Biquad VGA Lowpass Filter for High SNR and Linearity
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Solution Overview
Problem
Existing VGA filter architectures face challenges in integrating filtering and gain functions without compromising noise performance or linearity, particularly in analog signal chains where amplifying small wide-band signals and filtering large unwanted blockers are required.
Innovation Solution
A biquad filter architecture based on the Sallen-Key circuit is modified to incorporate a follower current and transconductance, allowing for low-noise, high-Q gain stages with minimal current consumption, thereby distributing gain and filtering along a signal chain with low-noise, high-Q biquad gain stages.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a VGA is placed in series with a lowpass filter to amplify small wide-band signals and filter large unwanted blockers, then the signal to noise ratio is improved, but very difficult non-linearity constraints are placed on the filter
Solution Approach 1:
The patent combines the VGA and lowpass filter into a single integrated circuit architecture where the filter provides both attenuation of blockers and gain amplification through its active components. This merging eliminates the need for a separate VGA stage, thereby improving SNR while avoiding the non-linearity problems that would arise from cascading separate amplifier and filter stages.
Solution Approach 2:
The filter circuit is designed to perform multiple functions simultaneously: it acts as both a lowpass filter to attenuate high-frequency blockers and a variable gain amplifier to amplify the desired baseband signal. The active components (operational amplifiers, transistors) provide both filtering and amplification capabilities within a single circuit block, reducing system complexity.
2Ease of operation
If an architecture consisting of first filtering the blockers and then amplifying the signal is used, then non-linearity problems are avoided, but noise constraints are placed upon the filter
Solution Approach 1:
By integrating the amplification function within the filter circuit itself, the patent eliminates the need for a separate post-filter amplifier stage. The filter's active components provide gain amplification after filtering, thereby avoiding non-linearity issues while simultaneously amplifying the signal before it would be affected by subsequent noise, thus resolving the noise constraint problem.
3Object-generated harmful factors
If high order filters are used to filter large unwanted blockers in a reverberant or high clutter background, then the elimination capacity is improved, but the filter complexity increases with several cascaded biquad filters
Solution Approach 1:
The patent implements a high-order filter (6th order or higher) by cascading multiple biquad filter sections, each providing a pair of complex conjugate poles. This segmentation allows the achievement of high elimination capacity for blockers while maintaining manageable complexity through modular design, where each biquad section can be independently designed and optimized.
Data Source
AI summary
A biquad gain stage, as well as a Variable Gain Amplifier is disclosed. The biquad gain stage comprises a plurality of transistors as well as conductances, and capacitances, as well as current sources. The resulting variable gain amplifier comprising a plurality of biquad gain stage cascaded in series allows to filter large unwanted blockers and to amplify a small wide-band signal. Both the gain and the filtering are distributed along a signal chain comprising a series of low-noise, high-Q biquad gain stages, each with limited current consumption and low component ratios.