Quadrature Phasor Measurement for Faster Capacitive Scanning
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Solution Overview
Problem
Existing methods for scanning touch screen electrodes, such as TDMA and CDMA, require multiple chip time intervals to distinguish individual nodes, leading to inefficiencies in signal-to-noise ratio (SNR) and interference estimation accuracy, particularly in sensor applications.
Innovation Solution
A capacitive sensing system using quadrature modulation (QM) drives two transmit electrodes in phase quadrature, allowing simultaneous measurement of twice the number of electrodes while maintaining SNR by computing phasors of the received signals, and combining with CDMA for enhanced decoding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If multiple transmit electrodes are driven simultaneously using CDMA, then the signal to noise ratio is improved, but the number of chips required to scan the whole screen still equals the number of X electrodes
Solution Approach 1:
The patent transitions from CDMA's single-dimensional (one chip per electrode) approach to quadrature modulation's two-dimensional approach by utilizing both in-phase and quadrature components simultaneously. This allows two transmit electrodes to be driven in parallel during each chip interval, effectively doubling the scanning throughput while preserving the SNR benefits of simultaneous multi-electrode driving.
2Measurement precision
If TDMA is used to drive one X electrode at a time, then individual nodes are distinctly measured, but the number of chip time intervals required equals the number of X electrodes
Solution Approach 1:
The patent segments the measurement process by separating the measurement of in-phase and quadrature components into distinct computational streams. By measuring both components simultaneously during each chip interval and then processing them separately to extract individual node capacitances, the system achieves both fast parallel scanning and precise individual node measurement.
Solution Approach 2:
The patent adds a temporal dimension to the measurement by utilizing quadrature phase shifts (0° and 90°) to encode information from multiple transmit electrodes. This allows the system to measure multiple nodes simultaneously in parallel while maintaining the ability to distinctly resolve individual node contributions through phase-based separation.
3Productivity
If quadrature modulation is used to double the number of TX electrodes driven at a time, then scanning speed is improved, but maintaining SNR and touch estimation accuracy becomes challenging
Solution Approach 1:
The patent introduces phasor representation as an intermediary that combines the in-phase and quadrature measurement results. By computing the magnitude and phase of the received signals as phasors, the system can simultaneously process contributions from multiple transmit electrodes while preserving the SNR information needed for accurate touch estimation and mutual capacitance measurement.
Data Source
AI summary
Methods and systems for capacitive sensing provide first and second transmit electrodes and a receive electrode positioned to have mutual capacitances between the first and second transmit electrodes and the receive electrode at mutual capacitance nodes, wherein the mutual capacitance at respective ones of mutual capacitance nodes changes when an interfering object is proximate, to apply simultaneously a first drive signal to the first transmit electrode and a second drive signal to the second transmit electrode, wherein the first and second drive signals are in quadrature with each other; to receive a receive signal via the receive electrode; measure a phasor of the receive signal; and identify a change in mutual capacitance based on the phasor of the receive signal.