Capacitive Touch Controller Frequency Hopping for Noise Rejection
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Solution Overview
Problem
Existing frequency hopping techniques for capacitive touch screens have limited resolution in selecting scanning frequencies, which can lead to inadequate noise rejection due to external noise harmonics, especially when all available frequencies are near noise harmonics.
Innovation Solution
A control circuit alternates the scanning frequency between at least two different frequencies within each half-cycle of the force signal, allowing for a fractional mixed frequency that increases the available frequency options and enhances noise rejection by canceling noise harmonics.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If frequency hopping technique is used to move scanning frequency away from noise harmonics, then noise rejection is improved, but the limited number of available scanning frequencies may result in all selectable frequencies being near noise harmonics
Solution Approach 1:
The patent applies dynamics by transitioning from static frequency selection to dynamic frequency modulation. The system alternates between multiple scanning frequencies (first frequency and second frequency) within each frame period, allowing the scanning frequency to change dynamically rather than remaining fixed. This dynamic approach enables the system to avoid noise harmonics more effectively by having multiple frequency options available during operation.
Solution Approach 2:
The patent implements periodic action by alternating between different scanning frequencies in a regular pattern within each frame period. The force driver applies forcing signals at different frequencies in successive intervals, creating a periodic frequency hopping pattern. This periodic alternation ensures that noise harmonics are not continuously present at the scanning frequency, improving noise rejection while maintaining a structured approach to frequency selection.
2Device complexity
If conventional frequency hopping with limited frequencies is used, then implementation is simple, but finer resolution selection of scanning frequency is not achieved
Solution Approach 1:
The patent applies segmentation by dividing the frame period into multiple sub-intervals, each using a different scanning frequency. Instead of using a single frequency for the entire frame, the system segments the timing into first and second intervals with different frequencies, thereby achieving finer frequency resolution without requiring a completely new implementation approach. This segmented frequency approach maintains relative simplicity while improving frequency selection precision.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach provides finer resolution in selecting optimal scanning frequencies, significantly improving noise rejection and allowing for complete or almost complete cancellation of external noise focused at specific harmonics.
Implementation Method 1
an integrator circuit configured to integrate the sense signal
Data Source
AI summary
A circuit includes a force driver to apply a force signal to a force node associated with a mutual capacitance to be sensed, and a charge to voltage converter having an input coupled to receive a sense signal from a sense node associated with the mutual capacitance to be sensed. The charge to voltage converter includes an integrator circuit to integrate the sense signal to sense the mutual capacitance, an input switch between the input of the charge to voltage converter and an input of the integrator circuit, and a reset switch between an output of the integrator circuit and the input of the integrator circuit. A control circuit controls generation of the force signal to alternate between at least two different frequencies and generates, for each half cycle of the force signal, a first signal for closing the input switch and a second signal for closing the reset switch.