Watch-Crystal RF Transmitter with On-Demand LC Oscillator Calibration
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Solution Overview
Problem
Existing wireless transmitters for low power applications face challenges in balancing power consumption, phase noise, and calibration requirements, often relying on high frequency crystal oscillators that consume more power and may not meet settling time and phase noise requirements, while low frequency oscillators may not provide sufficient stability for RF signal transmission.
Innovation Solution
A watch-crystal-based RF transmitter that uses a single low frequency crystal oscillator for both timekeeping and calibrating an LC oscillator, operating in an open-loop mode for data transmission and switching to a closed-loop calibration mode when necessary, with a control circuit determining calibration needs based on temperature and elapsed time, and updating a calibration table for future reference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high frequency crystal oscillator is used to provide reference frequency for RF signal transmission, then phase noise and frequency stability are improved, but power consumption increases
Solution Approach 1:
The patent segments the oscillator system into two distinct components: a low-frequency watch crystal oscillator (32.768 kHz) for timekeeping and a high-frequency LC oscillator for RF signal generation. The watch crystal provides periodic calibration signals to correct drift in the LC oscillator, allowing the system to achieve high frequency stability without continuously operating a high-frequency crystal oscillator, thus reducing power consumption.
Solution Approach 2:
The system performs preliminary calibration by using the watch crystal oscillator to periodically adjust and correct the frequency of the LC oscillator before RF transmission. This preliminary calibration action ensures that the LC oscillator maintains accurate frequency reference without requiring a continuously powered high-frequency crystal oscillator.
2Use of energy by moving object
If a low frequency crystal oscillator is used to reduce power consumption, then power consumption is improved, but phase noise and frequency stability deteriorate
Solution Approach 1:
The patent introduces a phase-locked loop (PLL) as an intermediary mechanism that couples the low-frequency watch crystal oscillator with the high-frequency LC oscillator. The PLL uses the stable low-frequency reference to control and stabilize the high-frequency LC oscillator, enabling the system to achieve high frequency stability through a low-power oscillator combined with active frequency multiplication and phase control.
3Reliability
If continuous calibration of the RF oscillator is performed to maintain frequency accuracy, then frequency stability is improved, but power consumption and settling time increase
Solution Approach 1:
The patent implements periodic calibration by using the watch crystal oscillator to periodically adjust the LC oscillator at predetermined intervals or when specific conditions are met (e.g., temperature changes, wake from sleep mode). This periodic calibration maintains frequency accuracy over time without requiring continuous calibration operations, thereby reducing average power consumption and minimizing settling time impact on transmission readiness.
Data Source
AI summary
Example radio frequency (RF) transmitters and associated methods are disclosed. One example RF transmitter includes an RF oscillator, a real-time clock (RTC) oscillator. and a control circuit. The control circuit is configured to determine whether a calibration of the RF oscillator is needed; electrically couple the RF oscillator to the RTC oscillator and initiate calibrating of the RF oscillator using the RTC oscillator when it is determined that the calibration is needed; and activate the RF oscillator to operate in an open-loop mode to generate an RF signal for data transmission. The calibration can be performed in a closed-loop mode before the data transmission or in an open-loop mode during the data transmission.