Fractional-N AFC for Reference Oscillator Error in Mobile PLLs
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Solution Overview
Problem
Existing frequency correction methods for reference oscillators in communications systems, such as those used in mobile terminals, require significant factory calibration and are not precise enough to meet the accuracy demands of standards like GSM, necessitating multiple iterations and the use of expensive temperature-compensated crystal oscillators.
Innovation Solution
A fractional-N based Automatic Frequency Control (AFC) system that employs two fractional-N phase-locked loops (FN-PLLs) and a translational PLL to generate local oscillator signals, using a single AFC value to correct frequency errors in the reference oscillator, which remains constant across varying frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a crystal oscillator with ±30-40 ppm accuracy is used, then the system can operate with a simple and inexpensive oscillator, but the frequency accuracy does not meet the ±0.02 ppm requirement of communications standards
Solution Approach 1:
The patent introduces a frequency correction algorithm as an intermediary mechanism between the crystal oscillator and the communication system. This algorithm processes the output frequency and applies corrections to achieve the required ±0.02 ppm accuracy, effectively mediating between the oscillator's native ±30-40 ppm accuracy and the system's accuracy requirements
Solution Approach 2:
The patent implements a feedback mechanism where the frequency correction algorithm continuously monitors and adjusts the oscillator output. By measuring the actual frequency deviation and applying corrective adjustments, the system maintains the required frequency accuracy without needing a more expensive TCXO component
2Measurement precision
If a precision D/A converter or capacitor array is used to correct frequency, then the frequency accuracy can be improved, but significant factory calibration is required and multiple iterations are needed
Solution Approach 1:
The patent replaces the mechanical/electrical calibration system (precision D/A converter controlling varactor capacitance) with a digital frequency correction algorithm. This algorithmic approach eliminates the need for complex factory calibration of hardware components while achieving the same frequency accuracy through software-based adjustments
Solution Approach 2:
The frequency correction algorithm performs self-calibration by automatically detecting and correcting frequency deviations without requiring external calibration equipment or manual adjustment. The system services its own frequency accuracy requirements through the algorithm's autonomous operation
3Measurement precision
If a temperature compensated crystal oscillator (TCXO) sub-assembly is purchased, then frequency accuracy can be improved, but the cost increases significantly and frequency correction is still required
Solution Approach 1:
The patent uses a simple, inexpensive crystal oscillator instead of an expensive TCXO, accepting that the basic oscillator has lower inherent accuracy. The frequency correction is achieved through a software algorithm rather than expensive hardware, effectively replacing a costly component with a combination of cheap hardware and processing
Data Source
AI summary
A fractional-N based Automatic Frequency Control (AFC) system for a mobile terminal is provided. In general, automatic frequency control is implemented in a frequency synthesizer to correct or compensate for a frequency error of an associated reference oscillator. The frequency synthesizer includes a first fractional-N phase-locked loop (FN-PLL) generating a baseband clock signal used by a baseband processor of the mobile terminal, a second FN-PLL generating a receiver local oscillator signal used by a receiver of the mobile terminal to downconvert a received radio frequency signal to a desired frequency, and a translational PLL generating a transmitter local oscillator signal used by a transmitter of the mobile terminal to provide a radio frequency transmit signal. The automatic frequency control is performed by applying a digital correction value, which is preferably multiplicative, to fractional-N dividers of the first and second FN-PLLs.