- Buyers Guide
A Compact RFID Reader Platform for UHF and Microwave Applications
A new line of compact RFID readers has been developed for UHF and microwave applications worldwide. WJ’s model SX2000 is the first in a series of compact EPC Class 1 RFID readers whose size and performance are enabling new RFID applications. The SX20xx RFID reader platform — only 3.37" x 2.13" x 0.30" and 3 ounces — offers exceptional performance commensurate with much larger units: 1 W max RF output, 14 dB power control range, two separate antenna ports, RS232 or TTL interface protocol, power conditioning and fused over-voltage protection. Table 1 shows the performance specifications for the SX2000. Extensive use of WJ chip sets, novel circuit design and utilization of a cavitized RF web with 0402 passives have enabled the small form-factor.
Certain RFID markets have already emerged. However, UHF and microwave markets for supply chain management are now starting a dramatic expansion. Recent announcements from major market movers like WalMart, the US Department of Defense (DoD) and Tesco have embraced RFID with implementation requirements starting in early 2005. For these large companies, the potential value of using RFID for more efficient supply chain logistics is tremendous, and today’s reader and tag technology is helping make RFID systems an affordable reality. Today, RFID is finding global applications in asset tracking, cold chain management (food and pharmaceuticals), security, environment logging (sensors) and defense.
The SX20xx reader family uses a homodyne receiver architecture to create a common platform for UHF and microwave EPC Class 1 systems. Table 2 lists the family of SX20xx products, that together cover the main bands of operation currently considered for US, EU, Japan and other global applications. To date the SX2000 and SX2001 are in production, and all other models have only been prototyped or produced in sample quantities.
The EPC Class 1 RFID system uses radar backscatter techniques to communicate between the reader and passive tags. The passive RFID tag couples bias power from the reader’s transmit signal. Once enough energy is coupled from the transmitted wave, the reader communicates with the tag using amplitude modulation, followed by a CW carrier. The tag responds by reflecting that incident CW signal at its modulation rate. The tag communicates with the reader through this backscatter approach, leading to the selection of a homodyne receiver architecture for the reader.
Figure 1 shows the RF transceiver block diagram of the SX20xx reader platform. The homodyne topology is the classic approach for backscatter radar where received signals are close in frequency to the transmitted carrier. WJ has used homodyne receivers for military applications in the past and has used this expertise to evaluate the various design trade-offs. A homodyne receiver performs a direct conversion of the received RF signals to a zero intermediate frequency (IF) baseband. Elimination of the IF reduces the need to perform image rejection filtering as is typical with other receiver approaches like superheterodyne. This architecture uses two WJ MH303 high compression point MMIC mixers in quadrature along with low pass filtering. Reducing CW carrier leakage into the RF port of the mixers is critical to receiver performance since the resulting phase discriminator generates unwanted DC signal offsets. Leakage of the high power transmit signal into the receive path can saturate the receiver and significantly degrade receive signal sensitivity. Some unique methods have been implemented for providing adequate isolation with typical receiver sensitivity levels better than –76 dBm.
One of the most aggressive design challenges was that of dramatic size reduction. Figure 2 shows the outline drawing for the SX20xx platform. The SX20xx platform is much smaller than readers currently on the market. Top-level trade-offs between performance requirements and design margin were reviewed extensively to give absolute design limits. The reader uses an inexpensive microprocessor to control reader operation, power management and data processing. The embedded controller executes firmware routines to command tags, including anti-collision, masking and other tag management operations. A 40 MHz temperature-compensated crystal oscillator (TCXO) is used as the clock signal for the microprocessor and serves the dual purpose of the reference in the phase-locked loop employed to generate the RF carrier signals. Utilization of the company’s GaAs MESFET and InGaP HBT integration technology, and extensive use of 0402 package type devices also helped minimize board layout area. Finally, one of the most important design features is that of a cavitized RF web. The RF web features are realized in the housing cover. The web contacts the top of the printed wiring board (PWB) and essentially breaks the area into many separate cavities. This is critical since the cavities are used to physically isolate different areas of the PWB. One important subtlety is the ability to place the small 0402 devices much closer together on the physical layout without the usual concern of mutual interaction or interference. Small MMCX coaxial connectors make the RF interface. There are two separate antenna ports. The readers are able to select between ports and thereby enable the use of two antennas in different locations. This can be used to the installer’s advantage to improve the probability of intercept or increase the tag reading zone.
