FM Mathematically is expressed as a sine of sine or cosine of sine by using the Bessel Functions then equation • The trigonometric series of above equation becomes e(t)= Ecmax{Jo(β) coswct + J1(β) cos (wc±Wm)t + J2(β) cos (wc±2Wm)t + J3(β) cos (wc±3Wm)t + J4(β) cos (wc±4Wm)t……..} • It has infinite no pairs of sidebands with a coefficients J called as Bessel function coefficient

  22. The FM waveform has a component at the carrier frequency and an unlimited series of frequency, above and below the carrier frequency • Characteristics of Bessel Function OR

  23. Bessel Function Coefficient Table

  24. Bessel Function

  25. FM Bandwidth The spectra of various values of β are shown in above fig ,in each case the spectral lines are spaced by fm BFM =2 n fm Hz Eigen Value (career component zero)

  26. Significant Sidebands – Spectrum • The table below shows the number of significant sidebands for various modulation Example: For  = 5, 16 sidebands (8 pairs).

  27. Observations from FM Spectrum of Bessel’s Function • Unlike AM, has large no. of sidebands which are separated from carrier by Fm, 2Fm,…. • J coefficients eventually decrease as in value as n increases • In AM increased depth of modulation increases side-band power & hence the total transmitted power. • In FM total transmitted power always remains constant, but with increased depth of modulation the required bandwidth is also increased. • Amplitude of carrier component does not remains same. • For certain values of modulation index, carrier component disappears completely, these are called Eigen values.

  28. FM Bandwidth • The n is highest order of side frequency for which the amplitude is significant • The order of side frequency is greater than ( β + 1) and the amplitude is 5% less • Hence , BFM = 2 n fm = 2 ( β +1) fm Hz Substituting β = Δf/ fm BFM = 2(Δf + fm) Hz……….. Carson’s Rule Carson’s rule is an approximation and gives transmission bandwidth that are slightly narrower than the bandwidths determined using the Bessel table • “FM is called as a constant bandwidth system”

  29. Problems • 1. Calculate BW of FM for modulating signals • 0.1 Khz • 1Khz • 10Khz With maximum deviation of 75Khz 2. A FM wave is expressed as V=10sin[5*+4sin1250t] Find: 1. Carrier Frequency & Modulating frequencies 2. Modulation Index & Maximum deviation

  30. MCQ’s • The amount of frequency shift in FM is directly proportional to the ---- of the modulating signal • The general name given to FM & PM is ------- modulation • The FM produced by phase modulator is known as----- • When the modulating signal crosses zero, the phase shift & frequency deviation in a phase modulator are a. At a maximum b. At a minimum c. Zero • The bandwidth of FM signal is proportional to ------ • The amplitudes of sidebands in an FM signal are dependent upon a mathematical process known as -------

  31. Types of FM • Narrow band FM • Narrow band FM is defined as the situation where the modulation index  is small. • From the table of Bessel functions it may be seen that for small , ( 0.3) there is only the carrier and significant sidebands, i.e. BW = 2fm. • FM with  0.3 is referred to as narrowband FM (NBFM). • Maximum modulating frequency is usually 3kHz • maximum frequency deviation is =75 kHz.

  32. Types of FM Wide band FM •  Wideband FM is defined as the situation where the modulation index  is larger. • For  > 0.3 there are more than 2 significant sidebands. As  increases the number of sidebands increases. This is referred to as wideband FM (WBFM). • Modulation frequencies extend from 30 Hz to 15 kHz. • Maximum permissible deviation is=75 kHz. • Wideband FM system need large bandwidth, typically 15 times that of narrowband FM system.

  33. Generation of FM using PM Modulating Wave x(t) FM Wave Integrator Phase Modulator Carrier Oscillator

  34. Relation between FM & PM

  35. METHODS OF FM GENERATION INDIRECT METHOD DIRECT METHOD ARMSTRONG METHOD REACTANCE MODULATOR VARACTOR MODULATOR

  36. DIRECT METHOD- In direct method, the modulating (base band) signal directly modulates the carrier .The carrier signal is generated using a LC oscillator circuit. Frequency of oscillator of carrier

  37. The instantaneous frequency of the carrier wave is directly varied in accordance with the message signal by means of an voltage controlled oscillator. The frequency determining network in the oscillator is chosen with high quality factor (Q-factor) and the oscillator is controlled by the incremental variation of the reactive components in the tank circuit of the oscillator.

  38. Varactor diode Generation A varactor diode is a semiconductor diode whose junction capacitance varies linearly with applied voltage when the diode is reverse biased.

  39. FM Transmitters NBFM ANTENNA WBFM Crystal oscillator frequency multiplier phase modulator Power amplifier Audio source

  40. Crystal oscillator- Crystal oscillator generates the stable carrier signal. Phase modulator- The phase modulator modulates the carrier signal and the massage signal in the low power range to generate a narrowband FM. Frequency multiplier- The frequency multiplier is used to increase the frequency deviation and carrier signal frequency to a desired level.

  41. Power amplifier- The power amplifier gives the required power level to the signal which passes through the antenna. Antenna- Antenna is a device which is used for sending and receiving the information.

  42. Indirect Method- Armstrong Method

  43. FM stereo Transmitter

  44. Broadcasting standards • VHF band 88-108 MHz • Deviation ± 75KHz • Channel Spacing 200KHz • Line of Sight Propagation • Modulating Frequency 50Hz - 15 KHz • Coverage 50 miles • Power output 100kW

  45. Imp links • https://www.elprocus.com/voltage-controlled-oscillator-working-application/ • http://www.radio-electronics.com/info/rf-technology-design/pm-phase-modulation/what-is-pm-tutorial.php • http://www.ques10.com/p/11412/how-can-you-use-a-varactor-diode-in-the-generation/ • http://www.diffen.com/difference/AM_vs_FM

  46. Unit 4: FM Reception

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