MRF49XA-ISLASHT datasheet(72/102 Pages) MICROCHIP

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MRF49XA

DS70590C-page 72

Preliminary

3.19

RX-TX Frequency Alignment

Method

The RX-TX frequency offset occurs due to the

differences in the actual reference frequency. To

minimize this error, the same crystal type and the same

PCB layout should be used for the crystal placement on

the RX and TX PCBs. Also, see Section 3.6, Crystal

Selection Guidelines.

To verify the possible RX-TX offset, it is recommended

to measure the CLK output of both transceivers with a

high level of accuracy. Do not measure the output at the

RFXTL pin as the measurement process itself might

change the reference frequency. As the carrier

frequencies are derived from the reference frequency,

having identical reference frequencies, and nominal

frequency settings at the TX and RX side, there should

be no offset if the CLK signals have identical

frequencies.

The crystal oscillator load capacitor bank value is to

fine-tune the oscillator and minimize the offset. So the

process is to measure the clock output and parallel

change the value to minimize the offset.

The actual RX-TX offset can be monitored by using the

AFC status data included in the STSREG of the

receiver. By reading out the STSREG, the actual

measured offset frequency can be reported. In order to

get accurate values, the AFC has to be disabled during

the read by clearing the FOFEN bit in AFCCREG.

The registers associated with RX-TX alignment

procedures are:

•STSREG (see Register 2-1)

• AFCCREG (see Register 2-3)

• RXCREG (see Register 2-7)

• PMCREG (see Register 2-13)

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