RF Transmitter Calibration Using Subsampling ADC
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Solution Overview
Problem
Radio frequency (RF) transmitters face non-idealities due to local oscillator (LO) leakage and in-phase (I) and quadrature (Q) signal path impairments, which degrade transmission performance by introducing unintended frequency content.
Innovation Solution
A radio frequency transceiver circuit with a calibration circuit that calculates correction factors for LO leakage and IQ impairments using subsampling to avoid introducing further non-idealities, allowing for improved calibration of the transmitter by downconverting looped-back signals from the transmitter circuit using a subsampling converter instead of a mixer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a mixer is used to downconvert looped-back signals for calibration, then frequency conversion is achieved, but additional non-idealities and complexity are introduced in the receive circuit
Solution Approach 1:
The patent extracts the harmful function of the mixer from the calibration path by using a subsampling ADC directly to downconvert the looped-back signal. This removes the mixer and its associated non-idealities (LO leakage, IQ impairment) from the receive circuit while preserving the essential frequency conversion function through the subsampling process.
Solution Approach 2:
The subsampling ADC acts as an intermediary that directly converts the high-frequency looped-back signal to baseband without requiring an intermediate mixing stage. This mediator approach eliminates the need for a separate mixer component and reduces the overall circuit complexity while achieving the same calibration objective.
2Measurement precision
If traditional calibration methods using mixers are employed, then LO leakage and IQ impairments can be measured, but further non-idealities are introduced that degrade calibration accuracy
Solution Approach 1:
The patent converts the potential harm of using complex downconversion circuitry into a benefit by using subsampling ADC, which inherently performs frequency conversion without introducing the traditional harmful non-idealities of mixers. The subsampling process itself becomes the beneficial mechanism that avoids LO leakage and IQ impairment while still enabling accurate measurement of these parameters in the transmit path.
3Reliability
If subsampling is used instead of a mixer for downconversion, then additional non-idealities are minimized, but the calculation complexity of correction factors may increase
Solution Approach 1:
The patent replaces the mechanical/analog mixing process with a digital subsampling approach. Instead of using a mixer with physical components that introduce non-idealities, the system uses digital signal processing in the subsampling ADC to achieve frequency conversion. This substitution moves the complexity from the analog domain to the digital domain, where it can be more precisely controlled and corrected.
Data Source
AI summary
A method of operating a radio frequency transceiver may include generating, by a transmit circuit of the transceiver, in-phase (I) and quadrature (Q) analog signals based on a digital calibration signal; mixing, by the transmit circuit, the I and Q analog signals with local oscillator (LO) and phase shifted LO signals to generate upconverted I and Q signals, the LO signal having a first frequency, and combining the upconverted I and Q signals to generate a combined signal; and converting, by a receive circuit of the transceiver, a signal based on the combined signal to a received digital signal using a sampling rate at a second frequency, the second frequency being less than the first frequency, wherein the converting downconverts frequency content in the combined signal.