RC Oscillator Capacitor Array for Linear Monotonic Frequency Tuning
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Solution Overview
Problem
Current variable capacitors in oscillators exhibit non-linear frequency-tuning steps due to the reciprocal relationship between capacitance and frequency, leading to inaccurate frequency adjustments and difficulties in compensating for temperature and process variations.
Innovation Solution
The use of binary-switched capacitor arrays with different unit capacitances, where each array adjusts its total capacitance using a binary code, and a thermometer code selects which arrays are connected, compensating for the non-linear relationship by resetting unit capacitance values to maintain a more linear frequency response.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If binary-weighted capacitors are used for variable capacitance tuning, then the capacitor array can be controlled with binary codes, but the frequency-tuning steps become non-linear due to the reciprocal relationship between capacitance and frequency
Solution Approach 1:
The capacitor array is divided into multiple sub-arrays, each with binary-weighted capacitors. A thermometer code selects which sub-arrays are active, while binary codes tune capacitors within each sub-array. This segmentation allows the system to maintain binary code control while achieving more linear frequency tuning by resetting unit capacitance values across different sub-arrays.
Solution Approach 2:
The patent changes the parameter of unit capacitance values across different sub-arrays. By resetting and adjusting unit capacitance values in each sub-array, the system compensates for the non-linear reciprocal relationship between capacitance and frequency, achieving more uniform frequency-tuning steps while maintaining binary code controllability.
2Device complexity
If a single binary-weighted capacitor array is used, then the circuit complexity is reduced, but temperature and process variations cause frequency inaccuracies that are difficult to compensate
Solution Approach 1:
The capacitor array is segmented into multiple sub-arrays, each with its own binary-weighted capacitors and unit capacitance values. This segmentation allows independent tuning and compensation for temperature and process variations in each sub-array, improving frequency accuracy while maintaining relatively simple circuit structures.
Solution Approach 2:
The patent implements preliminary compensation by designing sub-arrays with specific unit capacitance values that account for temperature and process variations. The thermometer code selection and binary code tuning are pre-configured to compensate for these variations before they affect frequency accuracy, reducing the need for trial-and-error adjustments.
3Ease of operation
If binary codes are used to control capacitor switching, then the control is simple and direct, but the frequency adjustments are non-uniform with larger changes at low binary values and smaller changes at high binary values
Solution Approach 1:
The capacitor array is divided into sub-arrays controlled by thermometer codes, with binary codes controlling capacitors within each sub-array. This segmentation transforms the single non-linear tuning curve into multiple smaller tuning segments, each with more uniform frequency steps, while maintaining binary code control simplicity.
Solution Approach 2:
The patent changes unit capacitance values across different sub-arrays to compensate for the non-uniform frequency tuning. By adjusting these parameters, each sub-array contributes to a more uniform overall frequency response, maintaining the simplicity of binary code control while achieving uniform frequency-tuning steps.
Data Source
AI summary
An RC oscillator has a variable capacitor that sets the output frequency. The variable capacitor has m binary-weighted switched capacitor arrays, and each binary-weighted switched capacitor array has binary-weighted capacitors. p binary bits are decoded into an m-bit thermometer code that selects one of the m binary-weighted switched capacitor arrays to use n binary bits to switch its binary-weighted capacitors. Other binary-weighted switched capacitor arrays have all their capacitors switched on, or all their capacitors switched off by the thermometer code. The smallest or unit capacitance of each binary-weighted switched capacitor array is adjusted to compensate for the non-linear reciprocal relationship of frequency being proportional to 1/RC. The unit capacitance is increased for each successive binary-weighted switched capacitor array to reset to the ideal linear relationship of the (p,n)-bit code to frequency.