Single-Sideband RF Modulation with Closed-Loop Spectral Purity Correction
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Solution Overview
Problem
Current radiofrequency generators face limitations in achieving high spectral purity over wide frequency bands due to the use of single sideband mixers, which introduce defects like LO leakage, image lines, and intermodulation lines, and existing correction methods are inadequate, especially when the modulation band widens or frequency varies.
Innovation Solution
A closed-loop system that separates modulation generation from carrier frequency generation, using a single sideband mixer, a high-linearity radio-frequency receiver, an analog-digital converter, and a digital calculation module to correct deviations and cancel defects like LO leakage and intermodulation lines, allowing for arbitrary waveform generation and flexible frequency coverage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a single sideband mixer is used to transpose the modulation around an oscillating carrier frequency, then frequency coverage is expanded, but spectral purity deteriorates due to LO leakage, image lines, and intermodulation lines
Solution Approach 1:
The patent implements a closed-loop feedback system where the output of the single sideband mixer is fed back through a radio-frequency receiver and analog-digital converter to a digital calculation module. This module dynamically calculates correction signals that compensate for LO leakage, image lines, and intermodulation lines, thereby maintaining high spectral purity across wide frequency ranges without sacrificing frequency coverage
Solution Approach 2:
The patent introduces a digital calculation module as an intermediary between the single sideband mixer and the final output. This module processes the feedback signal and generates correction signals that are applied to cancel the harmful effects of the mixer, acting as a mediator that preserves both frequency coverage and spectral purity
2Device complexity
If fixed or tabulated preset values are used to compensate for mixer faults, then device complexity is reduced, but spectral purity is insufficient with at best -50dBc over extended temperature and frequency range
Solution Approach 1:
The patent replaces static fixed or tabulated preset values with a dynamic closed-loop correction system. The digital calculation module continuously adjusts compensation parameters based on real-time feedback from the radio-frequency receiver and analog-digital converter, enabling the system to adapt to varying temperature and frequency conditions and achieve spectral purity far exceeding -50dBc
3Manufacturing precision
If closed-loop correction is implemented at the output of the single sideband mixer, then spectral purity improves, but the correction does not specifically address SSB mixer defects like LO leakage and intermodulation lines
Solution Approach 1:
The patent applies local quality by designing the digital calculation module to specifically target and correct SSB mixer defects such as LO leakage, image lines, and intermodulation lines. The correction algorithm is tailored to address these particular defects rather than applying generic linearization, ensuring high spectral purity while maintaining the ability to handle various modulation types and frequency ranges
4Manufacturing precision
If direct digital synthesis on narrow band low intermediate carrier frequency is used, then spectral purity is good, but modulation bandwidth is limited
Solution Approach 1:
The patent segments the frequency synthesis process into two stages: direct digital synthesis on a narrow band low intermediate carrier frequency for high spectral purity, followed by single sideband mixing for frequency transposition to achieve wide modulation bandwidth. The closed-loop correction system ensures that the segmentation process does not compromise spectral purity
Solution Approach 2:
The patent uses a single sideband mixer as an intermediary between the narrow band direct digital synthesis stage and the final wide band output. This intermediary enables frequency transposition while the closed-loop correction system compensates for any spectral degradation, allowing both good spectral purity and wide modulation bandwidth
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system achieves high spectral purity and flexibility in frequency coverage, enabling up to four times more modulation bandwidth per DAC and minimizing phase noise, while allowing for agile carrier frequency generation and adaptation to performance and cost constraints.
Implementation Method 1
a single sideband mixer making it possible to transpose the modulation around an oscillating carrier frequency
Implementation Method 2
a radio-frequency receiver receiving as input the loop return modulated analog signal
Implementation Method 3
an analog-digital converter receiving as input the analog output of the radio-frequency receiver
Implementation Method 4
a digital calculation module receiving as input the digital modulation signal and the digital output of the analog-digital converter, said module making it possible to compensate for the defects introduced by the mixer
Data Source
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AI summary
The device has a digital calculation module i.e. field programmable gate array (1), receiving a digital modulation signal. A local agile transposition oscillator (9) transposes an analog signal modulated in a measuring band of an analog-digital converter (10). The unit has a phase locked loop (12) whose multiplication ratio is adjustable. The loop permits the selection of a frequency sub-band by selecting its central frequency such that the converter measures gaps introduced by a single sideband mixer (5) irrespective of a frequency range in which the analog signal is modulated.