LINEAR TECHNOLOGY LTC6602 Technical data

Programmable Baseband Filter for Software-Defi ned
UHF RFID Readers

Design Note 432

Philip Karantzalis

Introduction

Radio frequency identifi cation (RFID) is an auto-ID tech­nology that identi fi es any object tha t 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 refl ec­tion coeffi cient of its antenna, thereby backscattering an
information signal to the reader.

Tag signal detection requires measuring the time interval
betwe en 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. RFID readers can
operate in a noisy RF environment where many readers
are in close proximit y. The three operating modes, single­interrogator, multiple interrogator and dense-interroga­tor, defi ne the spectral limits of reader and tag signals.
Software programmability of the receiver provides an
optimum balance of reliable multitag detection and high
data throughpu t. The programmable reader co ntains a high
linearit y direct conversion I and Q demodulator, low noise
amplifi ers, a dual baseband fi lter with variable gain and
bandwidth and a dual analog-to-digital converter (ADC).
The LTC6602 dual, matched, programmable bandpass
fi lter can optimize high performance RFID readers.

The LTC6602 Dual Bandpass Filter

The LTC6602 features two identical fi lter channels with
matched gain control and frequency-controlled lowpass,
and highpass networks. The phase shift through each
channel is matched to ±1 degree. A clock frequency, either
internal or external, positions the pass band of the fi lter
at the required frequency spectrum.

The lowpass and highpass corner frequencies, as well
as, the fi lter bandwidth are set by division ratios of the

clock frequency. The lowpass division ratio options are
100, 300 and 600 and the highpass division ratios are,
1000, 2000, 6000. Figure 1 shows a t ypical fi lter response
with a 90MHz internal clock and the division ratios set to
6000 and 600 for the highpass and lowpass, respectively.
A sharp 4th order elliptical stopband response helps
eliminate out-of-band noise. Controlling the baseband
bandwidth permits software defi nition of the operating
mode of the RFID receiver as it adapts to the operating
environment.

An Adaptable Baseband Filter for an RFID Reader

Figure 2 shows a simple LTC6602-based fi lter circuit that
uses SPI serial control to vary the fi lter’s gain and band­width to adapt to a compl ex set of data rates and en coding.
(The back scatter link frequency r ange is 40kHz to 640kH z
and the data rate range is 5kbps to 640kbps.)

For fi ne resolution positioning of the fi lter, the internal
clock frequency is set 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 set by serial

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10

0

–10

–20

–30

GAIN (dB)

–40

–50

–60

–80

Figure 1. Filter Response for a 15kHz–150kHz Passband

10
FREQUENCY (kHz)

100

10001

01/08/432

SPI control of the LTC6602. The cutoff range for the
highpass fi lter is 6.7kHz to 100kHz and 66.7kHz to 1MHz
for the lowpass fi lter. The optimum fi lter bandwidth set­ting can be adjusted by a software algorithm and is a
function of the data clock, data rate and encoding. The
fi lt er bandwidth must be suf fi ciently narr ow to max imize
the dynamic range of the ADC input and wide enough to
preserve signal transitions and pulse widths (the proper
fi lter setting ensures reliable DSP tag signal detection).

Figure 3 shows an example of the fi lter’s time domain
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

= 1/10μs = 100kHz). If the

CUTOFF

lowpass cutoff frequency is lower, the signal transition
and time interval will be distorted beyond recognition. The
setting of the highpass cutoff frequency is more qualita­tive than specifi c. The highpass cutoff frequency must
be lower than the reciprocal of the longest interval (for

5V 3V

0.1μF

I
CHANNEL
OUTPUT

Q
CHANNEL
OUTPUT

3V

CS

0.1μF

CS

SCLK

SDI

8-BIT DAC

3V

+

V

GND

LTC2630

I CHANNEL

INPUT

Q CHANNEL

INPUT

TRANSMITTER

MUTE INPUT

174k

V

OUT

68.1k

0.1μF

V

DDINVDDAVDDD

MUTE

DOUT

NC

R

BIAS

OCM

0.1μF

LTC6602

GNDV

CLKIO

PAR/SER

CLKCNTL

SDO

SCLK

SDI

FROM
ADC
V

REF

OUTPUT

the example shown, highpass f

< 1/20μs) and as

CUTOFF

hig h a s p ossi bl e t o d ecr ea se th e r ecei ve r’s low f re qu en cy
noise (of the baseband amplifi er and the down-converted
phase and amplitude noise). The lower half of Figure 3
shows the fi lter’s overall respon se (lowpass plus highpass
fi lter). Comparing the fi lter outputs with a 10kHz and a
30kHz highpass setting, the signal transitions and time
intervals of the 10kHz output are adequate for detecting
the symbol sequence (in an RFID environment, noise will
b e s u p e r i m p o s e d o n t h e o u t p u t s i g n a l ) . I n g e n e r a l , i n c r e a s ­ing the lowpass f
f

“enhances” signal transitions and intervals at the

CUTOFF

and/or decreasing the highpass

CUTOFF

expense of increased fi lter output noise.

Conclusion

The LTC6602 dual bandpass fi lter is a programmable
baseband fi lter for high performance UHF RFID readers.
Using 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 dense interrogator physical setting. The LTC6602 is
a very compact IC in a 4mm × 4mm QFN package and is
programmable with parallel or serial control.

References:

1. The RF in RFID, Daniel M. Dobkin, 9/07, Elsevier Inc.

2. Class-1 Generation-2 UHF RFID Protocol for Com-
munications at 860 MHz to 960 MHz, Version 1.1.0,
www.epcglobalinc.org/standards/specs/

TYPICAL TAG
SYMBOL SEQUENCE

LTC6602 100kHz LOWPASS

LTC6602 100kHz LOWPASS + HIGHPASS FILTER

SPI CONTROL OF LTC6602 SETS THE FILTER GAIN

AND THE LOWPASS AND HIGHPASS DIVISION RATIO

SCK SDI CS2CS1

Figure 2. An Adaptable RFID Baseband Filter
with SPI Control

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10kHz HIGHPASS30kHz HIGHPASS

10μs/DIV

Figure 3. Filter Transient Response to a Tag
Symbol Sequence

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