CC1021-RTR1 datasheet(30/92 Pages) TI

30 / 92 page

CC1021

SWRS045

Page 30 of 91

12.

Receiver

12.1. IF Frequency

The IF frequency is derived from the

crystal frequency as

[] )1

(

⋅

DIV

ADC

xoscx

where ADC_DIV[2:0] is set in the MODEM

The analog filter succeeding the mixer is

used for wideband and anti-alias filtering

which is important for the blocking

performance at 1 MHz and larger offsets.

This filter is fixed and centered on the

nominal IF frequency of 307.2 kHz. The

bandwidth of the analog filter is about 160

kHz.

Using crystal frequencies which gives an

IF frequency within 300 ñ 320 kHz means

that the analog filter can be used

(assuming low frequency deviations and

low data rates).

Large offsets, however, from the nominal

IF frequency will give an un-symmetric

filtering (variation in group delay and

different

attenuation)

the

signal,

resulting in decreased sensitivity and

selectivity. See Application Note AN022

Crystal Frequency Selection for more

details.

For IF frequencies other than 300 ñ 320

kHz and for high frequency deviation and

high data rates (typically ≥ 76.8 kBaud) the

analog filter must be bypassed by setting

FILTER_BYPASS = 1 in the FILTER

performance at 1 MHz and larger offsets

will be degraded.

The IF frequency is always the ADC clock

frequency divided by 4. The ADC clock

frequency should therefore be as close to

1.2288 MHz as possible.

12.2. Receiver Channel Filter Bandwidth

In order to meet different channel spacing

requirements, the receiver channel filter

bandwidth is programmable. It can be

programmed from 38.4 to 307.2 kHz.

The minimum receiver channel filter

bandwidth

depends

data

rate,

frequency

separation

and

crystal

tolerance.

The signal bandwidth must be smaller

than the available receiver channel filter

bandwidth. The signal bandwidth (SBW)

can be approximated by (Carsonís rule):

SBW = 2 · fm + 2 · frequency deviation

where fm is the modulating signal. In

Manchester

mode

the

maximum

modulating

signal

occurs

when

transmitting a continuous sequence of 0ís

(or 1ís). In NRZ mode the maximum

modulating

signal

occurs

when

transmitting a 0-1-0 sequence. In both

Manchester and NRZ mode 2·fm is then

equal to the programmed baud rate. The

equation for SBW can then be rewritten as

SBW = Baud rate + frequency separation

Furthermore, the frequency offset of the

transmitter and receiver must also be

considered. Assuming equal frequency

error in the transmitter and receiver (same

type of crystal) the total frequency error is:

f_error = ±2 · XTAL_ppm · f_RF

where XTAL_ppm is the total accuracy of

the crystal including initial tolerance,

temperature drift, loading and ageing.

f_RF is the RF operating frequency.

The minimum receiver channel filter

bandwidth (ChBW) can then be estimated

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