Digitally Tunable Capacitor PWM Weighting for Fine RF Tuning
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Solution Overview
Problem
Digitally tunable capacitors face limitations in resolution and performance due to spurious signals, low time or amplitude resolution, non-uniform step sizes, and manufacturing constraints, which affect their accuracy and effectiveness in applications like oscillators and RF transmitters.
Innovation Solution
The use of a pair of switchable capacitors with a larger capacitance ratio, driven by (M+1)-ary pulse width modulated signals, and an offset switchable capacitor to achieve high-resolution capacitance tuning, minimizing spurious signals and phase noise, with a pulse generator controlling the modulation signals to adjust the capacitance precisely.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If digital PWM is used to tune capacitance, then resolution is improved, but spurious signals are introduced
Solution Approach 1:
The patent applies periodic action by using a periodic PWM signal to modulate the capacitor. The PWM signal operates at a specific frequency with a defined duty cycle, creating a time-varying capacitance effect. This periodic modulation allows the capacitor to achieve fine resolution tuning while the regular periodic nature helps minimize spurious signals through predictable spectral characteristics.
Solution Approach 2:
The patent changes the duty cycle parameter of the PWM signal to adjust the effective capacitance. By varying the duty cycle from 0% to 100%, the capacitor can be tuned continuously within its range. This parameter change approach provides high resolution tuning capability while maintaining a single capacitor structure, avoiding the spurious signals associated with switching between multiple discrete capacitors.
2Device complexity
If a single capacitor with digital PWM is used, then device complexity is reduced, but resolution is limited by timeslot pulse width
Solution Approach 1:
The patent applies dynamics by making the capacitor switching behavior time-varying through PWM control. Instead of static switching between discrete capacitor values, the single capacitor is dynamically modulated using a PWM signal with variable duty cycle. This dynamic approach allows the effective capacitance to take on any value within the capacitor's range, providing high resolution tuning without requiring multiple discrete capacitor elements.
3Measurement precision
If high frequency master clock is used for PWM, then timeslot resolution is improved, but manufacturing constraints are violated
Solution Approach 1:
The patent applies partial action by using a moderate frequency master clock that provides sufficient timeslot resolution for the application requirements, rather than using the maximum possible frequency. This approach achieves the necessary capacitance tuning resolution while staying within semiconductor manufacturing capabilities and avoiding excessive frequency that would violate design rules or increase power consumption unnecessarily.
Data Source
AI summary
The disclosed device can contain a pair of switchable capacitors, one of which has the larger capacitance of the pair. Each of the switchable capacitors can include a capacitor in series with a switch. Both switchable capacitors can be connected in a parallel circuit that has a tunable capacitance. The ratio of the capacitances of the pair can approximately equal a ratio of mutually prime integers. In a particular case, the ratio of capacitances can approximately equal a ratio of two consecutive integers. The capacitance ratio can be called a weight or weight ratio. A switch controller can drive the pair of switchable capacitors with a pair of (M+1)-ary pulse width modulated signals, each of which has the same modulation period.