CC2400-RTR1 datasheet(47/83 Pages) TI

47 / 83 page

CC2400

SWRS042A

Page 47 of 83

38 Frequency Programming

The operating frequency is set by

programming the frequency word in the

FSDIV configuration register.

The frequency word is 12 bits and is

located

FSDIV.FREQ[11:0].

Writing/reading FSDIV[11:0] will give

the frequency directly in MHz. (The bits

FSDIV.FREQ[11:10] are hardwired to

‘10’ giving a fixed offset of 2048.)

FSDIV should only be modified while the

CC2400 is in IDLE mode. Otherwise the

PLL may go out of lock as a calibration is

only performed when exiting IDLE mode.

38.1 Transmit mode

transmit

mode

I/Q

direct

upconversion scheme is used (i.e. no

intermediate frequency for the modulated

baseband

signal).

MDMTST0.TX_1MHZ_OFFSET_N=1

must

therefore

set

during

the

chip

initialization sequence (ref. Figure 16).

When MDMTST0.TX_1MHZ_OFFSET_N=1

the transmit channel center frequency

(carrier frequency), fc, in MHz is given

directly by:

[]

[ ]

FREQ

2048

The two FSK modulation frequencies are

given by:

f0 = fc − fdev

f1 = fc + fdev

where fdev is the FSK frequency deviation.

fdev

programmed

with

MDMCTRL.MOD_DEV[6:0] and given by

(in kHz):

[]

9062

DEV

MOD

dev

⋅

The default value is MOD_DEV = 64 giving

250 kHz deviation.

The TX_GAUSSIAN_FILTER bit in the

GRMDM register controls the Gaussian

shaping of the modulation signal. See also

page 38.

38.2 Receive mode

Low side LO injection is used, hence:

fLO = fRF − fIF

where, fRF is the center frequency of the

channel and fIF = 1 MHz.

Thus, in receive mode the frequency

generated by the frequency synthesizer,

fc, must be programmed to be the LO

frequency.

39 Alternate TX IF setting

It is possible to configure

CC2400 to

operate with an intermediate frequency of

1 MHz in transmit mode. It is not generally

recommended to do this, as the TX

spectrum will have higher spur content

than when using the direct up conversion

mode. Using an intermediate frequency of

1 MHz in TX has the advantage of much

lower RX/TX switching time because the

VCO operates at the same frequency in

RX and TX.

1 MHz IF in TX mode is enabled by setting

MDMTST0.TX_1MHZ_OFFSET_N=0.

CC2400-RTR1 数据表(PDF) 47 Page - Texas Instruments

CC2400-RTR1 数据表(HTML) 47 Page - Texas Instruments

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