Since these small reader engines are destined for any number of mobile applications such as handheld readers, power consumption and management are of critical importance. The SX20xx products are designed to operate from a single +5 or +6 VDC bias supply. For example, at the UHF ISM band, the SX2000 uses a nominal +6 VDC bias with an RS232 interface and the SX2001 is a nominal +5 VDC bias version with a TTL interface. The reader receives external power, and provides internal conditioning. Low drop out (LDO) regulator circuits are utilized to provide bias throughout the reader. Low power consumption, high-efficiency devices were chosen where possible. Additionally, the controller is used to shut down bias voltages to devices that are not being used. During operation with +6 VDC the maximum current consumption is 650 mA with average current consumption being closer to 275 mA. The reader has a CW mode used to program tags that consumes additional power, and there is a power save mode consuming less than 35 mA.
The SX20xx products are designed to deliver +30 dBm (1 W) of output power to a 6 dBi gain antenna per FCC regulatory allowances. The SX2000 can actually deliver almost +32 dBm (1.5 W) of output power. This provides enough power to accommodate additional losses associated with real-world RFID installations. Sometimes antenna cabling adds transmit loss in excess of the antenna gain. In these cases, the SX2000 reader has enough headroom to boost transmit power at the antenna to the maximum allowed level. For low power applications, such as smart label printers, the transmit power at the antenna may only be +20 dBm (1/10 W). The 14 dB power control range of the unit is employed to reduce transmit power to these levels and calibrate unit-to-unit gain variations. In this case, the power amplifier output is turned down to conserve power while ensuring linear operation of the amplifier.
As realized for the existing EPC Class 1 standard, all of the SX20xx products are frequency-hopping transceivers falling under regulatory governance. In the US, these readers fall under the requirements of FCC Part 15, in the EU it is EN 302 208-1 and in Japan it is METI. Regulatory requirements drive transceiver architectures and packaging, and many of the regulatory requirements — like ETSI and METI — are in a state of change and not yet finalized. Initial concepts utilized an extremely low cost on/off switched modulator without additional filtering. While this approach provides adequate system-level performance and meets US FCC regulatory emissions requirements, it is not sufficient for all expected applications. Figure 3 shows the transmit spectrum for an SX2020 prototype using this method. Note that at 1 MHz from the carrier the spectrum is rejected 45 dBc. The company has since implemented a linear modulator design including some additional pulse shaping (filtering). The current design is more spectrally efficient and is able to meet international regulatory requirements like what is expected for METI. While the international compliance requirements are evolving they are expected to be somewhat more demanding than the current FCC requirements. Figure 4 shows the transmit spectrum for an SX2020 prototype using a linear modulator with pulse shaping. This approach adds over 20 dB more rejection, and suppression is more than 66 dBc at 1 MHz away from the carrier. This improved spectral efficiency enables the utilization of additional hop channels within the allocated frequency band. 25 channels spaced at 300.75 kHz are possible for the SX2020 prototype while still meeting the regulatory requirement of –36 dBm at the lower 948 MHz and upper 957 MHz band edges (see Figure 5).
WJ designed the SX20xx reader platform for Alien Technology® Inc. (Alien®). The SX20XX reader engines utilize WJ and Alien intellectual property and are only one part of Alien’s complete RFID system. The top-level system includes an antenna, a power supply, digital control, top-level packaging and software. WJ has a strategic partnership with Alien for EPC Class 1 readers, and the SX20xx products are provided to Alien on an original equipment manufacturer (OEM) basis. The following readers have been released to production and are being sold by Alien as models ALR 9932A (WJ SX2000) and ALR-9930 (WJ SX2001). Other models are in various stages of research and development. Alien may be contacted at www.alientechnology.com for further information on price and availability.