Frequency Modulation In Data Transmission
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The document discusses various methods of frequency modulation (FM) and amplitude modulation (AM), highlighting their characteristics and bandwidth differences. It explains the processes for FM generation, demodulation techniques such as frequency discrimination and phase locked loops, and the components involved in these systems. Additionally, it covers the importance of preemphasizers and deemphasizers in improving signal-to-noise ratio in transmission systems.
Frequency Modulation In Data Transmission
- 4. Modulation Continuous Wave Pulse Wave Modulation Modulation Linear Modulation Amplitude Modulation Non-linear Modulation Frequency Modulation Phase Modulation
- 5. Carrier Signal Baseband Signal Modulated Signal
- 6. There is two types of Frequency Modulation with respect to bandwidth:--- i> Wide Band Frequency Modulation ii> Narrow Band Frequency Modulation
- 7. em (t) _ NBFM Integrator Product ∑ Modulator + Accos(2Πfct) Phase Shifter Narrow – Band Phase Modulator
- 8. Theoretically the Bandwidth Of FM is infinite (for Sinusoidal Modulation). But practically FM wave is effectively limited to a number of significant side frequencies (almost 200 kHz). From Carson’s rule approximation Bandwidth of FM - - BT = 2 f + 2fm BT = 2 f (1 + 1/β)
- 10. In case of AM instantaneous phase contains baseband Signal. But in case of FM that contains baseband as well as higher order odd harmonics. AM signal is amplitude modulated , but FM is amplitude As well as frequency modulated. The bandwidth of FM wave is much larger than AM wave.
- 12. Indirect Method of FM Generation :----- Narrowband Base Band Frequency FM Integrator Phase Signal Multiplier Signal Modulator Crystal Oscillator
- 13. Indirect Method to Generate WBFM for Practical use:- Narrow Frequency Frequency Baseband FM Integrator Band Phase Multiplier Mixer Multiplier signal signal Modulator (*100) (*75) fc=100MHz Δf=75KHz f1 =0.1 MHz f2 =8.5 MHz Crystal Crystal Oscillator Oscillator
- 14. Direct Method of FM generation:---- Base band VCO Frequency Frequency Bandpass WBFM Mixer Signal Multiplier Multiplier Filter Fixed Oscillator DISADVANTAGE: very poor frequency stability due to unstable basic oscillator unlike crystal oscillator.
- 15. Feedback Scheme For Frequency Stabilization of a Direct Frequency Modulator:- Base Band Frequency stabilized VCO Signal FM wave LPF & Frequency Crystal Mixer Amplifier Discriminator Oscillator
- 16. Demodulation of Frequency Modulated Wave:- Frequency Demodulation is the process that enables us to extract the original modulating signal. There are two basic methods for demodulating the FM waves: 1. Based on Frequency Discrimination 2. Based on Phase Locked Loop
- 17. Frequency discriminator:- A frequency discriminator is an FM demodulator which must produce an output voltage linearly dependent on input frequency.
- 18. Frequency Discrimination(Slope Detection) technique: H(f) sc(t) Envelope sd(t) s(t) Detector t t S(t) Sc(t) Sd(t) t
- 19. Problems:- 1>The detector also responds to spurious amplitude variation of the input FM. 2> The range of linear slope is quite small. |H(f)| f0 fc f
- 20. Frequency Demodulation using Phase Locked Loop:- PLL is useful in FM demodulation in presence of large noise and low signal power. Thus it finds application in space vehicle to earth data links. Recently it is being used in Commercial FM receiver.
- 21. Phase Locked Loop:- A PLL is basically a negative feedback system. It consists of three major components: 1. A multiplier 2. A loop filter 3. A voltage controlled oscillator(VCO) These are connected together in the form of a feedback loop. FM wave e(t) s(t) X Loop Filter v(t) r(t) VCO
- 22. Circuit for Direct FM generation:- FM Sound Wave L OSCILLATOR C Condenser Microphone
- 23. Varactor Diode Modulator:- -Vcc Cc em(t) FM C L Varactor Diode
- 24. Practical Reactance Modulator:- +Vcc RFC R1 RFC R1 Reactance Oscilator C Modulator C1 FM output R2 R RE CC RE CE C2 C CE R2 CB Modulating signal
- 25. FM Receiving System:- Frequency changer circuit
- 26. FM receiver circuit:- The application circuit to generate FM(88-108 MHz) with stereo headphone amplifier
- 28. What Stereo Means :-
- 29. How then comes the virtualization ??
- 30. What do we need for the transmission? • We need two different signals L(left) & R(right) to be modulated, transmitted, received and demodulated duly without getting distorted
- 32. We need :---- a.An input signal b.Two preemphasizers c.A DSB-SC modulator d.A frequency doubler e.A pilot signal f. A summer g.An FM modulator
- 35. Why do we need L + R & L – R?? A mono phonic receiver will work with only the L+R component and there will no loss of AUDIO(so the signal will have the backward compatibility)
- 37. We need :---- a. An input signal (through a LIMITER DISCRIMINATOR) b. A Low Pass Filter(0-15 KHz) c. A Bandpass Filter(19 KHz) d. A Bandpass Filter( 23 – 53 KHz) e. A pilot signal f. A Frequency Doubler g. A Synchronous Detector h. Two Deephasizers
- 41. Preemphasizer refers to a system process designed to increase, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.
- 46. Deemphasizer refers to a system process designed to decrease, within a band of frequencies, the magnitude of some (usually higher) frequencies with respect to the magnitude of other (usually lower) frequencies in order to improve the overall signal-to-noise ratio by minimizing the adverse effects of such phenomena as attenuation distortion or saturation of recording media in subsequent parts of the system.