CC2400 Datasheet, PDF (47/84 Pages) Texas Instruments – 2.4 GHz Low-Power RF Transceiver

CC2400
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
in
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
In transmit mode an 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 be set during the chip
initialization sequence (ref. Figure 16).
When MDMTST0.TX_1MHZ_OFFSET_N=1
the transmit channel center frequency
(carrier frequency), f_c, in MHz is given
directly by:
f_c = FREQ[11:0] = 2048 + FREQ[9:0]
The two FSK modulation frequencies are
given by:
f₀ = f_c − f_dev
f₁ = f_c + f_dev
where f_dev is the FSK frequency deviation.
f_dev
is
programmed
with
MDMCTRL.MOD_DEV[6:0] and given by
(in kHz):
f_dev = ±3.9062 ⋅ MOD _ DEV [6 : 0]
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:
f_LO = f_RF − f_IF
where, f_RF is the center frequency of the
channel and f_IF = 1 MHz.
Thus, in receive mode the frequency
generated by the frequency synthesizer,
f_c, 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.
SWRS042A
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