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Chapter 5. Angle Modulation Husheng Li The University of Tennessee.

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Presentation on theme: "Chapter 5. Angle Modulation Husheng Li The University of Tennessee."— Presentation transcript:

1 Chapter 5. Angle Modulation Husheng Li The University of Tennessee

2 Phase and Frequency Modulation  Consider the standard CW signal  We define the total instantaneous angle

3 Phase and Frequency Modulation  Phase modulation (PM)  Frequency modulation (FM)

4 Characteristics of Angle Modulation  The amplitude of an angle modulated wave is constant.  The message resides in the zero crossings alone, providing the carrier frequency is large.  The modulated wave does not resemble the message waveform.

5 Narrowband PM and FM  We can expand the signal (using Taylor’s expansion)  The spectrum is given by  Hence, the signal has a bandwidth of 2W.

6 Example of Narrow Band Angle Modulation  Both PM and FM have carrier component.

7 Tone Modulation  We can allow a 90 degree difference in the modulating tones:  Βis called the modulation index for PM or FM with tone modulation.

8 Spectrum of Narrowband Tone Modulation  When the modulation index is very small, we have  The spectrum is given by

9 Spectrum of Arbitrary Modulation Index  For a single tone signal with arbitrary modulation index, the modulated signal can be written as where j_n(β) is the Bessel function.

10 Bessel Functions

11 Characteristic of FM Spectrum

12 Homework 5  Deadline Oct. 14, 2013

13 Spectrum with Different Modulation Indices We can either fix or fix

14 Multi-tone  Consider the case of multiple tones, e.g.,  The modulated signal can be written as

15 Periodic Modulation  When the signal is periodic, the Fourier series are given by  The modulated signal can be written as

16 Transmission Bandwidth  The generation and transmission of pure FM requires infinite bandwidth. Hence, our questions is: how much of the modulated signal spectrum is significant?  The Bessel function falls off rapidly for  There are M significant sideband pairs and 2M+1 significant lines all told. The bandwidth can be given by

17 Illustration

18 Arbitrary Modulated Signal Bandwidth  For arbitrary modulating signal, the required bandwidth is given by  An approximation: Carson’s rule (deviation ratio)

19 Case of Phase Modulation  We can also define the phase deviation.  We have

20 Linear Distortion  We consider an angle-modulated bandpass signal applied to a linear system:  The lowpass equivalent output spectrum is

21 Nonlinear Distortion  The output of signal through a nonlinear system is given by

22 Example: Clipper  A clipper has only two outputs  The output signal is given by

23 Procedure of Clipper

24 Direct FM  In direct FM, we use VCO to generate the frequency modulated by the signal.

25 Phase Modulator  Although we seldom transmit a PM wave, we are still interested in phase modulators because (1) the implementation is relatively easy; (2) the carrier can be supplied by a stable frequency source; (3) integrating the input signal to a phase modulator produces an FM output.

26 Switching-circuit Modulator  Larger phase shifts can be achieved by the switching-circuit modulator:

27 Indirect FM Transmitter  The integrator and phase modulator constitute a narrowband frequency modulator that generates an initial NBFM signal with instantaneous frequency:

28 Triangular-Wave FM  Triangular-wave FM is a modern and rather novel method for frequency modulation that overcomes the inherent problems of conventional CVOs and indirect FM systems.

29 Frequency Detection  A frequency detector, often called a discriminator, produces an output voltage that should vary linearly with the instantaneous frequency of the input.  Almost every circuit falls into one of the following four categories:  FM-to-AM conversion  Phase-shift discrimination  Zero-crossing detection  Frequency feedback

30 FM-to-AM Conversion  Any device of circuit whose output equals the time derivative of the input produces FM-to-AM conversion:

31 PHASE-SHIFT Discriminators  Phase-shift discriminators involve circuits with linear phase response, in contrast to the linear amplitude response for slope detection:

32 Quadrature Detector  A phase-shift discriminator built with a network having group delay and carrier delay:

33 Zero Crossing Detector

34 Interference  Interference refers to the contamination of an information-bearing signal by another similar signal, usually from a human source.  Interfering sinusoids: consider a receiver tuned to some carrier frequency. The total received signal is

35 Demodulated Output  Consider a weak interference. The demodulated output is

36 Deemphasis  The fact that detected FM interference is most severe at large values of |f_i| suggests a method for improving system performance with selective postdetection filtering, called deemphasis filtering.

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