Mixer Circuit Calibration for LO Feedthrough and Harmonic Rejection
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Solution Overview
Problem
Designing satisfactory mixers and local oscillator circuitry for electronic devices with wireless communications capabilities is challenging due to non-linearities that produce undesired spurious emissions and interference with in-band signals.
Innovation Solution
The mixer circuitry includes a configuration with four mixer transistors and three digital-to-analog converters (DACs) that are calibrated in phases to trim DC mismatch, impedance, and local oscillator feedthrough, minimizing first and second-order LO feedthrough and rejecting second harmonic conversion gain.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If mixer circuitry is used to modulate/demodulate signals in wireless communications, then signal transmission and reception capabilities are improved, but non-linearities in the mixer and local oscillator circuitry produce undesired spurious emissions and interference
Solution Approach 1:
The mixer circuitry is divided into multiple independent mixer stages (first mixer, second mixer) with separate local oscillator inputs. Each mixer stage can be independently calibrated using separate DACs to trim DC mismatch and feedthrough, allowing the harmful non-linearities to be addressed individually while maintaining the overall signal processing capability
Solution Approach 2:
The patent employs digital-to-analog converters (DACs) to dynamically adjust and trim DC bias voltages applied to the mixer transistors. By changing the DC operating parameters of the mixer circuitry through DAC control, the system can minimize LO feedthrough and suppress spurious emissions while maintaining proper mixing functionality
2Productivity
If local oscillator circuitry is used to provide clock signals to mixers, then frequency conversion capability is improved, but LO fundamental and harmonics feedthrough to the output occurs
Solution Approach 1:
The local oscillator signal path is segmented into multiple independent inputs (first LO input, second LO input) that feed different mixer stages. This segmentation allows separate calibration of each path using individual DACs, enabling selective suppression of LO feedthrough at different frequencies while maintaining frequency conversion capability
Solution Approach 2:
The patent implements preliminary calibration actions using DACs to trim DC mismatch and adjust bias conditions before the mixer operates with full LO signals. This preliminary adjustment minimizes LO feedthrough and harmonics before they can interfere with the output signal
Data Source
AI summary
Mixer circuitry can include a first pair of transistors coupled to a first tail node and configured to receive a local oscillator signal, a second pair of transistors coupled to a second tail node and configured to receive the local oscillator signal, a first digital-to-analog converter, a second DAC coupled between the first DAC and of the first pair of transistors, and a third DAC coupled between the first DAC and the second pair of transistors. During a first phase, control circuitry can sweep the first DAC to trim a first and/or other odd order local oscillator feedthrough. During a second phase, the control circuitry can sweep the second DAC to trim a second and/or other even order local oscillator feedthrough. During a third phase, the control circuitry can sweep the second and third DACs to reject signals associated with a second harmonic conversion gain of the mixer circuitry.