Polar Transmitter Feedthrough Compensation for Lower RF Distortion
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Solution Overview
Problem
Polar transmitters face significant distortion due to local oscillator feedthrough, which existing technologies struggle to fully correct, leading to AMAM and AMPM distortion and impacting Error Vector Magnitude (EVM) and spectrum performance, especially at lower output power levels.
Innovation Solution
A polar transmitter system that applies a complex digital offset, including both in-phase and quadrature offset correction factors, calculated and applied using Coordinate Rotation Digital Computers (CORDICs) to correct amplitude and phase signal components, effectively addressing local oscillator feedthrough and reducing distortion.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a polar transmitter is used to generate RF output, then communication functionality is provided, but local oscillator feedthrough causes AMAM and AMPM distortion
Solution Approach 1:
The patent applies preliminary correction by calculating and storing complex digital offset values before transmission occurs. The correction circuit pre-computes the necessary compensation for LO feedthrough based on measured or characterized offset values, then applies this correction to the I and Q signal components before they are converted to amplitude and phase for RF generation. This preliminary action prevents distortion rather than correcting it after the fact.
Solution Approach 2:
The patent creates a digital model or representation of the LO feedthrough effect through complex digital offset values that capture both magnitude and phase information. By working with digital copies of the offset characteristics rather than directly manipulating the physical LO signal, the system can apply precise correction through digital signal processing before the signals are converted to analog RF.
2Measurement precision
If existing correction techniques are applied, then some distortion reduction is achieved, but full correction is not possible leading to residual EVM degradation
Solution Approach 1:
The patent transitions from real-only offset correction to complex offset correction by adding the imaginary dimension. The complex digital offset includes both real and imaginary components, allowing correction of both in-phase and quadrature errors simultaneously. This dimensional expansion enables complete cancellation of LO feedthrough effects that cannot be achieved with real-valued correction alone.
Solution Approach 2:
The patent changes the correction parameter from simple real offset values to complex offset values that include both magnitude and phase information. By modifying the parameter representation to include the imaginary component, the system can fully characterize and correct the LO feedthrough distortion, achieving complete EVM performance restoration.
3Reliability
If complex digital offset correction is applied, then full LO feedthrough correction is achieved, but processing complexity increases
Solution Approach 1:
The patent replaces complex analog correction mechanisms with digital signal processing. Instead of using complex analog circuitry to correct LO feedthrough, the system uses digital computation of complex offsets and digital multiplication of the I and Q components. This substitution of digital for analog simplifies the overall system architecture while achieving complete correction.
Solution Approach 2:
The patent introduces complex digital offset values as an intermediary between the digital baseband signals and the analog RF generation. These complex offsets serve as a bridge that carries the correction information through the digital domain, enabling precise control of both magnitude and phase errors without requiring complex analog intervention circuits.
Data Source
AI summary
A circuit includes a polar transmitter to generate a radio frequency output from amplitude and phase signal components. The polar transmitter includes an amplifier to combine amplitude and phase signal components. A processor is coupled to the polar transmitter to provide the amplitude and phase signal components. The processor includes: a digital modulation circuit to generate a modulated digital signal including in-phase and quadrature signal components and a correction circuit to calculate and apply a complex digital offset for local oscillator feedthrough of the amplifier. The complex digital offset includes an in-phase offset correction factor and a quadrature offset correction factor.