DESIGN IDEAS L
Introduction
Radio frequency identification (RFID) is an auto-ID technology that identi-fies any object that contains a coded tag. A UHF RFID system consists of a reader (or interrogator) that transmits information to a tag by modulating an RF signal in the 860MHz–960MHz frequency range. Typically, the tag is passive—it receives all of its operating energy from a reader that transmits a continuous-wave (CW) RF signal. A tag responds by modulating the reflec-tion coefficient of its antenna, thereby backscattering an information signal to the reader.
Tag signal detection requires mea-suring the time interval between signal transitions (a data “1” symbol has a longer interval than a data “0” symbol). The reader initiates a tag inventory by nding a signal that instructs a tag to t its backscatter data rate and encoding. RFID readers can operate in a noisy RF environment where many readers are in clo proximity. The three operating modes, single-interrogator, multiple interrogator and den-interrogator, define the spectral limits of reader and tag sig-nals. Software programmability of the receiver provides an optimum balance of reliable multitag detection and high data throughput. The programmable reader contains a high linearity direct conversion I and Q demodulator, low noi amplifiers, a dual baband filter
with variable gain and bandwidth and a dual analog-to-digital converter (ADC). The LTC6602 dual, matched, program-mable bandpass filter can optimize high performance RFID readers.
Programmable Baband Filter for Software-Defined UHF RFID Readers
by Philip Karantzalis
G A I N (d B )
FREQUENCY (kHz)
1000
1
10–80
蓬莱市10
100–30 –40–50 –60–10 –20
0Figure 1. Filter respon for a 15kHz–150kHz passband
Figure 2. An Adaptable RFID baband filter with SPI control
L DESIGN IDEAS The LTC6602
Dual Bandpass Filter
The LTC6602 features two identical fil-ter channels with matched gain control and frequency-controlled lowpass and highpass networks. The pha shift through each channel is matched to ±1 degree. A clock frequency, either internal or external, positions the passband of the filter at the required frequency spectrum.
The lowpass and highpass corner frequencies, as well as, the filter band-width are t by division ratios of the clock frequency. The lowpass division
ratio options are 100, 300, 600 and the highpass division ratios are 1000, 2000, 6000. Figure 1 shows a typical filter respon with a 90MHz internal clock and the division ratios t to 6000 and 600 for the highpass and lowpass, respectively. A sharp 4th order ellipti-cal stopband respon helps eliminate out-of-band noi. Controlling the baband bandwidth permits software definition of the operating mode of the RFID receiver as it adapts to the operating environment.
An Adaptable Baband Filter for an RFID Reader
Figure 2 shows a simple LTC6602-bad filter circuit that us SPI rial control to vary the filter’s gain and bandwidth to adapt to a complex t of data rates and encoding. (The backscatter link frequency range is 40kHz to 640kHz and the data rate range is 5kbps to 640kbps.)
For fine resolution positioning of the filter, the internal clock frequency is t by an 8-bit LTC2630 DAC. A 0V to 3V DAC output range positions the clock frequency between 40MHz and 100MHz (234.4kHz per bit). The lowpass and highpass division ratios are t by rial SPI control of the LTC6602. The cutoff range for the highpass filter is 6.7kHz to 100kHz and 66.7kHz to 1MHz for the lowpass filter. The optimum filter bandwidth tting can be adjusted by a software algorithm and is a function of the data clock, data rate and encoding. The filter bandwidth must be sufficiently narrow to maximize the dynamic range of the ADC input and wide enough to prerve signal transitions and
pul widths (the proper filter t-
ting ensures reliable DSP tag signal
detection).
Figure 3 shows an example of the
filter’s time domain respon to a
typical tag symbol quence (a “short”
pul interval followed by a “long”
pul interval). The lowpass cutoff
frequency is t equal to the recipro-
cal of the shortest interval (f CUTOFF =
1/10µs = 100kHz). If the lowpass cutoff
frequency is lower, the signal transi-
tion and time interval will be distorted
beyond recognition. The tting of the
highpass cutoff frequency is more
qualitative than specific. The highpass
cutoff frequency must be lower than
the reciprocal of the longest interval (for
the example shown, highpass f CUTOFF
< 1/20µs) and as high as possible to
decrea the receiver’s low frequency
noi (of the baband amplifier and
the down-converted pha and ampli-
tude noi). The lower half of Figure
3 shows the filter’s overall respon
(lowpass plus highpass filter). Com-
paring the filter outputs with a 10kHz
and a 30kHz highpass tting, the
signal transitions and time intervals
of the 10kHz output are adequate for
detecting the symbol quence (in an长沙社保查询
RFID environment, noi will be su-
perimpod on the output signal). In
general, increasing the lowpass f CUTOFF
and/or decreasing the highpass f CUTOFF
“enhances” signal transitions and
intervals at the expen of incread
filter output noi.
Conclusion孙悟空台词
The LTC6602 dual bandpass filter is a
programmable baband filter for high
performance UHF RFID readers. Using现成的丸子怎么做汤
the LTC6602 under software control
provides the ability to operate at high
data rates with a single interrogator
or with optimum tag signal detection
in a multiple or den interrogator
physical tting. The LTC6602 is a
very compact IC in a 4mm × 4mm QFN
package and is programmable with
parallel or rial control. L
References:
1 The RF in RFID, Daniel M. Dobkin, 9/07, Elvier
Inc.
我的show
2 Class-1 Generation-2 UHF RFID Protocol for Com-幸福的流泪
munications at 860 MHz to 960 MHz, Version 1.1.0,
www.epcglobalinc/standards/specs/
Figure 3. Filter transient respon to a tag symbol quence
The LTC6602 dual bandpass
filter is a programmable
baband filter for high
performance UHF RFID
readers. Using the LTC6602
under software control
provides the ability to
手动挡遇红灯5档怎么降
operate at high data rates
with a single interrogator
or with optimum tag signal
述职报告模板
detection in a multiple or
den interrogator physical
tting. The LTC6602 is a
very compact IC in a 4mm
× 4mm QFN package and is
programmable with parallel
or rial control.
