6.9 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 4 Ω or 8 Ω (typical). The transmit power levels are selectable between 100 mW (half power) or 200 mW (full power) when configured for 5-V automatic operation. Selection of the transmit power level is set by bit B4 in the Chip Status Control register (0x00). When configured for 3-V automatic operation, the transmit power level is typically in the range of 33 mW (half power) or 70 mW (full power).

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 from 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 TRF7963A 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). In case of 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 must be corrected by using the TX Pulse Length 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. This means the pulse length can be adjusted from 73.7 ns to 18.8 µs in 73.7-ns increments.