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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
status register as described in
FREQEST
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
spectrum-shaping feature improves adjacent
channel
power
(ACP)
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
Antenna
T/R switch
Figure 29. Block Diagram of CC2500 Usage with External Power Amplifier
2-FSK
(GFSK).
This
and
occupied
Filter
PA
SWRS040C
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
CC2500EM 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 applications, 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 should 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.
Balun
T/R switch
CC2500
CC2500
Page 56 of 89
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