RF FIR Filter Input Termination for Low-Noise Down-Conversion
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Solution Overview
Problem
Discrete time filters (DTFs) are not suitable as the first stage of communication receivers due to their reactive input termination and high input noise bandwidth, which leads to noise addition and limited linearity, making them unsuitable for radio frequency (RF) input stages.
Innovation Solution
A communication receiver with a finite impulse response (FIR) DTF that includes an input circuit with parallel branches and a shunt inductance to resonate the reactive part of the equivalent input impedance around the sampling frequency, converting it to a real impedance, allowing for direct down-conversion and filtering of RF signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If discrete time filters are used as the first stage of communication receivers, then down-conversion and filtering can be performed, but the reactive input termination and large input noise bandwidth cause noise addition and limited linearity
Solution Approach 1:
The patent transforms the reactive input impedance to a real impedance by adjusting the operating parameters and circuit configuration. This allows the DTF to present a resistive input termination that is better matched to the source, reducing noise figure and improving linearity while maintaining the down-conversion function.
Solution Approach 2:
The patent introduces an intermediary circuit stage between the RF input and the DTF that transforms the reactive impedance to a real impedance. This intermediary structure enables the DTF to function as the first stage without suffering from the harmful effects of reactive termination and excessive noise bandwidth.
2Productivity
If discrete time filters are used as the first stage of communication receivers, then filtering and down-conversion can be performed, but linearity is limited due to the reactive input termination
Solution Approach 1:
The patent changes the impedance parameter from reactive to real by modifying the circuit configuration. This transformation improves the linearity of the input stage, making the DTF suitable for use as the first stage in communication receivers where high linearity is required.
3Manufacturing precision
If sampling capacitances are accurately matched in FIR DTFs, then filtering performance is improved, but device complexity increases
Solution Approach 1:
The patent divides the filtering function into multiple parallel branches, each with simpler capacitance requirements. This segmentation allows for easier manufacturing and matching of sampling capacitances while maintaining overall filtering performance, reducing the complexity compared to a single complex filter structure.
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 configuration enables the FIR DTF to perform both input termination and down-conversion of RF signals effectively, improving linearity and reducing noise bandwidth, making it suitable for use as the first stage of a communication receiver.
Implementation Method 1
The input circuit comprises parallel input shunt inductance configured to resonate a reactive part of the equivalent input impedance of the N parallel branches around the sampling frequency, thereby converting an equivalent input impedance of the at least one FIR DTF at the sampling frequency to a real impedance
Data Source
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AI summary
There is provided a communication receiver comprising: an input for receiving a radio frequency, RF, input signal; and at least one finite impulse response, FIR, discrete time filter, DTF. The at least one FIR DTF comprises: an input circuit comprising an input port for sampling the RF input signal at a sampling frequency that is comparable to the input RF input signal; and N parallel branches, each branch having a set of input unit sampling capacitances, where each unit sampling capacitance is independently selectively coupleable to an output summing node. The input circuit is configured to convert an equivalent input impedance of the at least one FIR DTF around the sampling frequency to a real impedance.