RF Filter Circuit Using Active IF Feedback for Blocker Rejection
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Solution Overview
Problem
Wideband wireless systems face challenges in accurate RF filtering due to the presence of undesired blockers, which require complex and costly high-Q filters and dual-conversion architectures, and struggle with harmonic rejection, especially when using square wave local oscillators, leading to signal smearing and increased system complexity.
Innovation Solution
The implementation of RF filtering techniques using an intermediate frequency (IF)-based active feedback signal path, which employs transconductors and mixers to transfer impedance from IF to RF, allowing for sharp filtering without the need for high-Q components and enabling harmonic cancellation by subtracting harmonic feedforward signals, thereby reducing the complexity and cost of RF filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If high-Q filters composed of discrete passive devices are used for RF filtering, then far-out blockers are attenuated, but system cost and device complexity increase
Solution Approach 1:
The patent introduces an intermediate frequency (IF) signal path as a mediator between the RF input and baseband processing. By down-converting RF signals to IF frequency and performing filtering at this intermediate stage, the system achieves effective blocker rejection without requiring complex high-Q RF filters. The IF filter serves as an intermediary filtering stage that simplifies the overall RF front-end design.
Solution Approach 2:
The patent replaces the traditional mechanical/discrete passive filter structure (high-Q filters composed of resistors, capacitors, inductors) with an active digital signal processing approach. Instead of using physical high-Q resonant circuits, the system uses active mixing stages and digital filtering algorithms to achieve the same spectral selection function, thereby reducing device complexity and enabling integration.
2Object-affected harmful factors
If dual-conversion architecture is used for image rejection, then image blockers are rejected, but device complexity and cost increase
Solution Approach 1:
The patent extracts and cancels the image frequency component separately from the desired signal path. By identifying the image frequency location (f_image = f_LO ± f_IF) and using a dedicated cancellation path with adjustable gain and phase control, the system removes the image blocker without requiring a complex dual-conversion architecture. This extraction and separate cancellation approach simplifies the overall system design.
Solution Approach 2:
The patent applies preliminary anti-action by pre-canceling the image frequency component before it interferes with the desired signal processing. The image rejection mechanism is implemented in advance through a cancellation path that generates an anti-phase signal at the image frequency, effectively neutralizing potential image blockers before they can contaminate the baseband output.
3Ease of manufacture
If square wave local oscillators are used for down-conversion, then cost is reduced, but harmonic distortion and signal smearing increase
Solution Approach 1:
The patent converts the harmful harmonic distortion products generated by square wave local oscillators into a manageable filtering problem. Rather than avoiding square wave LOs, the system accepts their inherent harmonics and uses sharp IF filtering to selectively reject the down-converted harmonic images. The harmful harmonics are transformed into predictable spectral components that can be filtered out at the IF stage, turning a disadvantage into a controllable parameter.
Solution Approach 2:
The patent introduces sharp IF filtering as an intermediary stage between the square wave LO down-conversion and baseband processing. This intermediate filtering stage acts as a mediator that separates the desired signal from the harmonic distortion products, allowing the use of simple square wave LOs while maintaining signal purity through the IF filter barrier.
4Object-affected harmful factors
If sharp IF filtering is applied after down-conversion, then adjacent blockers are attenuated, but desired signal droop increases
Solution Approach 1:
The patent employs dynamic tuning of the IF filter characteristics based on the selected channel frequency. By making the filter parameters adjustable and adaptive rather than fixed, the system can optimize the trade-off between adjacent blocker rejection and desired signal passband characteristics for each channel, preventing signal droop while maintaining blocker attenuation.
Solution Approach 2:
The patent changes the operating parameters of the IF filter dynamically according to the tuned channel. By adjusting filter center frequency, bandwidth, and Q-factor based on the selected channel, the system adapts the filtering characteristics to preserve the desired signal while maintaining effective adjacent blocker rejection, avoiding the fixed trade-off inherent in static filter designs.
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 achieves effective RF filtering with a decimation of unwanted signal content by 36 dB, reducing the need for external filtering and simplifying the RF tuner design, while maintaining high precision and stability, thus enhancing the overall performance and reducing hardware overhead.
Implementation Method 1
The resulting amplified signal is then fed into a down-conversion mixer that frequency translates the desired RF signal into an intermediate frequency (IF) signal
Implementation Method 2
employs transconductors and mixers to transfer impedance from IF to RF
Data Source
AI summary
A receiver, such as a television tuner, includes a radio frequency (RF) filter circuit. The RF filter circuit includes a filter, a first node, and a second node coupled to the filter, and a conversion signal path having an input coupled to the first node and an output coupled to the second node, the conversion signal path having an active mixer coupled between the first node and the second node. The active mixer can include, for example, a first transconductor and a first mixer coupled in series between the first node and the second node. The RF filter circuit further includes a feedback signal path having an input coupled to the second node and an output coupled to the first node, the feedback signal path including a second transconductor and a second mixer coupled in series between the second node and the first node.