CC1020-RTR1 datasheet(42/91 Pages) TI

42 / 91 page

CC1020

SWRS046

Page 42 of 92

offset. The new frequency must be

calculated and written to the FREQ

can be used for an FSK/GFSK signal, but

not for OOK. Application Note AN029

CC1020/1021 AFC provides the procedure

and equations necessary to implement

AFC.

The AFC feature reduces the crystal

accuracy requirement.

12.14. Digital FM

possible

read

back

the

instantaneous IF from the FM demodulator

as a frequency offset from the nominal IF

frequency. This digital value can be used

perform

pseudo

analog

demodulation.

The frequency offset can be read from the

GAUSS_FILTER register and is a signed

8-bit value coded as 2-complement.

The instantaneous deviation is given by:

F = GAUSS_FILTER·Baud rate / 8

The digital value should be read from the

order to get an analog audio signal. The

internal register value is updated at the

MODEM_CLK

rate.

MODEM_CLK

available

the

LOCK

when

LOCK_SELECT[3:0] = 1101 in the LOCK

the reading.

For audio (300 – 4000 Hz) the sampling

rate should be higher than or equal to 8

kHz (Nyquist) and is determined by the

MODEM_CLK. The MODEM_CLK, which

is the sampling rate, equals 8 times the

baud rate. That is, the minimum baud rate,

which can be programmed, is 1 kBaud.

However, the incoming data will be filtered

in the digital domain and the 3-dB cut-off

frequency is 0.6 times the programmed

Baud rate. Thus, for audio the minimum

programmed

Baud

rate

should

approximately 7.2 kBaud.

The

GAUSS_FILTER

resolution

decreases with increasing baud rate. A

accumulate and dump filter can be

implemented in the uC to improve the

resolution.

Note

that

each

GAUSS_FILTER

reading

should

synchronized to the MODEM_CLK. As an

example, accumulating 4 readings and

dividing the total by 4 will improve the

resolution by 2 bits.

Furthermore,

fully

utilize

the

GAUSS_FILTER

dynamic

range

the

frequency deviation must be 16 times the

programmed baud rate.

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