Post Mixer I/Q Equalization for High Image Rejection
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Solution Overview
Problem
Current broadband tuners face challenges in achieving high image rejection, linearity, and signal-to-noise ratio without increasing circuit size, particularly in TV tuners that require up to 70 dB of image rejection.
Innovation Solution
A post-mixing equalization technique is employed to equalize the amplitudes and align the phases of differential output signals from an I/Q mixer, using Variable Gain Amplifiers and polyphase filters to achieve greater than 70 dB of image rejection without enlarging the tuner's size.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If filters such as SAW filters, variable bandpass filters, and polyphase filters are used to reject image signals, then image rejection is improved, but device complexity and circuit size are increased
Solution Approach 1:
The patent replaces complex physical filtering mechanisms (SAW filters, variable bandpass filters) with an electronic signal processing approach using I/Q mixing and digital signal processing. The image rejection is achieved through mathematical operations (mixing with quadrature local oscillators, low-pass filtering, and combination of I and Q paths) rather than complex physical filters, thereby reducing circuit size while maintaining high image rejection capability
Solution Approach 2:
The patent changes the approach from physical filter parameters to signal processing parameters. By adjusting the quadrature phase relationship (90 degrees) between I and Q local oscillators and using programmable low-pass filter cutoff frequencies, the system achieves adaptable image rejection without requiring large physical filters. The image rejection ratio can be tuned by adjusting signal processing parameters rather than physical filter characteristics
2Measurement precision
If double-conversion tuners with fixed IF SAW filters are implemented, then image rejection of up to 70 dB is achieved, but device complexity and circuit size are increased
Solution Approach 1:
The patent extracts the image rejection function from complex physical filtering stages and implements it through a simplified single-conversion architecture with I/Q mixing. Instead of using double-conversion with fixed IF SAW filters, the invention separates the image rejection function into quadrature mixing operations and subsequent digital signal combination, removing the need for complex intermediate frequency filtering hardware
Solution Approach 2:
The patent substitutes mechanical/physical filtering systems (SAW filters, multiple conversion stages) with electronic signal processing operations. The image rejection is achieved through mathematical combination of I and Q path signals after quadrature mixing, replacing the need for fixed IF SAW filters and reducing overall device complexity
3Measurement precision
If single-conversion tuners with complex input tracking filters are used, then image rejection is improved, but device complexity is increased
Solution Approach 1:
The patent replaces complex input tracking filters with I/Q quadrature mixing and digital signal processing. Instead of using frequency-tuned physical filters to track and reject images across different channels, the invention uses quadrature local oscillators and programmable low-pass filters followed by digital combination of I and Q paths, achieving tracking-free image rejection with reduced circuit complexity
4Measurement precision
If wide band matching techniques are used before mixing operation, then image rejection is improved, but signal-to-noise ratio is reduced
Solution Approach 1:
The patent applies preliminary quadrature mixing with low-pass filtering before the critical signal combination stage, preparing the I and Q signals with proper frequency content and phase relationships. This preliminary processing ensures that when the signals are combined to achieve image rejection, the desired signal components are properly preserved while image components are suppressed, maintaining high signal-to-noise ratio alongside high image rejection
Data Source
AI summary
The invention provides a system and method for tuning broadband signals by using post mixer I/Q equalization. An Image Rejection Mixer (IRM) is used for mixing Radio Frequency (RF) signals and rejecting image signals from the desired RF signals. The IRM includes an I/Q mixer and a filter. The I and Q paths resulting from the mixing operation in the I/Q mixer are equalized in amplitude and phase by an I/Q equalizer. Thereafter, the image signals are rejected from the desired RF signals using the filter.