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Modulasi Sudut (2). Levy Olivia MT. 3.3.3 Implementation of Angle Modulators and Demodulators. Design an oscillator whose frequency changes with the input voltage. Voltage-controlled oscillator Varactor diode - capacitance changed with the applied voltage.

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modulasi sudut 2

Modulasi Sudut (2)

Levy Olivia MT

slide2

3.3.3 Implementation of Angle Modulators and Demodulators

  • Design an oscillator whose frequency changes with the input voltage.
  • Voltage-controlled oscillator
    • Varactor diode - capacitance changed with the applied voltage.
    • A inductor with the varactor diode is used in the oscillator circuit.
slide3

Let the capacitance of the varactor diode is given by

  • When m(t) = 0, the frequency of the tuned circuit is given by
  • In general for nonzero m(t), we have
  • Assuming that
  • We have
slide4

Indirect method for generation of FM and PM signals

    • generate a narrow band angle-modulated signal
    • change the narrow band signal to wideband signal
slide5

Generate wideband angle-modulated signals from narrow band angle-modulated signals

    • frequency multiplier
    • implemented by nonlinear device and bandpass filters
  • Using down converter
slide6
A nonlinear device followed by a bandpass filter tuned to the desired center frequency can be used as frequency multiplier.
  • For example, assume a nonlinear device has the function
  • The output signal will be
  • The frequency is multiplied by a factor of 2.
slide7

FM demodulation

    • generate an AM signal
    • use AM demodulator to recover the message signal
  • Pass the FM signal through a filter with response
  • If the input to the system is
  • the output
  • The above signal is an AM signal.
slide8

FM to AM converter: Tuned circuit implementation

But, usually the linear region of the frequency characteristic may not be wide enough.

slide9

Balanced discriminator

    • use two tuned circuits
    • connect in series to form a linear frequency response region.
slide12

FM demodulator with phase-locked loop (PLL)

  • Input :
  • VCO output:
  • Phase Comparator:
slide14

By taking the Fourier transform

  • Suppose that we design G(f) such that

v(t) is the demodulated signal

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