Texas Instruments CC2500, Manual

Page: 55 / 96

CC2500
SWRS040C Page 55 of 89
31.2 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. The blanking interval between each
frequency hop is then approximately 810 us.
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. The blanking interval
between each frequency hop is then
approximately 90 us. The VCO current
calibration result is available in
FSCAL2
and is
not dependent on the RF frequency. Neither is
the charge pump current calibration result
available in
FSCAL3
. The same value can
therefore be used for all frequencies.
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. The blanking interval
between
each frequency hop is then
approximately 240 us
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).
31.3 Wideband Modulation not Using
Spread Spectrum
Digital modulation systems under FCC part
15.247 includes 2-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 +1 dBm.
31.4 Data Burst Transmissions
The high maximum data rate of
CC2500
opens
up for burst transmissions. A low average data
rate link (e.g. 10 kBaud), 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 kBaud) 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.
31.5 Continuous Transmissions
In data streaming applications the
CC2500
opens up for continuous transmissions at 500
kBaud effective data rate. As the modulation is
done with 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.)