Switched-Capacitor Loop Filter Using Predistortion for PLL Spurs
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Solution Overview
Problem
Fractional-N phase-locked loops (PLLs) suffer from quantization noise and spurs due to fractional control modules, limiting their loop bandwidth and lock time, which is undesirable for high-performance applications.
Innovation Solution
A switched-capacitor loop filter is employed in a closed-loop system, utilizing two switched-capacitor networks and control signals to adjust the output voltage, enabling quantization noise cancellation through predistortion and charge sharing, thereby reducing reference feedthrough and propagation delay.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If the loop bandwidth of the PLL is reduced to reduce quantization noise and spurs, then spectral purity is improved, but lock time increases and VCO noise reduction is compromised
Solution Approach 1:
The patent converts the harmful quantization noise and spurs generated by the fractional control module into a beneficial effect by using them to control the switched-capacitor networks. The quantization noise modulates the switched capacitors in a way that creates predistortion, which cancels the harmful spurs when combined with the charge pump output, thus transforming the harmful factor into a useful noise cancellation mechanism
Solution Approach 2:
The switched-capacitor networks act as an intermediary between the fractional control module and the charge pump. They receive the quantization noise from the fractional control module and transform it into a predistorted signal that cancels the spurs. This intermediary component enables the system to maintain wide loop bandwidth while achieving noise cancellation, resolving the contradiction between lock time and spectral purity
2Loss of time
If the loop bandwidth is increased to reduce VCO noise and speed up lock time, then lock time is reduced, but quantization noise and spurs increase
Solution Approach 1:
The patent enables the system to operate with wide loop bandwidth (fast lock time) by converting the harmful quantization noise into a beneficial predistortion signal. The switched-capacitor networks use the quantization noise to modulate their capacitance values, creating a cancellation signal that eliminates the spurs even at wide bandwidths, thus resolving the contradiction between lock time and spectral purity
Solution Approach 2:
The switched-capacitor networks dynamically adjust their capacitance values based on the quantization noise from the fractional control module. This dynamic adaptation allows the system to maintain optimal performance across varying operating conditions, enabling wide loop bandwidth operation without suffering from increased quantization noise and spurs
3Object-generated harmful factors
If switched-capacitor networks are used for noise cancellation, then quantization noise is reduced, but device complexity increases
Solution Approach 1:
The switched-capacitor networks perform multiple functions simultaneously: they act as the loop filter, generate predistortion for noise cancellation, and dynamically adjust based on quantization noise. This multi-functionality reduces the need for separate components, achieving noise cancellation without proportionally increasing overall system complexity
Solution Approach 2:
The patent changes the operational parameters of the switched-capacitor networks, using them to switch between different capacitance values based on control signals. This parameter modulation enables the networks to achieve noise cancellation through predistortion while maintaining a relatively simple structure compared to traditional analog loop filters with multiple passive components
Data Source
AI summary
A loop filter has a first switched-capacitor network and a second switched-capacitor network. The first switched-capacitor network is coupled to an input node of the loop filter. The second switched-capacitor network is coupled to the input node of the loop filter. The input node of the loop filter is arranged to receive an input from a charge pump.