Digital Cancellation of CIM3 Distortion in RF Transmitters
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional RF digital transmitters suffer from third counter intermodulation (CIM3) distortion due to nonlinearity in passive attenuator circuits, which generates in-band interference that is difficult to cancel effectively.
Innovation Solution
A digital cancellation method is implemented using adjustable cancellation coefficients determined through a calibration process, where a single-sideband tone is used to optimize the coefficients to minimize the power of the CIM3 component in the output signals by generating and subtracting distortion terms that are equal in magnitude but opposite in polarity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a passive attenuator (PAD) circuit is used to combine switched currents in individual cells, then the transmitter can achieve the desired signal combining function, but third order nonlinearity in the PAD generates in-band CIM3 distortion that is difficult to cancel
Solution Approach 1:
The invention applies preliminary action by pre-calculating and storing cancellation coefficients in a lookup table before actual transmission. The lookup table contains pre-computed correction values that compensate for the known nonlinear characteristics of the PAD circuit, allowing the system to eliminate CIM3 distortion without real-time complex calculations.
Solution Approach 2:
The invention changes parameters by adjusting the digital input signals based on pre-stored cancellation coefficients. These coefficients modify the amplitude and phase parameters of the input signals to counteract the nonlinear distortion introduced by the PAD circuit, effectively canceling the CIM3 distortion through parameter transformation.
2Adaptability or versatility
If digital cancellation is implemented without pre-stored coefficients, then real-time adaptation is possible, but the complexity of calculating and adjusting cancellation coefficients in real-time increases significantly
Solution Approach 1:
The invention resolves this contradiction by performing the complex cancellation coefficient calculations in advance and storing them in a lookup table. During actual operation, the system only needs to retrieve pre-computed coefficients rather than performing real-time calculations, thereby maintaining adaptability while significantly reducing computational complexity and device requirements.
Solution Approach 2:
The invention uses copying by creating a lookup table that stores pre-computed cancellation coefficients for various operating conditions. Instead of recalculating coefficients in real-time, the system copies the appropriate pre-stored coefficients from the lookup table based on current operating parameters, greatly simplifying the real-time processing burden.
3Object-generated harmful factors
If the cancellation coefficients are made adjustable, then the power of the CIM3 component can be minimized, but the system requires additional calibration and control mechanisms
Solution Approach 1:
The invention applies preliminary action by pre-determining the optimal cancellation coefficients through calibration and storing them in a lookup table. The calibration process is performed once during system setup or manufacturing, and the resulting coefficients are stored for reuse during normal operation, eliminating the need for continuous calibration mechanisms while still achieving minimal CIM3 component power.
Solution Approach 2:
The system applies self-service by using the pre-stored cancellation coefficients to automatically compensate for distortion without requiring external calibration during operation. The lookup table provides self-contained correction data that the system uses independently, reducing the need for ongoing external control and calibration mechanisms.
Data Source
Figure 1A~1B
Figure 2A~2C
Figure 3
AI summary
An apparatus includes a first circuit to receive a first input data, a second input data and coefficients, generate a first distortion term and a second distortion term based, respectively on the first input data and the coefficients and the second input data and the coefficients, and change a polarity of the first distortion term and the second distortion term. A first subtraction circuit subtracts the first distortion term from the first input data and generates first difference data, and a second subtraction circuit subtracts the second distortion term from the second input data and generates second difference data. A transmit data-path generates a RF output. The first difference data and the second difference data compensate, based on the polarity changes of the first distortion term and the second distortion term, respectively, one or more impairments of the RF output.