RF-DAC Phase Modulator Using 45° LO Offsets for Harmonic Rejection
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional phase modulators in RF transmitters suffer from high harmonic distortion, limited modulation bandwidth, and lack of frequency agility, leading to inefficiencies and compliance issues with noise limitation requirements in modern communications systems.
Innovation Solution
A wideband, frequency agile radio frequency digital-to-analog converter (RF-DAC) based phase modulator is introduced, utilizing three RF-DACs driven by LO clocks phased 45 degrees apart to cancel 3rd-order and 5th-order harmonics, reducing the need for bulky band-pass filters and enabling integration in a single IC chip.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If conventional phase modulators are used in RF transmitters, then the device can be implemented with simpler architecture, but the harmonic distortion is high and modulation bandwidth is limited
Solution Approach 1:
The conventional single-phase modulator is segmented into three separate RF-DACs, each handling a different phase component (0°, 45°, 90°). This segmentation allows independent optimization of each path and enables harmonic cancellation through destructive interference of unwanted frequency components while preserving the fundamental signal.
Solution Approach 2:
The patent introduces asymmetric phase shifting of 45 degrees between the three RF-DAC paths, which is unconventional compared to traditional quadrature (90-degree) systems. This asymmetric configuration creates specific harmonic relationships that enable cancellation of 3rd and 5th order harmonics while maintaining signal integrity.
2Object-generated harmful factors
If conventional phase modulators with band-pass filters are used, then harmonic distortion can be reduced, but the device becomes bulky and integration is difficult
Solution Approach 1:
The patent extracts the harmonic filtering function from the traditional bulky band-pass filter and relocates it to the digital domain through the three-phase RF-DAC architecture. The harmonic cancellation is achieved through signal processing rather than physical filtering, eliminating the need for large inductors and capacitors.
Solution Approach 2:
The patent replaces the mechanical/physical band-pass filter system with a digital signal processing system. Instead of using physical components (inductors, capacitors) to filter harmonics, the system uses digital modulation and phase cancellation techniques to achieve the same effect in a compact, integrable form.
3Productivity
If non-constant envelope modulation schemes are used to maximize spectral efficiency, then data rate increases, but the PA output power must be backed off significantly
Solution Approach 1:
The patent changes the modulation approach by using three-phase RF-DACs with 45-degree phase shifts, which enables better control of the RF signal characteristics. This parameter change allows the system to maintain higher PA efficiency while supporting modern modulation schemes through improved signal conditioning and harmonic rejection.
Data Source
AI summary
A wideband, frequency agile, radio frequency digital-to-analog converter (RF-DAC) based phase modulator includes first, second, and third RF-DACs, each configured to upconvert an input I/Q digital baseband signal pair to a local oscillator (LO) frequency but with the first RF-DAC being driven by a first set of LO clocks, the second RF-DAC being driven by a second set of LO clocks that is forty-five degrees out of phase with respect to the first set of LO clocks, and the third RF-DAC being driven by a third set of LO clocks that is a further forty-five degrees out of phase with respect to the second set of LO clocks. First, second, and third upconverted analog signals produced by the first, second, and third RF-DACs are combined to reinforce the fundamental LO component while canceling 3rd-order and 5th-order LO harmonics.