ADF7021 datasheet(24/44 Pages) AD

24 / 44 page

ADF7021

Preliminary Technical Data

Rev. PrI | Page 24 of 44

As with the external Rx/Tx switch, an additional LPF or BPF

could be required to suppress harmonics in the transmit

spectrum or to improve the resilience of the receiver against

out-of-band interferers.

TRANSMIT PROTOCOL AND CODING

CONSIDERATIONS

PREAMBLE

SYNC

WORD

FIELD

DATA FIELD

CRC

Figure 25. Typical Format of a Transmit Protocol

A dc-free preamble pattern is recommended for FSK demodula-

tion. The recommended preamble pattern is a dc-free pattern

such as a 10101010 … pattern. Preamble patterns with longer

run-length constraints such as 11001100… can also be used.

However, this results in a longer synchronization time of the

received bit stream in the receiver.

Manchester coding can be used for the entire transmit protocol.

However, the remaining fields that follow the preamble header

do not have to use dc-free coding. For these fields, the ADF7021

can accommodate coding schemes with a run length of up to

6 bits without any performance degradation.

If longer run-length coding must be supported, the ADF7021

has several other features that can be activated. These involve a

range of programmable options that allow the envelope detector

output to be frozen after preamble acquisition.

DEVICE PROGRAMMING AFTER INITIAL POWER-UP

Table 13 lists the minimum number of writes needed to set up

the ADF7021 in either Tx or Rx mode after CE is brought high.

Additional registers can also be written to tailor the part to a

particular application, such as setting up sync byte detection or

enabling AFC. When going from Tx to Rx or vice versa, the

user needs to write only to Register 1 to alter the LO by 100 kHz

and to toggle the Tx/Rx bit.

Table 13. Minimum Register Writes Required for Tx/Rx Setup

Mode

Registers

Reg 0

Reg 1

Reg 2

Reg 3

Reg 0

Reg 1

Reg 3

Reg 4

Reg 5

Tx <-> Rx

Reg 0

Figure 26 and Figure 27 show the recommended programming

sequence and associated timing for power-up from standby mode.

2.0mA

3.65mA

12mA

TIME

REG.

READY

WR1

VCO

WR3

WR6

WR5

19mA TO

22mA

tOFF

tON

XTAL

WR0

WR4

t10

AGC/

RSSI

t11

CDR

t12

AFC

t13

RxDATA

t14

Figure 26. Rx Programming Sequence and Timing Diagram

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