ADF7241 datasheet(30/72 Pages) AD

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ADF7241

Rev. 0 | Page 30 of 72

PHY_RDY

VALID IEEE802.15.4-2006 FRAME

FRAME IN TX_BUFFER

tx_mac_delay +

mac_delay_ext

RC_STATUS

PACKET

RECEIVED

PACKET

TRANSMITTED

t27,t28

t26

Figure 37. IEEE 802.15.4 Auto RX-to-TX Turnaround Mode

IEEE 802.15.4 AUTOMATIC RX-TO-TX

TURNAROUND MODE

The ADF7241 features an automatic RX-to-TX turnaround mode

when it is operating in IEEE 802.15.4-2006 packet mode

(Register rc_cfg, Field rc_mode = 0). The automatic RX-to-TX

turnaround mode facilitates the timely transmission of

acknowledgment frames.

Figure 37 illustrates the timing of the automatic RX-to-TX

turnaround mode. When enabled by setting Register buffercfg,

Field auto_rx_to_tx_turnaround, the ADF7241 automatically

enters the TX state following the reception of a valid IEEE

802.15.4-2006 frame. After the combined transmit MAC delay

(tx_mac_delay + mac_delay_ext), the ADF7241 enters the TX

state and transmits the frame stored in TX_BUFFER. After the

transmission is complete, the ADF7241 enters the PHY_RDY

state. There is a 38 μs delay between the reception of the last

symbol and the generation of the rx_pkt_rcvd interrupt. The

transmit MAC delay timeout period begins immediately after

the reception of the last symbol. Therefore, the host MCU has

up to t28 μs (see Table 12) after a frame has been received to cancel

the transmit operation by means of issuing an RC_IDLE,

RC_PHY_RDY, or RC_RX command.

POWER AMPLIFIER

The integrated power amplifier (PA) is connected to the RFIO2P

and RFIO2N RF ports. It is equipped with a built-in harmonic

filter to simplify the design of the external harmonic filter. The

output power of the PA is set with Register extpa_msc, Field

pa_pwr with an average step size of 2 dB. The step size increases

at the lower end of the control range. Refer to Figure 26 for the

typical variation of output power step size with the control word

value. The PA also features a high power mode, which can be

enabled by setting Register pa_bias, Field pa_bias_ctrl = 63 and

PA Ramping Controller

The PA ramping controller of the ADF7241 minimizes spectral

splatter generated by the transmitter. Upon entering the TX state,

the ramping controller automatically ramps the output power of

the PA from the minimum output power to the specified nominal

value. In packet mode, transmission of the packet commences

after the ramping phase. When the transmission of the packet is

complete or the TX state is exited, the PA is turned off immedi-

ately. It is also possible to allow the PA to ramp down its output

power using the same ramp rate for the ramp-up phase, by

setting Register ext_ctrl, Field pa_shutdown_mode to 1.

Figure 38 illustrates the shape of the PA ramping profile and its

timing. It follows a linear-in-dB shape. The ramp time depends on

the output power setting in Register extpa_msc, Field pa_pwr

and is specified with Register pa_rr, Field pa_ramp_rate

according to the following equation:

t_ramp = 2pa_rr.pa_ramp_rate × 2.4 ns × extpa_msc.pa_pwr

External PA Interface

The ADF7241 has an integrated biasing block for external PA

circuits as shown in Figure 39. It is suitable for external PA circuits

based on a single GaAs MOSFET and a wide range of integrated

PA modules. The key components are shown in Figure 40. A

switch between Pin VDD_BAT and Pin PAVSUP_ATB3 controls

the supply current to the external FET. PABIOP_ATB4 can be

used to set a bias point for the external FET. The bias point is

controlled by a 5-bit DAC and/or a bias servo loop.

To have the external PA interface under direct control of the host

MCU, set Register ext_ctrl, Field extpa_auto_en = 0. The host

MCU can then use Register pd_aux, Field extpa_bias_en to enable

or disable the external PA. If Register ext_ctrl, Field extpa_auto_en

= 1, the external PA automatically turns on when entering, and

turns off when exiting the TX state. If this setting is used, the host

MCU should not alter the configuration of Register pd_aux, Field

extpa_bias_en.

ADF7241 Datasheet(PDF) 30 Page - Analog Devices

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