Transmitter - Analog Section - Texas Instruments TRF7970A Manual

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C = C = 2 × (C – C
1 2 L
A 27-pF capacitor must be placed on pins 30 and 31 to ensure proper crystal oscillator operation.
Any crystal used with TRF7970A should meet the minimum characteristics in
As an alternative, an external clock oscillator source can be connected to pin 31 to provide the system
clock; pin 30 can be left open.
6.7 Transmitter – Analog Section
The 13.56-MHz oscillator generates the RF signal for the PA stage. The power amplifier consists of a
driver with selectable output resistance of nominal 4 Ω or 8 Ω. The transmit power level is set by bit B4 in
the Chip Status Control register (0x00). The transmit power levels are selectable between 100 mW (half
power) or 200 mW (full power) when configured for 5-V automatic operation. The transmit power levels
are selectable between 33 mW (half power) or 70 mW (full power) when configured for 3-V automatic
operation.
The ASK modulation depth is controlled by bits B0, B1, and B2 in the Modulator and SYS_CLK Control
register (0x09). The ASK modulation depth range can be adjusted between 7% to 30% or 100% (OOK).
External control of the transmit modulation depth is possible by setting the ISO Control register (0x01) to
direct mode. While operating the TRF7970A in direct mode, the transmit modulation is made possible by
selecting the modulation type ASK or OOK at pin 12. External control of the modulation type is made
possible only if enabled by setting B6 in the Modulator and SYS_CLK Control register (0x09) to 1.
In normal operation mode, the length of the modulation pulse is defined by the protocol selected in the
ISO Control register (0x01). With a high-Q antenna, the modulation pulse is typically prolonged, and the
tag detects a longer pulse than intended. For such cases, the modulation pulse length needs to be
corrected by using the TX Pulse Length Control register (0x06).
If the register contains all zeros, then the pulse length is governed by the protocol selection. If the register
contains a value other than 0x00, the pulse length is equal to the value of the register multiplied by
73.7 ns; therefore, the pulse length can be adjusted between 73.7 ns and 18.8 µs in 73.7-ns increments.
Copyright © 2011–2017, Texas Instruments Incorporated
) = 2 × (18 pF – 4.5 pF) = 27 pF
S
Pin 30
C
1
Figure 6-7. Crystal Block Diagram
Table 6-9. Minimum Crystal Recommendations
PARAMETER
Frequency
Mode of operation
Type of resonance
Frequency tolerance
Aging
Operation temperature range
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SLOS743L – AUGUST 2011 – REVISED MARCH 2017
C
S
TRF7970A
Pin 31
Crystal
C
2
SPECIFICATION
13.56 MHz or 27.12 MHz
Fundamental
Parallel
±20 ppm
< 5 ppm/year
–40°C to 85°C
TRF7970A
TRF7970A
Table 6-9.
Detailed Description
(1)
29