ADF7241 datasheet(27/72 Pages) AD

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ADF7241

Rev. 0 | Page 27 of 72

RF FREQUENCY SYNTHESIZER BANDWIDTH

The ADF7241 radio controller optimizes the RF frequency synthe-

sizer bandwidth based on whether the device is in the RX or the

TX state. If the device is in the RX state, the frequency synthe-

sizer bandwidth is set by the radio controller to ensure optimum

blocker rejection. If the device is in the TX state, the radio

controller sets the frequency synthesizer bandwidth based on

the required data rate to ensure optimum modulation quality.

RF CHANNEL FREQUENCY PROGRAMMING

The frequency of the synthesizer is programmed with the

frequency control word, ch_freq[23:0], which extends over

The frequency control word, ch_freq[23:0], contains a binary

representation of the absolute frequency of the desired channel

divided by 10 kHz.

Writing a new channel frequency value to the frequency control

word, ch_freq[23:0], takes effect after the next frequency synthe-

sizer calibration phase. The frequency synthesizer is calibrated

by default during the transition into the PHY_RDY from the

idle state as well as in the TX, RX and CCA states. Refer to the

RF Frequency Synthesizer Calibration, Transmitter, and

Receiver sections for further details. To facilitate fast channel

frequency changes, a new frequency control word can be

written in the RX state before a packet has been received. The

next RC_RX or RC_TX command initiates the required

frequency synthesizer calibration and settling cycle. Similarly, a

new frequency control word can be written after a packet has

been transmitted while in the TX state and the next RC_RX or

RC_TX command initiates the frequency synthesizer

calibration and settling cycle.

REFERENCE CRYSTAL OSCILLATOR

The on-chip crystal oscillator generates the reference frequency

for the frequency synthesizer and system timing. The oscillator

operates at a frequency of 26 MHz. The crystal oscillator is

amplitude controlled to ensure a fast start-up time and stable

operation under different operating conditions. The crystal and

associated external components should be chosen with care

because the accuracy of the crystal oscillator can have a significant

impact on the performance of the communication system. Apart

from the accuracy and drift specification, it is important to con-

sider the nominal loading capacitance of the crystal. Crystals

with a high loading capacitance are less sensitive to frequency

pulling due to tolerances of external capacitors and the printed

circuit board parasitic capacitances. When selecting a crystal, these

advantages should be balanced against the higher current

consumption, longer start-up time, and lower trimming range

resulting from a larger loading capacitance.

The total loading capacitance must be equal to the specified

load capacitance of the crystal and comprises the external

parallel loading capacitors, the parasitic capacitances of the

XOSC26P and XOSC26N pins, as well as the parasitic capaci-

tance of tracks on the printed circuit board.

The ADF7241 has an integrated crystal oscillator tuning capacitor

that facilitates the compensation of systematic production

tolerance and temperature drift. The tuning capacitor is con-

trolled with Register xto26_trim _cal, Field xto26_trim (0x371).

The tuning range provided by the tuning capacitor depends on

the loading capacitance of a specific crystal. The total tuning

range is typically 25 ppm.

ADF7241 Datasheet(PDF) 27 Page - Analog Devices

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