LTC6602: Programmable Baseband Filter for Software-Defined UHF RFID Readers

Introduction

Radio-Frequency Identification (RFID) is an automatic identification technology used to identify any object containing an encoded tag. A UHF RFID system consists of a reader (or interrogator) that transmits information to a tag by modulating an RF signal operating in the 860MHz to 960MHz frequency range. Typically, the tag is passive—it receives all of its operating energy from the reader transmitting a continuous wave (CW) RF signal. The tag responds by modulating the reflection coefficient of its antenna, thereby backscattering an information signal to the reader. Tag signal detection requires measuring the time interval between signal transitions (a '1' symbol's time interval is longer than a '0' symbol's time interval). The reader initiates a tag inventory by transmitting a signal that tells the tag to set its backscatter data rate and encoding. RFID readers must operate in noisy RF environments where many readers are in close proximity. Three operating modes (single interrogator, multiple interrogator, and dense interrogator) define the spectral limits for reader and tag signals. The software programmability of the receiver provides the best balance for reliable multi-tag detection and high data throughput. Programmable readers include a high linearity direct conversion I and Q demodulator, a low noise amplifier, a dual-channel baseband filter with variable gain and bandwidth, and a dual-channel analog-to-digital converter (ADC). The LTC®6602 dual-channel, matched, programmable low-pass and high-pass filter optimizes high-performance RFID readers.

LTC6602 Dual-Channel Bandpass Filter

The LTC6602 features two identical filter channels that employ matched gain control and programmable low-pass and high-pass networks. The phase match between each channel is specified to ±1 degree. A clock frequency (internal or external) sets the filter's passband to the desired spectrum. The low-pass and high-pass corner frequencies and filter bandwidth are set by the filter's clock frequency division ratio. Low-pass division ratio options are 100, 300, and 600, while high-pass division ratios are 1000, 2000, and 6000. Figure 1 shows a typical filter response curve (using a 90MHz internal clock with high-pass and low-pass division ratios set to 6000 and 600, respectively). A sharp, fourth-order elliptic stopband response helps reject out-of-band noise. Software control of the baseband receiver operating modes makes the RFID reader software-defined, enabling it to adapt to its operating environment.

A Software-Defined Adaptive Baseband Filter for RFID Readers

Figure 2 shows a simple filter circuit based on the LTC6602 that uses SPI serial control to adjust the filter's gain and bandwidth to accommodate a complex set of data rates and encodings. (The backscatter link frequency range is 40kHz to 640kHz, with data rates ranging from 5kbps to 640kbps.)

To achieve fine resolution for the filter's settings, an 8-bit LTC2630 DAC is used to set the internal clock frequency. The 0V to 3V DAC output range sets the clock frequency between 40MHz and 100MHz (234.4kHz per bit). The low-pass and high-pass division ratios are set using the LTC6602's serial SPI control. The high-pass filter's cutoff frequency ranges from 6.7kHz to 100kHz, while the low-pass filter's cutoff frequency ranges from 66.7kHz to 1MHz. The optimal filter bandwidth setting can be adjusted using a software algorithm and is a function of the data clock, data rate, and data encoding.

A narrow filter bandwidth is required to maximize the ADC input's dynamic range and maintain signal transitions and pulse widths (correct filter settings ensure reliable DSP tag signal detection). Figure 3 shows an example of the filter's time-domain response to a typical tag symbol sequence (preceded by a 'short' pulse interval, followed by a 'long' pulse interval). The low-pass cutoff frequency is set equal to the reciprocal of the shortest interval (fCUTOFF = 1/10µs = 100kHz). If the low-pass cutoff frequency is too low, signal transitions and time intervals will be distorted and unreadable. The high-pass cutoff frequency setting is less definitive and more qualitative. The high-pass cutoff frequency must be set below the reciprocal of the longest interval (for the example shown, high-pass fCUTOFF < 1/20µs), and should be set as high as possible to minimize low-frequency noise in the receiver (baseband amplifier and downconverter phase and amplitude noise). The bottom half of Figure 3 shows the overall filter response with a 10kHz low-pass and a 30kHz high-pass setting. With a 10kHz output, signal transitions and time intervals are sufficiently preserved for symbol detection (noise will be superimposed on the output signal in an RFID environment). In general, increasing the low-pass fCUTOFF and/or decreasing the high-pass fCUTOFF will 'sharpen' the signal transitions and intervals at the expense of increased filter output noise.

Conclusion

The LTC6602 dual-channel baseband filter is a programmable baseband filter for high-performance UHF RFID readers. Using the LTC6602 under software control provides the capability to operate at high data rates (with single interrogator, or in multi- or dense-interrogator environments) for optimal tag signal detection. The LTC6602 is a very compact IC in a 4mm x 4mm QFN package and can be programmed using either parallel or serial control for optimal tag signal detection.

References

1. The RF in RFID, Daniel M. Dobkin, 9/07, Elsevier Inc.
2. Class-1 Generation-2 UHF RFID Protocol for Communications at 860 MHz to 960 MHz, Version 1.1.0, www.epcglobalinc.org/standards/specs/

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