LTC6602IUF#PBF Linear Technology, LTC6602IUF#PBF Datasheet - Page 23

• Manufacturer Part Number: LTC6602IUF#PBF
• Description: IC FILTER BANDPASS/LOWPASS 24QFN
• Manufacturer: Linear Technology

Specifications of LTC6602IUF#PBF

• Filter Type: Bandpass
• Frequency - Cutoff Or Center: 300kHz
• Number Of Filters: 2
• Max-order: 5th
• Voltage - Supply: 2.7 V ~ 3.6 V
• Mounting Type: Surface Mount
• Package / Case: 24-QFN
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

APPLICATIONS INFORMATION
The LTC6602, an Adaptable Baseband Filter for an
RFID Reader
A radio-frequency identification (RFID) system is an auto-id
technology that identifies any object that contains a coded
tag. An RFID system consists of a reader (or interrogator)
and a tag. An RFID system capable of identifying multiple
tags at a maximum operating distance operates in the
UHF frequency range. A UHF reader transmits informa-
tion to a tag by modulating an RF signal in the 860MHz
to 960MHz frequency range. Typically a tag is passive,
meaning that it receives all of its operating energy from a
reader that transmits a continuous wave (CW) RF signal
to power a tag. A tag responds by modulating the reflec-
tion coefficient of its antenna, thereby backscattering an
information signal to the reader. Reliable detection of a
tag signal requires communication protocols that define
the physical and operating interaction between readers
and tags. The latest UHF RFID protocol, the Electronic
Product Code™ (EPC) global class-1 generation 2 standard
(C1G2), have been accepted worldwide and is also known
as ISO 18000-6C. The C1G2 standard defines a reader to
tag and a tag to reader communication using a flexible
set of signal modulation, data encoding, data rates and
command procedures. C1G2 specifies reader and tag data
symbols using pulse-interval encoding. Tag signal detec-
tion requires measuring 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
sending a signal that instructs a tag to set its backscatter
data rate and encoding. C1G2 certified RFID readers can
operate in an RF environment where many readers are in
close proximity. The three operating modes of C1G2, single
interrogator, multiple interrogator and dense interrogator,
define the spectral limits of reader and tag signals for an
optimum balance of reliable multitag detection and high
data throughput (for more information on C1G2, consult
the references at the end of this design note). The advan-
tages of C1G2 complex protocols can be realized by using
a reader whose receiver contains a high linearity direct
conversion I and Q demodulator, a low noise amplifier, a
dual baseband filter with variable gain and bandwidth and
a dual analog to digital converter (ADC).
Certified C1G2 UHF RFID readers can adapt to a great
variety of operating conditions. To achieve operating
flexibility a reader’s baseband circuits must include an
adaptable bandwidth filter. Figure 17 shows an LTC6602
based filter circuit that uses SPI control to vary the filter’s
bandwidth to adjust for the C1G2 complex set of data rates,
encoding and modulation. The filter’s clock frequency is
set by the SPI control of 8-bit LTC2630 DAC (digital to
analog converter). The DAC voltage through a resistive
divider sets the current into the LTC6602 R
resistive divider sets the clock frequency range for a DAC
voltage range 0V to 3V. For the resistor values in Figure 17
(191k and 61.9k) the clock frequency range is 40MHz to
100MHz (234.4kHz per bit). The lowpass and highpass
division ratio is set by the SPI control of the LTC6602. The
cutoff range for the highpass filter is 6.7kHz to 100kHz and
for the lowpass filter is 66.7kHz to 1MHz. The optimum
filter bandwidth setting can be adjusted by a software
algorithm and is a function of the reader’s data clock, data
rate, encoding and modulation. The filter bandwidth must
be sufficiently narrow to maximize the dynamic range to
the ADC input and wide enough to preserve signal transi-
tions and pulse width. If the filter setting is optimum then
a DSP algorithm can reliably detect tag data. Figure 18a
shows the filter’s time response to a typical tag symbol
sequence (a “short” pulse interval followed by a “long”
pulse interval). The lowpass cutoff frequency is set equal
to the reciprocal of the shortest interval (f
= 100kHz). If the lowpass cutoff frequency is lower the
signal transition and time interval will be distorted beyond
recognition by any tag signal detection algorithm. The set-
ting 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 Figure 18
example, highpass f
high as possible to decrease the receiver’s low frequency
noise (baseband amplifier and down-converted phase and
amplitude noise). Figures 18a and 18b show the filter’s
total response (lowpass plus highpass filter). The filter’s
output is shown with 30kHz and a 10kHz highpass cutoff
frequency setting. Comparing the filter outputs with a
10kHz and a 30kHz highpass setting, the signal transitions
and time intervals of the 10kHz output are adequate for
CUTOFF
< 1/20µs < 50kHz) and as
LTC6602
CUTOFF
BIAS
= 1/10µs
pin. The
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