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PLL and Noise in Analog Systems

PLL and Noise in Analog Systems. Analog and Digital Communications Autumn 2005-2006. LPF. VCO. FM Detection: Phase Lock Loops. Basic Task of PLL:. Lock the instantaneous angle of a VCO with the instantaneous angle of an external signal. i.e., lock (or track) the phase and frequency.

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PLL and Noise in Analog Systems

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  1. PLL and Noise in Analog Systems Analog and Digital Communications Autumn 2005-2006 CS477: Analog and Digital Communications

  2. LPF VCO FM Detection: Phase Lock Loops Basic Task of PLL: Lock the instantaneous angle of a VCO with the instantaneous angle of an external signal. i.e., lock (or track) the phase and frequency. CS477: Analog and Digital Communications

  3. Phase Lock Loops • Used in • Modulators and demodulators • Synchronizers • Frequency synthesizers • Multiplexers • Locks or synchronizes the external angle with the output angle of VCO • A negative feedback system based on phase comparison CS477: Analog and Digital Communications

  4. LPF PLL: Phase Comparator CS477: Analog and Digital Communications

  5. Integrator Linearized PLL Models Assumptions: 1. Incoming frequency is approximately the same as the VCO free running frequency 2. Phase error is small such that: Consider phases as the signals for further analysis CS477: Analog and Digital Communications

  6. Linearized PLL Models CS477: Analog and Digital Communications

  7. If is a lowpass filter, i.e., Then is a first-order lowpass filter First order PLL requires loop gain to be greater than the frequency deviation Thus second order PLLs are used in practice. FM Detection using Linear PLL Model (for FM input) CS477: Analog and Digital Communications

  8. (Non-linear channel) Non-linearities in FM FM transmission is robust against channel variations! We need a BPF; what are the requirements? (Carson’s rule) With this BPF, output is: Unlike AM, FM can combat amplitude non-linearities! CS477: Analog and Digital Communications

  9. LPF Noise • White Noise • Filtered/Colored Noise CS477: Analog and Digital Communications

  10. Noise Equivalent Bandwidth Average power in filtered noise: (Noise equivalent bandwidth) CS477: Analog and Digital Communications

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