14.8.5 PHY_TX_PWR

The register bit PA_BOOST increases transmit gain by 5dB.

This register bit enables the PA boost mode where the TX output power is increased by approximately 5dB when PA_BOOST = 1. In PA boost mode, the PA linearity is decreased compared to the normal mode when PA_BOOST = 0. This leads to higher spectral side lobes of the TX power spectrum and higher power of the harmonics. Consequently, the higher TX power settings do not fulfill the regulatory requirements of the European 868.3MHz band regarding spurious emissions in adjacent frequency bands (see ETSI EN 300 220-1, ERC/REC 70-03, and ERC/DEC/(01)04).

The register bits GC_PA control the PA gain.

These register bits control the gain of the PA by changing its bias current. GC_PA needs to be set in TRX_OFF mode only. It can be used to reduce the supply current in TX mode when a reduced TX power is selected with the TX_PWR control word. A reduced PA bias current causes lower RF gain and lowers the 1dB compression point of the PA. Hence, it is advisable to use a reduced bias current of the PA only in combination with lower values of TX_PWR. A reasonable combination of register bits TX_PWR and GC_PA is shown in Table 14-54.

The register bits TX_PWR determine the TX output power of the radio transceiver.

These register bits control the transmitter output power measured at pins RFP/RFN. The value of TX_PWR describes the power reduction relative to the maximum output power. The resolution is 1dB per step. Since TX_PWR adjusts the gain in the TX path prior to the PA, the PA bias setting is not optimal for increased values of TX_PWR regarding PA efficiency.

The PA power efficiency can be improved when PA bias is reduced (decreased GC_PA value) along with the TX power setting (increased TX_PWR value). A recommended combination of TX power control (TX_PWR), PA bias control (GC_PA), and PA boost mode (PA_BOOST) is listed in Table 9-15. It is a recommended mapping of intended TX power to the 8-bit word in register 0x05 (PHY_TX_PWR).

1. Spectral side lobes remain < -54dBm / 100kHz measured with an RMS detector outside F_C ± 3MHz. Power settings may be used at channel 1 and 2 according to IEEE802.15.4c.
2. Spectral side lobes remain < -54dBm / 100kHz measured with an RMS detector outside F_C ± 1MHz. Power settings may be used at channel 0 and 3 according to IEEE802.15.4c.

Values of the above table are based on a mode dependent setting of register bits GC_TX_OFFS (register 0x16, RF_CTRL_0).