Digital Filtering for Analog Gain and Phase Errors in RF Systems
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Solution Overview
Problem
In wireless communication systems, digital step attenuation (DSA) often introduces phase and gain errors in RF signals, particularly in 3G/4G systems using QAM constellations, leading to decoding errors due to signal distortion, and these errors can vary with frequency, requiring effective correction mechanisms.
Innovation Solution
A parameterized filter with complex filter parameters is integrated into the digital signal path to correct frequency-dependent phase and gain errors, configured for each DSA attenuation step, using a frequency-dependent corrector filter and frequency-independent corrector circuitry to address errors across the target frequency band.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If digital step attenuation is used to dynamically control RF signal power, then signal power adjustment capability is improved, but phase and gain errors are introduced that distort the signal
Solution Approach 1:
The patent applies preliminary action by pre-calculating and storing correction values in lookup tables before actual operation. The system measures phase and gain errors across the frequency band, computes correction factors, and stores them in LUTs indexed by DSA attenuation words. During operation, the correction filter simply retrieves and applies pre-computed values, eliminating real-time computation complexity while maintaining high precision signal correction.
Solution Approach 2:
The patent introduces a digital correction filter as an intermediary component between the DSA attenuator and the ADC. This filter acts as a mediator that compensates for the phase and gain errors introduced by the DSA, allowing the system to maintain both dynamic power adjustment capability and signal accuracy. The filter uses complex coefficients stored in lookup tables to correct errors without requiring changes to the DSA hardware itself.
2Measurement precision
If a full N-tap complex filter is used to correct frequency dependent errors, then correction accuracy is improved, but device complexity increases
Solution Approach 1:
The patent reduces implementation complexity by pre-computing and storing filter coefficients in lookup tables before operation. During actual use, the system only needs to perform simple table lookups based on the DSA attenuation word and apply the retrieved coefficients, rather than performing complex real-time calculations. This approach maintains high correction accuracy while significantly simplifying the real-time processing requirements.
Solution Approach 2:
The patent changes the operational parameters of the filter by indexing it with the DSA attenuation word rather than treating it as a fully independent N-tap filter. This parameterization allows the same filter structure to adapt to different attenuation settings by simply changing which lookup table entries are used, reducing the complexity of managing multiple full filters for different attenuation levels.
3Measurement precision
If filter taps are estimated through tone sweeping across the frequency band, then frequency dependent error characterization is improved, but measurement time increases
Solution Approach 1:
The patent performs the time-consuming tone sweeping and filter tap estimation as a preliminary calibration step during system initialization or manufacturing, rather than during normal operation. The measured data is stored in lookup tables for rapid retrieval during operation. This separates the detailed measurement process from the operational process, allowing accurate error characterization to be performed once while maintaining fast operational response times.
Data Source
AI summary
A circuit for digital filtering an analog signal converted to digital, including an analog circuit to generate an analog signal, the analog signal including phase and/or gain errors. An analog-to-digital converter (ADC) to convert the analog signal to a digital signal output to a digital signal path. A frequency-dependent corrector filter included in the digital signal path, and configured as a parameterized filter, the parameterized filter configurable based on the DSA control signal with at least one complex filter parameter for each DSA attenuation step, to correct frequency-dependent errors in phase and/or gain.