RF Harmonic Cancellation Circuit for TV Tuners
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Solution Overview
Problem
Conventional wideband wireless systems face challenges in accurate radio frequency filtering due to the presence of undesired blockers, which can lead to signal smearing and increased system costs, particularly in harmonic reject filtering where odd-order harmonics of the LO frequency fold onto the desired signal, making it difficult to filter out without significant complexity and cost.
Innovation Solution
The implementation of an RF filter block with a low-pass filter and tracking harmonic notch filters, which includes a circuit design with a primary conversion signal path and a harmonic feedforward signal path to downscale and subtract odd-order harmonic content from the RF signal, effectively canceling these harmonics and reducing system complexity and cost.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If conventional RF filtering using High-Q filters and discrete passive devices is used, then far-out blockers are attenuated, but system cost increases and filter tuning complexity increases
Solution Approach 1:
The patent replaces mechanical/discrete passive filter tuning mechanisms with an electronic digital control system. The filter tuning word is generated digitally based on the desired RF channel frequency, eliminating the need for manual mechanical tuning of discrete passive devices. This substitution reduces tuning complexity while maintaining far-out blocker attenuation performance.
2Reliability
If RF filtering is implemented to reduce input power to LNA, then effective linearity is enhanced, but system cost increases due to external filters
Solution Approach 1:
The patent merges the RF filtering function with the on-chip LNA and down-conversion mixer into an integrated RF tuner component. The filtering is performed using on-chip passive devices controlled by digital signals, eliminating the need for external discrete passive filters. This integration maintains effective linearity enhancement while reducing system cost and complexity.
3Object-affected harmful factors
If image rejection filtering is performed at IF frequency, then adjacent blockers are attenuated, but the desired signal may droop and system complexity increases
Solution Approach 1:
The patent performs image rejection filtering at the RF frequency stage before down-conversion, rather than at the IF stage. By using a bandpass filter tuned to the desired RF channel frequency to pre-filter the RF signal before it enters the LNA and mixer, the system achieves adjacent blocker attenuation while avoiding signal droop issues that would occur with sharp IF filters. This preliminary filtering action maintains signal integrity.
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 improved harmonic rejection with reduced system complexity and cost, providing at least 30 dB of filtering, thereby minimizing the need for external filters and enhancing the overall efficiency of RF-based devices.
Implementation Method 1
The down-converter mixer produces frequency terms that are the sums and differences between the positive and negative values of the frequencies of signals found in the RF spectrum, including the frequency fCH of the desired RF signal and the frequency fBL of an undesired blocker. The relevant frequency terms are the difference products fCH-fLO, fLO-fCH, fBL-fLO, and fLO-fBL.
Implementation Method 2
Assuming that a low-pass filter (LPF) following the down-converter mixer attenuates the sum terms produced by the down-converter mixer
Data Source
AI summary
A radio frequency (RF) receiver, such as a television tuner, includes a harmonic cancellation circuit. The harmonic cancellation circuit includes a primary signal path to generate a first intermediate frequency (IF) signal by mixing an RF signal with a reference signal and a harmonic feedforward signal path to generate a second IF signal representing signal content of the RF signal near an nth-order harmonic of a frequency fLO of the first reference signal, n comprising a positive integer. The harmonic cancellation circuit further includes a summation stage to generate a third IF signal based on a difference between the first IF signal and the second IF signal.