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LT5571 View Datasheet(PDF) - Linear Technology

Part Name
Description
MFG CO.
'LT5571' PDF : 16 Pages View PDF
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LT5571
APPLICATIONS INFORMATION
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 VCM = 0.25V was chosen in order to save 30mA
current, resulting in a modulator supply current of about
73mA. This is achieved by sourcing 5mADC average DAC
current into 50Ω resistors R1A and R1B. As anti-aliasing
filter, an RCRC filter was chosen using R1A, R1B, C1A, C1B,
R2A, R2B, C2A and C2B. This results in a second-order
passive low-pass filter with –3dB cutoff at 790kHz. This
filter cutoff is chosen high enough that it will not affect
the RFID baseband signals in the fastest mode (TARI =
6.25µs, see [1]) significantly, 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 VCM
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.
LT5571
RF
VCC
5V
C
BALUN
LOMI
FROM
Q-CHANNEL
LOPI
0mA TO 10mA
DAC
10mA TO 0mA
0.25VDC
R2A
2500.25VDC BBPI
R1A
50
C1A
2.2nF
C2A
470pF
R1B
C1B GND
50
2.2nF
C2B
470pF
0.25VDC
R2B 0.25VDC BBMI
250
1.8pF
1.8pF
GND
5571 F15
Figure 15. Recommended Baseband Interface for RFID Applications (Only I Channel is Drawn)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
865.4
865.6 865.8 866.0 866.2 866.4
FREQUENCY (MHz)
CH BANDWIDTH: 100kHz
CH SPACING: 100kHz
CH PWR: –4.85dBm
ACP UP: –33.74dBc
ACP LOW: –37.76dBc
ALT1 UP: –71.15dBc
ALT1 LOW: –64.52dBc
ALT2 UP: –72.80dBc
ALT2 LOW: –72.42dBc
5571 F16a
0.3
0.2
0.1
0
–0.1
–0.2
–0.3
0
50
100 150 200 250
TIME (µs)
5571 F16b
Figure 16a and 16b. RFID SSB-ASK Spectrum with Mask and Corresponding RF Envelope for TARI = 25µs
[1] EPC Radio Frequency Identity Protocols, Class-1 Generation-2 UHF RFID Protocol for
Communications at 860MHz – 960MHz, version 1.0.9.
14
5571f
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