CC2480 datasheet(31/48 Pages) TI

31 / 48 page

CC2480

I-phase

Q-phase

Figure 11: I / Q Phases when transmitting a zero-symbol chip sequence, TC = 0.5 µs

10.2

Demodulator, Symbol Synchronizer and Data Decision

The block diagram for the

CC2480 demodulator

is shown in Figure 12. Channel filtering and

frequency offset compensation is performed

digitally. The signal level in the channel is

estimated to generate the RSSI level. Data

filtering

also

included

for

enhanced

performance.

With the ±40 ppm frequency accuracy

requirement from [1], a compliant receiver

must be able to compensate for up to 80 ppm

or 200 kHz. The

CC2480 demodulator tolerates

up to 300 kHz offset without significant

degradation of the receiver performance.

Soft decision is used at the chip level, i.e. the

demodulator does not make a decision for

each chip, only for each received symbol. De-

spreading is performed using over-sampling

symbol correlators. Symbol synchronization is

achieved by a continuous start of frame

delimiter (SFD) search.

The

CC2480 demodulator also handles symbol

rate errors in excess of 120 ppm without

performance degradation. Resynchronization

is performed continuously to adjust for error in

the incoming symbol rate.

Digital

IF Channel

Filtering

ADC

Digital

Data

Filtering

Frequency

Offset

Compensation

Symbol

Correlators and

Synchronisation

RSSI

Generator

I / Q Analog

IF signal

Data

Symbol

Output

RSSI

Average

Correlation

Value (may be

used for LQI)

Figure 12: Demodulator Simplified Block Diagram

10.3

Frame Format

CC2480 has hardware support for parts of the

IEEE 802.15.4 frame format. This section

gives a brief summary to the IEEE 802.15.4

frame format, and describes how

CC2480 is

set up to comply with this.

Figure 13 [1] shows a schematic view of the

IEEE 802.15.4 frame format. Similar figures

describing

specific

frame

formats

(data

frames, beacon frames, acknowledgment

frames and MAC command frames) are

included in [1].

CC2480 Data Sheet SWRS074

Page 31 of 43

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