MPR-2010AR
Page 6
Doc# 041308
AWID PROPRIETARY
is how the tags’ internal data are “conveyed” to the reader. The reader in turn monitors
the perturbation of the RF energy field, and thereby receives the varying degree of
signal reflected from the tags.
One of the unique design features for a (single-antenna) RFID reader is that it must
simultaneously transmit a strong CW signal and at the same time, receive a weak
reflected signal with little frequency separation. In a traditional design, such functions
are implemented 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-
2010AR uses a similar circuit to accomplish the same function, but in a much smaller
physical size.
Figure 1 Block Diagram, Single-Antenna RFID Reader
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
such 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
FREQUENCY
SYNTHESIZER
I/Q
DEMOD
Driver
AMP
Power
AMP
COUPLER
XMIT/
RCVE
A/D
CPU
I/O
XMTR
CIRCULATOR
BPF
RCV
IQ
RF
I
Q
I
Q
FREQ AMPL
1
2
3
