ADF7021 datasheet(37/64 Pages) AD

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Data Sheet

ADF7021

Rev. B | Page 37 of 64

Maximum AFC Range

The maximum frequency correction range of the AFC loop is

programmable on the ADF7021. This is set by R10_DB[24:31].

The maximum AFC correction range is the difference in

frequency between the upper and lower limits of the AFC

tuning range. For example, if the maximum AFC correction

range is set to 10 kHz, the AFC can adjust the receiver LO

within the

fLO ± 5 kHz range.

However, when RF_DIVIDE_BY_2 (R1_DB18) is enabled, the

programmed range is halved. The user should account for this

halving by doubling the programmed maximum AFC range.

The recommended maximum AFC correction range should be

≤ 1.5 × IF filter bandwidth. If the maximum frequency correction

range is set to be > 1.5 × IF bandwidth, the attenuation of the IF

filter can degrade the AFC loop sensitivity.

The adjacent channel rejection (ACR) performance of the

receivers can be degraded when AFC is enabled and the AFC

correction range is close to the IF filter bandwidth. However,

because the AFC correction range is programmable, the user

can trade off correction range and ACR performance.

When AFC errors are removed using either the internal or

external AFC, further improvement in receiver sensitivity can

be obtained by reducing the IF filter bandwidth using the

IF_BW bits (R4_DB[30:31]).

AUTOMATIC SYNC WORD DETECTION (SWD)

The ADF7021 also supports automatic detection of the sync or

ID fields. To activate this mode, the sync (or ID) word must be

preprogrammed into the ADF7021. In receive mode, this

preprogrammed word is compared to the received bit stream.

When a valid match is identified, the external SWD pin is

asserted by the ADF7021 on the next Rx clock pulse.

This feature can be used to alert the microprocessor that a

valid channel has been detected. It relaxes the computational

requirements of the microprocessor and reduces the overall

power consumption.

The SWD signal can also be used to frame the received packet

by staying high for a preprogrammed number of bytes. The data

packet length can be set in R12_DB[8:15].

The SWD pin status can be configured by setting R12_DB[6:7].

R11_DB[4:5] are used to set the length of the sync/ID word, which

can be 12, 16, 20, or 24 bits long. A value of 24 bits is recommended

to minimize false sync word detection in the receiver that can

occur during recovery of the remainder of the packet or when

noise/no signal is present at the receiver input. The transmitter

must transmit the sync byte MSB first and the LSB last to ensure

proper alignment in the receiver sync-byte-detection hardware.

An error tolerance parameter can also be programmed that

accepts a valid match when up to 3 bits of the word are

incorrect. The error tolerance value is assigned in R11_DB[6:7].

ADF7021 Datasheet(PDF) 37 Page - Analog Devices

ADF7021 Datasheet(HTML) 37 Page - Analog Devices

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