LINEAR TECHNOLOGY LT5516 Technical data

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Baseband Circuits for an RFID Receiver – Design Note 381

Philip Karantzalis

Introduction

Radio Frequency Identifi cation (RFID) technology uses
radiated and refl ected RF power to identify and track a
variety of objects. A typical RFID system consists of a
reader and a transponder (or tag). An RFID reader con­tains an RF transmitter, one or more antennas and an RF
receiver. An RFID tag is simply an uniquely identifi ed IC
with an antenna.

Communication between a reader and a tag is via back­scatter refl ection, similar to a radar system, in the UHF
frequencies from 860MHz to 960MHz. This design note
describes a high performance RFID receiver.

A Direct Conversion Receiver

Figure 1 shows the block diagram of a direct conversion
RF receiver—the receiver demodulates an RF carrier
directly into a baseband signal without an intermediate
frequency down-conversion (a zero IF receiver).

The antenna, shared by both the transmitter and receiver,
detects an RF carrier and passes it through a bandpass

®

fi lter to an LT

5516 demodulator’s RF input.

The LT5516 direct conversion demodulator frequency
range of 800MHz to 1.5GHz includes the UHF range used

by RFID readers (860MHz to 960MHz). The excellent
linearity of the LT5516 provides for high sensitivity to
low level signals, even in the presence of large interfer­ing signals.

The LT6231 low noise dual op amp acts as a differential
to single-ended amplifi er to drive the single-ended input
of the lowpass fi lter.

Analog baseband fi ltering is performed by the LT1568, a
low noise, precision RC fi lter building block. The LT1568
fi lter provides a simple solution for designing lowpass
and bandpass fi lters with cutoff frequencies from 100kHz
to 10MHz. These cutoff frequencies are suffi cient for the
250kHz to 4MHz signal spectrum typically used in UHF
RFID systems.

The differential output of an LT1568 drives the inputs of
an LTC2298 ADC. The LTC2298 is a 65Msps, low power
(400mW), dual 14-bit analog to digital converter with 74dB
signal-to-noise ratio (SNR). The digital signal processor
(DSP) that follows the ADC analyzes the received signal
from multiple tags and provides additional fi ltering.

, LTC and LT are registered trademarks of Linear Technology

Corporation. All other trademarks are the property of their respective owners.

01/06/381

RX

ANTENNA

BPF

LOW NOISE

BASEBAND
AMPLIFIER

LT6231LT5516 DEMODULATOR

LO

MATCHED I AND Q

LOWPASS FILTERS

LT1568 LT1568

B

A

B

A

Figure 1. A Direct Conversion Receiver for an RFID Reader

DUAL ADC LTC2298

DSP

DN381 F01

A Low Noise Differential to Single-Ended Amplifi er

Figure 2 shows an LT6231 difference amplifi er used to
convert the LT5516 differential I or Q output to a single­ended output. The addition of external 270pF capacitors
across the 60Ω resistors limits the demodulator’s output
to 10MHz to prevent any high frequency int erfer ence from
reaching the LT6231 amplifi er.

A C c o u p l i n g t o t h e b a s e b a n d a m p l i fi e r i s u s e d b e c a u s e D C
coupling is not necessary for the amplitude shift keying
(ASK) RFID signal.

V

R2

OUT

GAIN =

5V

60Ω 60Ω

LT5516 I OR Q OUTPUT

=

+

–

V

– V

IN

IN

5pF

200Ω

V

IN

V

IN

–

+

5 ≤ GAIN ≤ 40

C1

0.1µF

C5
270pF

5V

C3

0.1µF

C6
270pF

R1

140Ω

R3

140Ω

Figure 2. A Low Noise I or Q Baseband Interface

5V

–
+

R4
R4 = R2

C2

2.7pF

R2

1/2
LT6231

0.1µF

TO LT1568
I OR Q FILTER

C4

2.7pF

DN381 F02

V

OUT

1.5V ADC
REFERENCE

or V

(V

CMA

CMB

)

The highpass pole provided by the AC coupling capaci­tors and the amplifi er input resistors is set to 8kHz. The
differential amplifi er’s input resistors are set to 140Ω
in order to minimize the input referred noise. The noise
fl oor at the amplifi er’s output is 4.3nV/√Hz times the
amplifi er gain (gain ≥ 5). The 1.5V reference provided
by the LTC2298 ADC is used to level shift the amplifi er’s
output to the mid-supply point of the following 3V fi lter
and ADC circuits.

A Matched I and Q Filter and a Dual ADC

Figure 3 shows t wo LT1568 fi l t e r b u i l d i n g b l o c k s c o n n e c t e d
as dual, matched, fourth order fi lters. The LT1568 fi lter’s
single-ended inpu t to differ ential outpu t conversion gain
is 6dB. The LT1568 circuit implements an elliptic lowpass
fi lter function with equal resistor values (see Figure 3).
Stopband attenuation at 2(f

) is 34dB. I and Q fi lter

–3dB

matching is assured by the inherent matching of the
LT1568s’ A and B sides.

The input voltage range of an LTC2298 is adjustable to
2V

P-P

or 1V

P-P

.

Conclusion

Using only fi ve ICs (LT5516, LT6231, two LT1568 and an
LTC2298), a high performance UHF RFID receiver can be
designed with the fl exibility to adapt and be optimized
to meet the requirements of the present and emerging
RFID standards.

R12

1k

I

IN

3V

+

0.1µF 0.1µF

R11

1k

Q

IN

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C

N1

R22

1k

R32

1k

V

V+INVA

SB

INVB

LT1568

SA

R31

1k

R21

1k

ALL LT1568 RESISTORS =

f

–3dB

TO V

OUTB

OUTB

OUTA

OUTA

TO V

IN MHz

CMA

EN

GNDB

GNDANCV

CMB

1MHz
f

–3dB

47pF

INVB

R34

R33

R24

1k

1k

SB

OUTB

LT1568

SA

OUTA

1k

R23

1k

R14

1k

3V

–

V

–

R13

C

• 1k
47pF

+

V

V+INVA

1k

N2

I

OUT

Q

OUTB

OUTA

OUT

25Ω

25Ω

EN

GNDB

GNDANCV

25Ω

25Ω

12pF

–

V

–

2.2µF

2.2µF

12pF

Figure 3. A Matched, 1MHz, 4th Order, I and Q Lowpass Filter and ADC Driver

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call [phone], Ext. 2020

3V

+

A

INA

–

A

INA

V

CMA

1.5V

LTC2298

V

CMB

1.5V

–

A

INB

+

A

INB

DN381 F03

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