Single-Chip Low Power RF Transceiver for Narrowband Systems ...

f ref
f xosc
=
REF _ DIV + 1
FSK frequency deviation is programmed in
the DEVIATION register. The deviation
programming
is divided into a mantissa
( TXDEV_M[3:0])
(TXDEV_X[2:0]).
and an exponent
Generally REF_DIV should be as low as
possible
but the following requirements
must be met
f c
9 . 8304 ≥ f ref > [ MHz]
256
in the frequency band 402 – 470 MHz, and
f c
9 . 8304 ≥ f ref >
512
[ MHz]
in the frequency band 804 - 940 MHz.
The PLL output frequency equations
above
give the carrier frequency, fc , in
transmit
mode (centre frequency). The two
FSK modulation frequencies are given by:
11.1. Dithering
Spurious signals will occur at certain
frequencies depending on the division
ratios in the PLL. To reduce the strength of
these spurs, a common technique is to
use a dithering signal in the control of the
f0 = fc − fdev
f = f + f
1 c dev
CC1021
where fdev is set by the DEVIATION
register:
f
dev
= f
ref
⋅TXDEV
_ M ⋅ 2
in the frequency band 402 – 470 MHz
and
f
dev
= f
ref
⋅TXDEV
_ M ⋅ 2
in the frequency
band 804 - 940 MHz.
OOK (On-Off Keying) is used if
TXDEV_M[3:0] = 0000.
The TX_SHAPING bit in the DEVIATION
register controls Gaussian shaping of the
modulation signal.
In receive mode the frequency must be
programmed to be the LO frequency. Low
side LO injection is used, hence:
fLO = fc − fIF
( TXDEV _ X −16)
( TXDEV _ X −15)
where fIF is the IF frequency (ideally 307.2
kHz).
frequency dividers. Dithering is activated
by setting the DITHER bit in the FREQ_0
registers. It is recommended to use the
dithering in order to achieve the best
possible performance.
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