CC2500_06 Datasheet, PDF (49/84 Pages) Texas Instruments – Single Chip Low Cost Low Power RF Transceiver

CC2500
30.2 Frequency Hopping and Multi-
Channel Systems
The 2.400 – 2.4835 GHz band is shared by
many systems both in industrial, office and
home environments. It is therefore
recommended to use frequency hopping
spread spectrum (FHSS) or a multi-channel
protocol because the frequency diversity
makes the system more robust with respect to
interference from other systems operating in
the same frequency band. FHSS also combats
multipath fading.
CC2500 is highly suited for FHSS or multi-
channel systems due to its agile frequency
synthesizer and effective communication
interface. Using the packet handling support
and data buffering is also beneficial in such
systems as these features will significantly
offload the host controller.
Charge pump current, VCO current and VCO
capacitance array calibration data is required
for each frequency when implementing
frequency hopping for CC2500. There are 3
ways of obtaining the calibration data from the
chip:
1) Frequency hopping with calibration for each
hop. The PLL calibration time is approximately
720 µs.
2) Fast frequency hopping without calibration
for each hop can be done by calibrating each
frequency at startup and saving the resulting
FSCAL3, FSCAL2 and FSCAL1 register values
in MCU memory. Between each frequency
hop, the calibration process can then be
replaced by writing the FSCAL3, FSCAL2 and
FSCAL1 register values corresponding to the
next RF frequency. The PLL turn on time is
approximately 90 µs.
3) Run calibration on a single frequency at
startup. Next write 0 to FSCAL3[5:4] to
disable the charge pump calibration. After
writing to FSCAL3[5:4] strobe SRX (or STX)
with MCSM0.FS_AUTOCAL = 1 for each new
frequency hop. That is, VCO current and VCO
capacitance calibration is done but not charge
pump current calibration. When charge pump
current calibration is disabled the calibration
time is reduced from approximately 720 µs to
approximately 150 µs.
There is a trade off between blanking time and
memory space needed for storing calibration
data in non-volatile memory. Solution 2) above
gives the shortest blanking interval, but
requires more memory space to store
calibration values. Solution 3) gives
approximately 570 µs smaller blanking interval
than solution 1).
30.3 Wideband Modulation not Using
Spread Spectrum
Digital modulation systems under FCC part
15.247 includes FSK and GFSK modulation.
A maximum peak output power of 1 W (+30
dBm) is allowed if the 6 dB bandwidth of the
modulated signal exceeds 500 kHz. In
addition, the peak power spectral density
conducted to the antenna shall not be greater
than +8 dBm in any 3 kHz band.
Operating at high data rates and high
frequency separation, the CC2500 is suited for
systems targeting compliance with digital
modulation systems as defined by FCC part
15.247. An external power amplifier is needed
to increase the output above 0 dBm.
30.4 Data Burst Transmissions
The high maximum data rate of CC2500 opens
up for burst transmissions. A low average data
rate link (e.g. 10 kbps), can be realized using a
higher over-the-air data rate. Buffering the
data and transmitting in bursts at high data
rate (e.g. 500 kbps) will reduce the time in
active mode, and hence also reduce the
average current consumption significantly.
Reducing the time in active mode will reduce
the likelihood of collisions with other systems,
e.g. WLAN.
30.5 Continuous Transmissions
In data streaming applications the CC2500
opens up for continuous transmissions at 500
kbps effective data rate. As the modulation is
done with an I/Q up-converter with LO I/Q-
signals coming from a closed loop PLL, there
is no limitation in the length of a transmission.
(Open loop modulation used in some
transceivers often prevents this kind of
continuous data streaming and reduces the
effective data rate.)
30.6 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
PRELIMINARY Data Sheet (Rev.1.2) SWRS040A
Page 49 of 83