Programmable RC Filter Capacitor Array for High-Frequency Resolution
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Solution Overview
Problem
Existing programmable active RC filters face low bandwidth resolution at high frequencies due to the parabolic relationship between filter bandwidth and capacitance, making it difficult to achieve linear control over filter bandwidth with a binary control word, especially at high bandwidth values.
Innovation Solution
The solution involves dividing the filter bandwidth into multiple 'bands' by re-adjusting each 'bit unit' in the capacitor array, allowing for a more linear relationship between filter bandwidth and the binary control word, achieved by switching capacitors in parallel and using logic circuitry to gate less significant bits based on more significant bits in the control word.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a conventional binary-weighted capacitor array is used to control filter bandwidth, then the filter can be programmed with a binary control word, but the bandwidth resolution becomes very low at high bandwidth values due to the parabolic relationship between bandwidth and capacitance
Solution Approach 1:
The capacitor array is divided into multiple banks (e.g., five banks), each bank being switchable independently. This segmentation allows different subsets of capacitors to be activated based on the desired bandwidth range, effectively creating multiple resolution zones across the bandwidth spectrum.
Solution Approach 2:
Different regions of the capacitor array are assigned different weights and activation conditions. Less significant bits control smaller capacitor values for fine resolution at lower bandwidths, while more significant bits control larger capacitor values for coarser but necessary steps at higher bandwidths. This local differentiation optimizes resolution where needed.
2Measurement precision
If the number of control bits is increased to improve bandwidth resolution, then finer control over bandwidth is achieved, but the device complexity and circuit size increase significantly
Solution Approach 1:
The capacitor array dynamically reconfigures its effective capacitance values based on the control word input. By using switch banks that can be selectively activated, the same physical capacitor array provides different resolution characteristics across different bandwidth ranges, effectively adapting to the control requirements without adding permanent circuit complexity.
Solution Approach 2:
The segmented capacitor array serves multiple functions: it provides both coarse bandwidth adjustment (using higher-significance bit banks) and fine bandwidth adjustment (using lower-significance bit banks) within the same circuit structure. This multi-functionality allows a single array to replace what would otherwise require multiple separate capacitor sets or a much larger single array.
3Ease of manufacture
If binary-weighted capacitors are used with equal weighting factors, then the circuit design is simplified, but the bandwidth points corresponding to successive binary steps are far apart at high bandwidths
Solution Approach 1:
The capacitor banks are designed with asymmetric weighting patterns rather than uniform binary weighting. Each bank's capacitors have weights tailored to the specific bandwidth range they control, creating an asymmetric distribution that compensates for the non-linear bandwidth-capacitance relationship and produces more uniform bandwidth steps across the full range.
Data Source
AI summary
An active RC filter (20) includes a first resistive element (23) and a first capacitor array (10/50) which co-acts with the first resistive element (23) to determine a bandwidth characteristic of the programmable active RC filter circuit (20). The total filter capacitance is programmed by switching various first capacitors (4-0, 1, 2 . . . 7) of a first capacitor array (10) in parallel between first and second terminals of the first capacitor array in response to a control word (B0, 1, 2 . . . 7) to determine a first portion of the bandwidth characteristic, and by switching various second capacitors (7-0, 1, 2 . . . 6) of the first capacitor array between the first and second terminals of the first capacitor array in parallel with various ones of the first capacitors (4-0, 1, 2 . . . 7) of the first capacitor array (10) in response to less significant bits (B0, 1, 2 . . . 6) of the control word (B0, 1, 2 . . . 7) to determine a second portion of the bandwidth characteristic.