User's Manual

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MPR-1510 - 6 - Doc# 041313

AWID PROPRIETARY

through the use of a circulator. As shown in Figure 1, there is a 3-port device between

the Coupler and the band pass filter, which is called a circulator. A circulator is

physically constructed by a permanent magnet, a Y junction on a high-dielectric

ferromagnetic substrate, and a ferromagnetic enclosure to complete the flux field. A

circulator permits flow of RF energy in one direction only, e.g. from port 1 to 2, 2 to 3,

and 3 to 1. When one of the ports is terminated (matched condition), the other two are

isolated in the reverse direction. Many fixed-site RFID readers use circulators to ensure

that the power amplifier output flows from the amplifier (port 1) to the antenna (port 2),

and the received signal flows from the antenna (port 2) to the receiver (port 3). When

properly matched, a circulator can provide typically 15 to 18 dB of isolation between the

power amplifier output (port 1) and the receiver input (port 3), thereby reducing any in-

band interference from transmitter output to receiver input. MPR reader uses a similar

circuit to accomplish the same function, but in a much smaller physical size.

It should be noted that some fixed-site reader designs use separate transmit and receive

antennas to resolve this T/R signal isolation problem. Figure 2 is a block diagram of a

dual-antenna RFID reader. On the surface, this design has the advantage of allowing a

low-level design on the receive chain, which means lower compression point for mixers,

lower saturation point for amplifiers, and the possibility of using a front-end amplifier to

enhance receiver sensitivity. Such dual-antenna design becomes problematic in a

mobile environment, where signal strength is not easily controlled. A well-designed dual-

antenna reader can usually provide 25 to 30 dB of isolation between the two signal

paths, reducing the unwanted signal in the receive chain to –20 dBm. However, when

the RFID reader antenna is facing a tag placed on a large metallic object at a distance of

12 inches, the reflected transmitter signal at the receiver input can be as high as 13dBm,

thereby eliminating any advantage of the dual-antenna design.

In actual circuit implementation, AWID developed a proprietary circuit to duplicate the

functions of the circulator, with improved directivity and isolate

Figure 1 Block Diagram, Single-Antenna RFID Reader

FREQUENCY

SYNTHESIZER

1.1.3

1.1.4

I/Q

DEMOD

Driver

AMP

Power

AMP

COUPLER

XMIT/

RCVE

A/D

CPU I/O

XMTR

CIRCULATOR

BPF

RCV

FREQ

AMPL