PLL Carrier Offset Control for 4Fmod Spur Mitigation
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Solution Overview
Problem
Current wireless communication technologies face challenges in mitigating RF impairments, particularly with regards to LTE carrier frequencies and 4Fmod spurious signals caused by VCO pushing and pulling, which lead to spectral degradation and interference issues.
Innovation Solution
A wireless communication unit and method that employs a phase locked loop (PLL) with a controller to introduce frequency offsets in both the carrier frequency and baseband transmit signals, using digital signals to adjust the carrier frequency and compensate for VCO pushing, thereby reducing 4Fmod interference by shifting frequencies to avoid adjacent band interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If frequency offset is introduced to avoid 4Fmod spurious signals, then spectral degradation is reduced, but PLL lock stability may be compromised
Solution Approach 1:
The system applies preliminary frequency offset compensation before the 4Fmod spurious signals are generated. The controller calculates the expected frequency pushing effect and applies an opposite frequency offset to counteract it, preventing the harmful signals from occurring in the first place while maintaining PLL lock stability
Solution Approach 2:
The system uses feedback mechanisms where the controller continuously monitors the carrier frequency and adjusts the frequency offset dynamically. This closed-loop control ensures that frequency pushing is compensated in real-time, maintaining both spectral purity and PLL lock stability under varying operating conditions
2Object-generated harmful factors
If carrier frequency is adjusted to compensate for VCO pushing, then spectral degradation is reduced, but device complexity increases
Solution Approach 1:
The patent introduces a controller as an intermediary component that manages the frequency offset application. This controller acts as a mediator between the VCO and the frequency generation circuit, simplifying the overall system architecture by centralizing the compensation logic in a single programmable unit rather than requiring complex hardware modifications throughout the system
Solution Approach 2:
The system compensates for spectral degradation by dynamically changing the frequency offset parameter based on operating conditions. The controller adjusts this parameter programmatically, allowing flexible compensation without requiring complex hardware changes. This parameter-based approach enables adaptation to different VCO pushing characteristics across frequency bands while maintaining a relatively simple device structure
3Object-generated harmful factors
If frequency offset is applied to move 4Fmod interference out of band, then adjacent band interference is reduced, but manufacturing precision requirements increase
Solution Approach 1:
The system performs preliminary frequency offset calculation and application before transmission. The controller pre-calculates the appropriate frequency offset based on known VCO pushing characteristics and applies it proactively, reducing the need for high-precision real-time adjustments and thereby lowering manufacturing precision requirements for hardware components
Solution Approach 2:
The frequency offset is made dynamic and adaptable rather than fixed. The controller can adjust the frequency offset parameter based on actual operating conditions, frequency band, and power level. This dynamic approach allows the system to compensate for variations in VCO pushing without requiring extremely precise fixed-frequency hardware design, thereby reducing manufacturing precision requirements
Data Source
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AI summary
A circuit comprises a frequency generation circuit (322) comprising a phase locked loop, PLL, arranged to generate a carrier frequency; and a controller (308) operably coupled to the frequency generation circuit and arranged to determine a frequency location of one or more signals output by the frequency generation circuit (322) and provide a control signal thereto to adjust the carrier frequency generated by the frequency generation circuit (322). The controller (308) is arranged to: cooperate with the PLL to introduce a frequency offset in the generated carrier frequency in a first frequency direction; and introduce a compensating frequency offset in a baseband transmit signal in a second frequency direction opposite to the first frequency direction.