RTC Calibration via TCXO Frequency Ratio for GPS TTFF
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Solution Overview
Problem
Low power real-time clocks in GPS navigation systems suffer from temperature-dependent errors, leading to rapid degradation of time accuracy when the receiver is off, affecting the time to first fix (TTFF) performance.
Innovation Solution
A system and method that calibrate the real-time clock (RTC) using a temperature-compensated crystal oscillator (TCXO) to determine a frequency ratio, allowing the computing device to estimate and correct the RTC time, maintaining accuracy over extended periods with minimal power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a low power real time clock (RTC) based on a watch-class crystal oscillator is used, then power consumption is reduced to μW level, but time accuracy degrades rapidly due to temperature dependent errors of 200 PPM
Solution Approach 1:
The patent introduces a TCXO (temperature compensated crystal oscillator) as an intermediary high-accuracy time reference that operates periodically to calibrate the RTC. The TCXO serves as a mediator between the accurate GPS time and the power-consumption optimized RTC, transferring time information without requiring the RTC to continuously maintain high accuracy on its own.
Solution Approach 2:
The system performs periodic calibration of the RTC using the TCXO at scheduled intervals rather than continuously maintaining high accuracy. This periodic action allows the RTC to operate in low-power mode most of the time while receiving accuracy corrections periodically, resolving the contradiction between continuous accuracy maintenance and power consumption.
2Use of energy by moving object
If the receiver is turned off to save power, then power consumption is reduced, but the accuracy of RTC time relative to GPS system time degrades rapidly
Solution Approach 1:
The system performs preliminary calibration actions before the receiver needs to be turned off. By calibrating the RTC against the TCXO in advance and storing the calibration parameters, the system ensures that when power is restored, the RTC already has accurate time information without requiring continuous operation during the powered-off period.
Solution Approach 2:
The RTC circuitry performs self-calibration using the TCXO without requiring external intervention or continuous GPS signal processing. The calibration process is autonomous, allowing the system to maintain accuracy during powered-off periods through periodic self-correction when powered on.
3Measurement precision
If a temperature compensated crystal oscillator (TCXO) is used to calibrate the RTC, then time accuracy is improved, but device complexity increases
Solution Approach 1:
The TCXO circuitry serves multiple functions: it acts as a time reference for GPS processing, provides calibration signals for the RTC, and maintains accurate timing during both powered-on and powered-off states. This multi-functionality justifies the added complexity by eliminating the need for separate dedicated calibration hardware.
Solution Approach 2:
The patent merges the TCXO calibration function with the existing GPS timing infrastructure. The ratio counter circuitry that compares RTC and TCXO frequencies is integrated into the existing signal processing path, combining multiple functions into unified circuit blocks rather than adding completely separate calibration systems.
Data Source
AI summary
Systems and methods for calibrating real time clock are provided. A representative receiver includes a GPS device comprising a real time clock (RTC) circuitry that generates RTC clock signals and a temperature compensated crystal oscillator (TCXO) that generates TCXO clock signals. A ratio counter circuitry receives both the RTC clock signals and the TCXO clock signals and determines a frequency ratio by comparing the RTC clock signals and the TCXO clock signals. A computing device receives the frequency ratio and estimates a current RTC frequency based on the received frequency ratio. The computing device is configured to calibrate an estimated RTC time being maintained at the RTC circuitry based on an estimated RTC frequency from a prior estimation, the current RTC frequency and an elapsed time of the RTC circuitry.