Texas Instruments 3138 155 232931 - Manual (Page 59)

CC2420
SWRS041B Page 59 of 89
• The RXCTRL1.RXBPF_LOCUR
control bit should be set to 1.
The simplest way of making a PER
measurement will be to use another
CC2420 as the reference transmitter.
However, this makes it difficult to measure
the exact receiver performance.
Using a signal generator, this may either
be set up as O-QPSK with half-sine
shaping or as MSK. If using O-QPSK, the
phases must be selected according to [1].
If using MSK, the chip sequence must be
modified such that the modulated MSK
signal has the same phase shifts as the O-
QPSK sequence previously defined.
For a desired symbol sequence s
0
, s
1
, … ,
s
n-1
of length n symbols, the desired chip
sequence c
0
, c
1
, c
2
, …, c
32n-1
of length 32n
is found using table lookup from Table 3
on page 24. It can be seen from
comparing the phase shifts of the O-QPSK
signal with the frequency of a MSK signal
that the MSK chip sequence is generated
as:
(c
0
xnor c
1
), (c
1
xor c
2
), (c
2
xnor c
3
), … ,
(c
32n-1
xor c
32n
) where c
32n
may be
arbitrarily selected.
35 PCB Layout Recommendations
Following Texas Instruments’s reference
design is highly recommended.
In our reference design, the top layer is
used for signal routing, and the open
areas are filled with metallisation
connected to ground using several vias.
Layer 2 has not been used in our CC2420
reference designs. Layer 3 is used for
power routing and the bottom layer serves
as ground plane with a little routing.
The area under the chip is used for
grounding and must be well connected to
the ground plane with several vias.
The ground pins should be connected to
ground as close as possible to the
package pin using individual vias. The de-
coupling capacitors should also be placed
as close as possible to the supply pins
and connected to the ground plane by
separate vias. Supply power filtering is
very important.
The external components should be as
small as possible (0402 is recommended)
and surface mount devices must be used.
Caution should be used when placing the
microcontroller in order to avoid
interference with the RF circuitry.
A Development Kit with a fully assembled
Evaluation Module is available. It is
strongly advised that this reference layout
is followed very closely in order to get the
best performance.
The schematic, BOM and layout Gerber
files for the reference designs are all
available from the Texas Instruments
website.
36 Antenna Considerations
CC2420 can be used together with various
types of antennas. A differential antenna
like a dipole would be the easiest to
interface not needing a balun (balanced to
un-balanced transformation network).
The length of the λ/2-dipole antenna is
given by:
L = 14250 / f
where f is in MHz, giving the length in cm.
An antenna for 2450 MHz should be 5.8
cm. Each arm is therefore 2.9 cm.
Other commonly used antennas for short-
range communication are monopole,
helical and loop antennas. The single-
ended monopole and helical would require
a balun network between the differential
output and the antenna.
Monopole antennas are resonant
antennas with a length corresponding to
one quarter of the electrical wavelength