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Analog Communication

Analog Communication. Presented by: Ashutosh Rastogi Assistant Professor BBDNITM,Lucknow. Outline. Angle Modulation: Frequency Modulation Phase Modulation. Frequency Modulation.

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Analog Communication

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  1. Analog Communication Presented by: AshutoshRastogi Assistant Professor BBDNITM,Lucknow BBDNITM,Lucknow

  2. Outline Angle Modulation: • Frequency Modulation • Phase Modulation BBDNITM,Lucknow

  3. Frequency Modulation • Variation of the instantaneous value of frequency of carrier wave w.r.t. to the instantaneous value of amplitude of message signal is termed as Frequency Modulation. Carrier wave Message signal FM modulated wave BBDNITM,Lucknow

  4. One more picture of FM wave: BBDNITM,Lucknow

  5. Other important topics that we had already discussed in the classroom: • Mathematical analysis of FM and PM • NBFM • WBFM • Frequency Deviation • Carson rule BBDNITM,Lucknow

  6. Advantages of Frequency Modulation • Resilience to noise • Easy to apply modulation at a low power stage of the transmitter: it is not necessary to use a linear form of amplification to increase the power level of the signal to its final value. • It is possible to use efficient RF amplifiers with frequency modulated signals: It is possible to use non-linear RF amplifiers to amplify FM signals in a transmitter and these are more efficient than the linear ones required for signals with any amplitude variations (e.g. AM and SSB). This means that for a given power output, less battery power is required and this makes the use of FM more viable for portable two-way radio applications. BBDNITM,Lucknow

  7. Assignment • What are the advantages and disadvantages of FM over AM? describe in detail. BBDNITM,Lucknow

  8. AM vs. FM • AM requires a simple circuit, and is very easy to generate. • AM is simple to tune, and is used in almost all short wave broadcasting. • The area of coverage of AM is greater than FM (longer wavelengths (lower frequencies) are utilized-remember property of HF waves?) • In AM Class A or AB amplifiers are used whereas in FM Class C amplifiers are used. BBDNITM,Lucknow

  9. AM vs. FM • However, it is quite inefficient, and is susceptible to static and other forms of electrical noise. • The main advantage of FM is its audio quality and immunity to noise. Most forms of static and electrical noise are naturally AM, and an FM receiver will not respond to AM signals. • The audio quality of a FM signal increases as the frequency deviation increases (deviation from the center frequency), which is why FM broadcast stations use such large deviation. • The main disadvantage of FM is the larger bandwidth it requires BBDNITM,Lucknow

  10. Generation Method of FM • Direct method or parameter variation method • Indirect method or Armstrong method BBDNITM,Lucknow

  11. Direct method • Direct Method: Modulate a VCO with m(t) • Varactor diode method for FM generation Modulating signal x(t) C L0 C0 Cd V0(d.c. bias) Varactor diode method BBDNITM,Lucknow

  12. Varactor diode - capacitance changed with the applied voltage. • An inductor with the varactor diode is used in the oscillator circuit. • Generates FM signal. • Works only for Narrowband FM generation. BBDNITM,Lucknow

  13. Mathematical analysis of varactor diode method vd=V0+x(t) BBDNITM,Lucknow

  14. Problem with the direct method • The frequency of carrier wave become unstable. • The non-linearity of the varactor diode produces a frequency variation due to the harmonics of the baseband signal therefore the signal get distorted. BBDNITM,Lucknow

  15. Indirect method or Armstrong method • Proposed by armstrong • Approximate narrowband FM Carrier+ SSB(90 degree out of phase) Problem : Phase noise in the system Taylor series expansion BBDNITM,Lucknow

  16. Wideband FM generation BBDNITM,Lucknow

  17. Detection of FM • Demodulator • Frequency to amplitude converter circuit • Frequency discriminators BBDNITM,Lucknow

  18. FM detection techniques • Slope Detector • Zero-Crossing Detector • PLL for FM Detection • Quadrature Detection BBDNITM,Lucknow

  19. Slope Detector for FM BBDNITM,Lucknow

  20. Zero-Crossing Detector Input FM Signal Pulse Train BBDNITM,Lucknow

  21. Output of Differentiator used as a trigger MonostableMultivibrator Output of low-pass filter BBDNITM,Lucknow

  22. PLL Demodulation for FM BBDNITM,Lucknow

  23. Quadrature Detection Technique • Most popular detection technique • Can be implemented on a single IC at very low cost. • The phase difference between original message signal and the signal at the output of the phase shift network can be detected. • Output of the phase detector will be proportional to the instantaneous frequency of the input FM signal. BBDNITM,Lucknow

  24. Quadrature Detection Technique BBDNITM,Lucknow

  25. Phase-shift Quadrature FM demodulation BBDNITM,Lucknow

  26. Thanks BBDNITM,Lucknow

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