PLL loop filter with switched-capacitor resistor

Abstract

A phase locked loop circuit with improved noise characteristics includes a voltage controlled oscillator (12) that generates an oscillating output signal in response to a voltage controlled input. A reference source provides the reference frequency signal. A phase detector (30) is operatively connected to the voltage controlled oscillator and a reference source and produces an output proportional to the phase difference between the oscillation output signal and the reference signal. A loop filter (46) connects the phase detector output to the voltage control input. The loop filter consists of a switch-capacitor equivalent resistance (Q1, Q2, CR).

Description

Field of invention: [0001] This invention relates to phase-locked loop filter circuits, and more particularly, to loop filters with switched-capacitor resistors. Background of the invention: [0002] Phase-locked loop circuits are widely used in circuits that require the generation of high-frequency periodic signals whose frequencies are precise multiples of the frequency of some very stable low-noise reference frequency signal. The phase-locked loop circuit is also used in occasions where the phase of the output signal must track the phase of the reference signal, which is also the origin of its name "phase-locked loop". [0003] Phase-locked loop circuits are used in radio transmitters and receivers to generate local oscillator signals. This local oscillator signal is used for channel selection and is therefore a multiple of a stable, low noise and often temperature compensated reference generator. PLL circuits are also used in clock correction applications in digital co...

AI Technical Summary

Problems solved by technology

This resistor generates thermal noise large enough to affect the phase noise spectrum of the PLL output signal

In the vicinity of the loop bandwidth, the loop filter resistor noise may dominate the overall phase noise

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