Texas Instruments CC2500 User Manual

CC2500
S
WRS040
C
Page
56
of
89
31.6
Crystal Drift Compensation
The
CC2500
has a very fine frequency
resolution (see
Table
9
)
.
This feature can be
used to compensate for frequency offset and
drift.
The frequency offset between an ‘external’
transmitter and the receiver is measured in the
CC2500
and can be read back from the
FREQEST
status register as described in
Section
14.1
. The measured frequency offset
can be used to calibrate the frequency using
the ‘external’ transmitter as the reference. That
is, the received signal of the device will match
the receiver’s channel filter better. In the same
way the
centre frequency of the transmitted
signal will match the ‘external’ transmitter’s
signal.
31.7
Spectrum Efficient Modulation
CC2500
also has the possibility to use
Gaussian shaped
2
-
FSK (GFSK). This
spectrum
-
shaping feature improves adjacent
channel power (AC
P) and occupied
bandwidth. In ‘true’
2
-
FSK systems with abrupt
frequency shifting, the spectrum is inherently
broad. By making the frequency shift ‘softer’,
the spectrum can be made significantly
narrower. Thus, higher data rates can be
transmitted in the
same bandwidth using
GFSK.
31.8
Low Cost Systems
A differential antenna will eliminate the need
for a balun, and the DC biasing can be
achieved in the antenna topology, see
Figure
3
.
The CC25XX Folded Dipole reference
design
[8]
contains schematics and layout files
for a CC2500EM with a folded dipole PCB
antenna. Please see DN004
[9]
for more
details on this design.
A HC
-
49 type SMD crystal is used in the
CC2500E
M reference design
[4]
. Note that the
crystal package strongly influences the price.
In a size constrained PCB design a smaller,
but more expensive, crystal may be used.
31.9
Battery Operated Systems
In low power app
lications, the SLEEP state
with the crystal oscillator core switched off
should be used when the
CC2500
is not active.
It is possible to leave the crystal oscillator core
running in the SLEEP state if start
-
up time is
critical.
The WOR functionality shoul
d be used in low
power applications.
31.10
Increasing Output Power
In some applications it may be necessary to
extend the link range. Adding an external
power amplifier is the most effective way of
doing this.
The power amplifier should be inserted
between the
antenna and the balun, and two
T/R switches are needed to disconnect the PA
in RX mode. See
Figure
29
.
Figure
29
. Block D
iagram of
CC2500
Usage with Externa
l Power A
mplifier
CC2500
Balun
Filter
Antenna
T/R switch
T/R switch
PA

Texas Instruments CC2500 Specifications

General

Brand	Texas Instruments
Model	CC2500
Category	Transceiver
Language	English