lt5571 Linear Technology Corporation, lt5571 Datasheet - Page 14

• Manufacturer Part Number: lt5571
• Description: 620mhz - 1100mhz High Linearity Direct Quadrature Modulator
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
LT5571
Example: RFID Application
Figure 15 shows the interface between a current drive DAC
and the LT5571 for RFID applications. The SSB-ASK mode
requires an I/Q modulator to generate the desired spectrum.
According to [1], the LT5571 is capable of meeting the
“Dense-Interrogator” requirements with reduced supply
current. A V
current, resulting in a modulator supply current of about
73mA. This is achieved by sourcing 5mA
current into 50Ω resistors R1A and R1B. As anti-aliasing
fi lter, an RCRC fi lter was chosen using R1A, R1B, C1A, C1B,
R2A, R2B, C2A and C2B. This results in a second-order
passive low-pass fi lter with –3dB cutoff at 790kHz. This
fi lter cutoff is chosen high enough that it will not affect
[1] EPC Radio Frequency Identity Protocols, Class-1 Generation-2 UHF RFID Protocol for
Communications at 860MHz – 960MHz, version 1.0.9.
14
CM
Figure 16a and 16b. RFID SSB-ASK Spectrum with Mask and Corresponding RF Envelope for TARI = 25µs
= 0.25V was chosen in order to save 30mA
Figure 15. Recommended Baseband Interface for RFID Applications (Only I Channel is Drawn)
0mA TO 10mA
10mA TO 0mA
–100
DAC
–10
–20
–30
–40
–50
–60
–70
–80
–90
0
865.4
CH BANDWIDTH: 100kHz
CH SPACING: 100kHz
CH PWR: –4.85dBm
ACP UP: –33.74dBc
ACP LOW: –37.76dBc
865.6
R1A
50Ω
R1B
50Ω
FREQUENCY (MHz)
0.25V
0.25V
865.8
DC
DC
C1A
2.2nF
C1B
2.2nF
866.0
ALT1 UP: –71.15dBc
ALT1 LOW: –64.52dBc
ALT2 UP: –72.80dBc
ALT2 LOW: –72.42dBc
DC
250Ω
250Ω
GND
R2A
R2B
866.2
average DAC
0.25V
0.25V
5571 F16a
DC
DC
866.4
C2A
470pF
C2B
470pF
V
BBMI
RF
5V
BBPI
CC
LT5571
the RFID baseband signals in the fastest mode (TARI =
6.25µs, see [1]) signifi cantly, and at the same time achiev-
ing enough alias attenuation while using a 32MHz sampling
frequency. The resulting Alt80-CPR (the alias frequency at
897.875MHz falls outside the RF frequency range of Figure
16a) is –92dBc for TARI = 6.25µs. The SSB-ASK output
signal spectrum is plotted in Figure 16a, together with the
Dense-Interrogator Transmit mask [1] for TARI = 25µs. The
corresponding envelope representation is given in Figure
16b. The Alt1-CPR can be increased by using a higher V
at the cost of extra supply current or a lower baseband drive
at the cost of lower RF output power. The center frequency
of the channel is chosen at 865.9MHz (“channel 2”), while
the LO frequency is chosen at 865.875MHz.
1.8pF
1.8pF
C
–0.1
–0.2
–0.3
0.3
0.2
0.1
0
0
BALUN
50
LOMI
GND
100
TIME (µs)
150
LOPI
200
5571 F16b
FROM
Q-CHANNEL
250
5571 F15
5571f
CM