RFID Distance Measurement Using Intermediate Frequency Phase Shift
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Solution Overview
Problem
Existing methods for measuring the distance to an RFID tag using phase shift of reflected waves are limited by the wavelength of the carrier wave, making it difficult to accurately measure distances beyond half the wavelength, particularly with short wavelengths like 30 cm, which is problematic for practical applications such as UHF band communication.
Innovation Solution
A communication processing device that overlaps an intermediate frequency signal with the carrier wave and detects phase differences in the reflected wave to measure distance, allowing for more accurate distance measurement beyond the limitations of traditional methods.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If phase shift measurement using carrier wave is used, then distance measurement capability is provided, but measurement precision deteriorates beyond half wavelength
Solution Approach 1:
The patent changes the measurement parameter from carrier wave phase shift to intermediate frequency signal phase shift. By downconverting the high-frequency carrier wave (e.g., 2.45 GHz) to a lower intermediate frequency (e.g., 100 kHz), the wavelength increases from approximately 12 cm to 1.5 km, enabling accurate distance measurement over much longer ranges while maintaining measurement precision.
2Adaptability or versatility
If radio wave transmission is performed, then communication with tags is enabled, but false responses from distant tags occur
Solution Approach 1:
The patent implements feedback by continuously measuring the distance to the RFID tag using intermediate frequency phase shift detection during communication. The communication processing device uses this distance information to determine whether the tag is within the expected communication range, and selectively processes or discards responses based on the measured distance, thereby preventing false information processing from distant tags.
3Speed
If carrier wave frequency is increased, then communication speed is improved, but measurement range is limited to half wavelength
Solution Approach 1:
The patent segments the communication process into two distinct phases: (1) high-frequency carrier wave transmission for fast data communication, and (2) intermediate frequency signal transmission for accurate distance measurement. This segmentation allows each phase to optimize for its specific function - speed for communication and range for measurement - without compromising either performance.
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
Enables reliable distance measurement up to 10 meters, preventing false information processing by ensuring only tags within the communication range are recognized, effectively addressing the limitations of previous methods.
Implementation Method 1
a process in which a command is transmitted to a tag such that a carrier wave having a constant frequency is modulated while continuously transmitted and a process of transmitting an unmodulated carrier wave (called a Continuous Wave or CW for short) to receive a response from the tag
Implementation Method 2
The tag that receives the command responds to the command by changing impedance of a circuit thereof
Implementation Method 3
detecting a phase difference of an extracted intermediate frequency signal from the intermediate frequency signal in the carrier wave output
Data Source
AI summary
A radio frequency signal oscillator and an intermediate frequency signal oscillator are incorporated in a communication processing device, and a carrier wave with which an intermediate frequency signal is overlapped is generated using the oscillators and transmitted from an antenna. Mixers separate and extract an I signal and a Q signal from the intermediate frequency signal in the signal received by the antenna. A phase difference detector detects a phase difference of the intermediate frequency signal in a reflected wave to the intermediate frequency signal in the carrier wave using the I signal and the Q signal after the reception of the reflected wave from a tag is started. A distance calculator calculates a distance from the antenna to the tag using the phase difference and a wavelength of the intermediate frequency signal.