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An adaptive fundamental frequency filter for RFID readers
Radio Frequency Identification (RFID) is an automatic identification technology that allows objects to be recognized through a coded label. In UHF RFID systems, a reader transmits an RF signal in the 860MHz to 960MHz range and communicates with a passive tag. The tag draws all its power from the continuous wave (CW) signal sent by the reader and responds by modulating the reflection coefficient of its antenna, effectively backscattering the data signal back to the reader.
To accurately detect volume signals, the system must measure the time intervals between signal transitions. A "1" symbol typically has a longer interval than a "0" symbol. The reader initiates the process by sending a signal that specifies the backscatter data rate and encoding format used by the tag.
RFID readers often operate in environments with multiple readers nearby, which can cause interference. To manage this, three operating modes—single interrogator, multi-interrogator, and dense interrogator—are used to define spectral limits for both the reader and the tags. Software programmability in the receiver ensures a balance between reliable detection and high data throughput.
The programmable RFID reader includes advanced components such as a high linearity direct conversion I and Q demodulator, low noise amplifiers, a dual-channel fundamental frequency filter with variable gain and bandwidth, and a dual-channel analog-to-digital converter (ADC). One key component is the LTC6602, a dual-channel bandpass filter designed for high-performance RFID applications.
The LTC6602 features two identical filter channels with matched gain control and frequency-dependent low-pass and high-pass networks. Each channel maintains a phase shift of ±1 degree, ensuring consistent performance across both channels. The internal or external frequency can be set to position the passband at the desired spectrum.
The corner frequencies and bandwidth of the filter are determined by the scale ratio of the internal frequency. Options include low-pass scale ratios of 100, 300, and 600, and high-pass ratios of 1000, 2000, and 6000. For example, with a 90MHz internal frequency and scale ratios of 6000 and 600, the filter response shows a fourth-order elliptical cutoff that helps suppress out-of-band noise.
Controlling the fundamental frequency bandwidth enables the RFID receiver to adapt to different working environments. This flexibility is crucial for optimizing performance in various scenarios, whether it's a single-reader setup or a complex multi-reader environment.
The LTC6602-based filter circuit uses SPI serial control to adjust the gain and bandwidth, accommodating a wide range of data rates and encodings. Backscatter links can vary from 40kHz to 640kHz, while data rates range from 5kbps to 640kbps. An 8-bit DAC (LTC2630) sets the internal frequency, allowing precise tuning between 40MHz and 100MHz.
The low-pass and high-pass filters have adjustable cutoff ranges, with the low-pass covering 66.7kHz to 1MHz and the high-pass ranging from 6.7kHz to 100kHz. Software algorithms optimize the filter bandwidth based on factors like data frequency, data rate, and encoding. The goal is to maximize ADC dynamic range while preserving signal integrity.
In a time-domain example, the filter processes a sequence of short and long pulse intervals. Setting the low-pass cutoff frequency to 100kHz ensures accurate detection of the shortest interval. If the cutoff is too low, signal transitions may become distorted. The high-pass cutoff is more qualitative, but it should be set below the reciprocal of the longest interval to avoid distortion while minimizing low-frequency noise.
Comparing outputs with different high-pass settings, a 10kHz cutoff provides sufficient signal clarity for detecting symbol sequences, even in noisy RFID environments. Adjusting the low-pass and high-pass cutoffs involves trade-offs between signal quality and noise levels.
Overall, the LTC6602 is a compact, programmable dual-channel bandpass filter ideal for UHF RFID readers. It offers flexibility in both single and multi-reader setups, and its small 4mm x 4mm QFN package makes it suitable for a wide range of applications. With support for parallel or serial control, the LTC6602 enhances the performance and reliability of modern RFID systems.