Texas Instruments 3138 155 232931 - Manual (Page 58)

CC2420
SWRS041B Page 58 of 89
34.6 Low-cost systems
As the CC2420 provides 250 kbps multi-
channel performance without any external
filters, a very low-cost system can be
made.
A differential antenna will eliminate the
need for a balun, and the DC biasing can
be achieved in the antenna topology.
34.7 Battery operated systems
In low power applications, the CC2420
should be powered down when not being
active. Extremely low power consumption
may be achieved when disabling also the
voltage regulator. This will require
reprogramming of the register and RAM
configuration.
34.8 BER / PER measurements
CC2420 includes test modes where data is
received infinitely and output to pins
(RX_MODE 2, see page 40). This mode
may be used for Bit Error Rate (BER)
measurements. However, the following
actions must be taken to do such a
measurement:
• A preamble and SFD sequence
must be used, even if pseudo
random data is transmitted, since
receiving the DSSS modulated
signal requires symbol
synchronisation, not bit
synchronisation like e.g. in 2FSK
systems. The SYNCWORD may be
set to another value to fit to the
measurement setup if necessary.
• The data transmitted over air must
be spread according to [1] and the
description on page 24. This
means that the transmitter used
during measurements must be
able to do spreading of the bit
data to chip data. Remember that
the chip sequence transmitted by
the test setup is not the same as
the bit sequence, which is output
by
CC2420 .
• When operating at or below the
sensitivity limit, CC2420 may loose
symbol synchronisation in infinite
receive mode. A new SFD and
restart of the receiver may be
required to re-gain
synchronisation.
In an IEEE 802.15.4 system, all
communication is based on packets. The
sensitivity limit specified by [1] is based on
Packet Error Rate (PER) measurements
instead of BER. This is a more accurate
measurement of the true RF performance
since it mirrors the way the actual system
operates.
It is recommended to perform PER
measurements instead of BER
measurements to evaluate the
performance of IEEE 802.15.4 systems.
To do PER measurements, the following
may be used as a guideline:
• A valid preamble, SFD and length
field must be used for each
packet.
• The PSDU (see Figure 17 on
page 36) length should be 20
bytes for sensitivity measurements
as specified by [1].
• The sensitivity limit specified by [1]
is the RF level resulting in a 1%
PER. The packet sample space
for a given measurement must
then be >> 100 to have a
sufficiently large sample space.
E.g. at least 1000 packets should
be used to measure the
sensitivity.
• The data transmitted over air must
be spread according to [1] and the
description on page 24. Pre-
generated packets may be used,
although [1] requires that the PER
is averaged over random PSDU
data.
• The CC2420 receive FIFO may be
used to buffer data received
during PER measurements, since
it is able to buffer up to 128 bytes.
• The
MDMCTRL1.CORR_THR
control register is by default set to
20, as described in the
Demodulator, Symbol
Synchroniser and Data Decision
section.