Fractional-N PLL Charge Pump for Quantization Noise Reduction
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Solution Overview
Problem
Fractional-N phase-locked loops (PLLs) face issues with noise due to quantization noise from the feedback clock phase, caused by nonlinearity in the charge pump and mismatch between current sources, which corrupts the output clock and is difficult to reduce without introducing additional noise sources.
Innovation Solution
A linearizing phase frequency detector (PFD) is implemented with constant-width down pulses and pulse-amplitude modulation in the charge pump, along with a quantization noise reduction technique using a delta-sigma modulator to generate a feedback clock that reduces noise by aligning pulse edges with the VCO clock, and a self-correcting mechanism to manage current source units for effective noise suppression.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If a fractional-N PLL uses an ever-changing frequency divider ratio to maintain average VCO clock to reference clock frequency ratio, then the PLL provides wide bandwidth and fast settling time, but quantization noise of the feedback clock phase is injected through the PFD and charge pump, becoming the dominant noise source
Solution Approach 1:
The patent converts the harmful quantization noise by using a delta-sigma modulator to shape the noise spectrum, pushing quantization noise out of the PLL passband. The noise is not eliminated but transformed and relocated to higher frequencies where it does not corrupt the output clock within the bandwidth of interest.
Solution Approach 2:
The patent changes the modulation parameters by using a second delta-sigma modulator specifically for the DAC control, which modulates the DAC quantization noise out of the PLL passband. This parameter change in modulation strategy allows noise to be pushed to frequencies outside the bandwidth of interest.
2Object-generated harmful factors
If a separate bank of current sources is added to implement a canceling DAC, then quantization noise is reduced, but thermal noise, 1/f noise, switch charge injection, error charge due to component mismatch, device leakage current and supply current are introduced
Solution Approach 1:
The patent makes the charge pump current sources serve dual functions: both the primary charge pumping operation and the quantization noise cancellation. The same up and down current sources are used for both the PLL charge transfer and the DAC noise cancellation, eliminating the need for separate current banks and their associated noise sources.
Solution Approach 2:
The patent merges the noise cancellation function with the existing charge pump structure. The DAC control signals directly modulate the existing up and down current sources, combining the phase detection, charge pumping, and noise cancellation functions into a unified structure that avoids adding separate noisy current banks.
3Device complexity
If the charge pump exhibits nonlinearity due to size mismatch between up and down current sources, then the circuit is simpler, but high frequency quantization noise is modulated down into the pass band of the PLL, corrupting the output clock
Solution Approach 1:
The patent introduces feedback through the delta-sigma modulator that monitors and compensates for the effects of charge pump nonlinearity. The modulator adjusts the DAC control signals based on the quantization noise characteristics, effectively counteracting the noise modulation caused by current source mismatch and maintaining linear operation.
Data Source
Figure 1A~1C
Figure 2~3
Figure 4~5C
AI summary
A first current source supplies a first charge amount responsive to a first pulse signal from the phase frequency detector and a second current source supplies a second charge amount according to a fixed value and a variable value. The variable value corresponds to a phase difference between a first feedback clock signal and a hypothesized feedback clock signal with reduced quantization noise. The first and second charge amounts are of opposite polarity. A single set of first and second current sources perform the functions of charge pump and noise reduction DAC.