Receiver I/Q Imbalance Calibration Using Tapped Polar Modulator Signal
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Solution Overview
Problem
Existing wireless receivers face challenges in accurately determining In-Phase/Quadrature (I/Q) imbalance due to costly and complex calibration processes, especially in polar transmitter architectures where the local oscillator is used for both transmission and feedback, requiring precise frequency control across a wide range.
Innovation Solution
A receiver with a test signal generator that produces a test signal not requiring frequency lock, allowing the local oscillator to tune to a difference frequency within the receiver's passband filter, enabling on-board I/Q imbalance measurement using an inexpensive frequency source, such as a ring oscillator, and a simple algorithm to calculate gain and phase imbalances.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a precise on-board signal source is introduced for auto calibration, then I/Q imbalance determination accuracy is improved, but device cost and area increase significantly
Solution Approach 1:
The patent uses a copy of the transmitted signal from the polar modulator as the test signal instead of requiring a separate precise signal source. The test signal is generated by tapping the output of the polar modulator, which already contains the necessary frequency and phase characteristics. This copying approach eliminates the need for expensive precision signal generators while maintaining calibration accuracy.
Solution Approach 2:
The polar modulator's output signal serves multiple functions: it is used for normal transmission and simultaneously serves as the test signal for I/Q imbalance calibration. This multi-functionality eliminates the need for dedicated calibration hardware, reducing device cost and area while maintaining measurement precision.
2Device complexity
If the same phase modulated LO is used for both transmission and feedback receiver in polar transmitter architecture, then device complexity is reduced, but the received signal maps only to a pure radial trajectory making calibration difficult
Solution Approach 1:
The patent applies a frequency offset to the test signal before it is fed back to the receiver. This preliminary frequency shifting transforms the radial trajectory into a circular trajectory in the I/Q plane, enabling proper calibration measurements. The frequency offset is applied in advance to prevent the measurement problem rather than correcting it during processing.
Solution Approach 2:
The patent changes the frequency parameter of the test signal by applying an offset frequency. This parameter change transforms the signal trajectory from a radial line (which cannot be calibrated) to a circular trajectory (which enables I/Q imbalance measurement). The frequency offset converts an uncalibratable signal into a calibratable one.
3Measurement precision
If production test approach with test signal injection is used, then I/Q imbalance determination is achieved, but additional effort and cost are imposed and stability is assumed
Solution Approach 1:
The system performs self-calibration by using its own transmitted signal as the test signal. The polar modulator generates the test signal internally, and the receiver processes it to determine I/Q imbalance. This self-service approach eliminates the need for external production test equipment and ongoing calibration efforts, reducing both cost and complexity while removing the stability assumption.
Data Source
AI summary
A receiver having a test signal generator configured to generate a test signal that free of a requirement to be frequency locked, and to measure a frequency of the test signal; and a local oscillator signal source configured to tune a local oscillator signal to a difference frequency with respect to the measured test signal frequency, wherein the difference frequency falls within a passband of a passband filter of the receiver.