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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)

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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.

• A inductor with the varactor diode is used in the oscillator circuit.

• 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

• Indirect method for generation of FM and PM signals

• generate a narrow band angle-modulated signal

• change the narrow band signal to wideband signal

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

• frequency multiplier

• implemented by nonlinear device and bandpass filters

• Using down converter

• 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.

• 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.

FM to AM converter: Tuned circuit implementation

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

• Balanced discriminator

• use two tuned circuits

• connect in series to form a linear frequency response region.

• FM demodulator with feedback

• FM demodulator with phase-locked loop (PLL)

• Input :

• VCO output:

• Phase Comparator:

• Linearized model of the PLL

• or

• By taking the Fourier transform

• Suppose that we design G(f) such that

v(t) is the demodulated signal