CC2400-RTR1 datasheet(42/84 Pages) TI1

42 / 84 page

CC2400

SWRS042A

Page 42 of 83

AFC settling time

MDMTST0.AFC_SETTLING[1:0]

# Bits

Table 17. Minimum number of bits for the averaging filter

32 Automatic Frequency Control

CC2400 has a built-in optional feature

called

AFC

(Automatic

Frequency

Control). This feature can be used to

measure and compensate for frequency

drift.

The average frequency offset of the

received signal (from the nominal IF) can

read

from

the

FREQEST.RX_FREQ_OFFSET[7:0]

8-bit

value

that

can

used

compensate for frequency offset between

an external transmitter and the receiving

device. The frequency offset is given by:

∆F= RX_FREQ_OFFSET x 5.2 [kHz]

The receiver can be calibrated against an

external transmitter (another

CC2400 or an

external test signal) by changing the

operating frequency according to the

measured offset. The new frequency must

be calculated by the microcontroller and

written

the

MDMCTRL.MOD_OFFSET[5:0]

After

this

compensation

the

center

frequency of the received signal will better

match the digital channel filter bandwidth.

The compensation, as described above,

also

automatically

compensates

the

transmitter, i.e. the transmitted signal will

match the ‘external’ transmitter’s signal.

However, compensating the transmitter

signal may cause additional spurs in the

spectrum.

Chipcon

therefore

recommends only compensating in RX

mode.

This feature reduces the requirement on

the crystal accuracy, which is important

when

using

the

narrower

channel

bandwidths. For a further description of

this feature please refer to page 53

(Crystal drift compensation).

Figure 21 shows how the value of the

FREQEST.RX_FREQ_OFFSET[7:0]

offset

for

different

values

MDMTST0.AFC_SETTLING[1:0]

Chipcon recommends using a value of 4.

The

following

procedure

should

followed

when

using

the

AFC

compensate

for

frequency

offset

between transmitter and receiver:

1. Read

the

FREQEST.RX_FREQ_OFFSET[7:

complement value.

2. Use the equation on this page to

calculate the frequency offset in

kHz.

3. The microcontroller then needs to

calculate the equivalent value to

write

the

MDMCTRL.MOD_OFFSET[5:0]

For example:

The value read from the

FREQEST.RX_FREQ_OFFSET[7:0]

This corresponds to –32 x 5.2 = -166.4

kHz.

The MOD_OFFSET register should

therefore be set to –166.4 kHz / 15.625

kHz = -10.6496 ≈ -11. –11 equals 0x35 in

hexadecimal.

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