High-IF Superheterodyne Receiver With Complex BPF Image Rejection
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Solution Overview
Problem
Conventional RF receivers suffer from sensitivity to 1/f noise, dc offsets, and image folding issues due to their homodyne architecture, which worsen with technology scaling, and require bulky and expensive SAW filters to filter out blocking interferers, limiting their integration and performance.
Innovation Solution
A reconfigurable superheterodyne receiver employing a 3rd order complex IQ charge-sharing band-pass filter for image rejection and 1st order feedback based RF band-pass filtering, eliminating the need for SAW filters and addressing homodyne receiver disadvantages by using inverter-based gain stages and discrete-time filtering.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a homodyne receiver architecture is used, then high-level integration and low-pass filtering capability are achieved, but sensitivity to 1/f noise, dc offsets, and image folding issues worsen with technology scaling
Solution Approach 1:
The patent inverts the conventional homodyne architecture by using a superheterodyne approach with image-reject filtering. Instead of directly downconverting to baseband (homodyne), the system first converts to an intermediate frequency where image components can be filtered out before final demodulation, thereby eliminating the sensitivity issues while maintaining integration benefits
Solution Approach 2:
The filtering function is segmented into multiple stages: a first complex band-pass filter for image rejection at IF, and a second complex band-pass filter for channel selection. This segmentation allows each filter to specialize in specific frequency rejection tasks, improving overall performance while maintaining compact integration
2Reliability
If SAW filters are used to filter out blocking interferers, then receiver performance is improved, but device size and cost increase
Solution Approach 1:
The patent replaces bulky mechanical SAW filters with integrated complex band-pass filters implemented using standard CMOS circuitry. The first complex BPF uses quadrature mixing and digital signal processing to achieve the same blocking interferer rejection function in a compact, scalable form factor suitable for modern integrated circuits
Solution Approach 2:
The filter characteristics are made reconfigurable through digital control of the complex band-pass filters. By changing filter parameters (center frequency, bandwidth, Q-factor) through digital means, the system achieves adaptable blocking interferer rejection without requiring physical filter changes or additional hardware
3Measurement precision
If the gain of the LNA is kept high to handle weak desired signals, then signal detection capability is improved, but gain compression and higher-order nonlinearities increase due to blocking interferers
Solution Approach 1:
The system performs preliminary filtering of blocking interferers using the first complex band-pass filter before the signal reaches the LNA and subsequent gain stages. This preliminary action removes harmful out-of-band signals that would otherwise cause gain compression and nonlinearities, allowing the LNA to operate at high gain without distortion
Solution Approach 2:
The first complex band-pass filter acts as an intermediary between the antenna and the LNA, selectively passing the desired signal band while attenuating blocking interferers. This intermediary filtering protects the LNA from overload conditions while maintaining high gain for weak desired signals
Data Source
AI summary
A novel and useful reconfigurable superheterodyne receiver that employs a 3rd order complex IQ charge-sharing band-pass filter (BPF) for image rejection and 1st order feedback based RF BPF for channel selection filtering. The operating RF input frequency of the receiver is 500 MHz to 1.2 GHz with a varying high IF range of 33 to 80 MHz. The gain stages are inverter based gm stages and the total gain of the receiver is 35 dB and in-band IIP3 at mid gain is +10 dBm. The NF of the receiver is 6.7 dB which is acceptable for the receiver without an LNA. The architecture is highly reconfigurable and follows the technology scaling.