Texas Instruments 3138 155 232931 manual PCB Layout Recommendations, Antenna Considerations

• 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

CC2420

signal has the same phase shifts as the O- QPSK sequence previously defined.

For a desired symbol sequence s0, s1, … , sn-1of length n symbols, the desired chip sequence c0, c1, c2, …, c32n-1of 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:

(c0 xnor c1), (c1 xor c2), (c2 xnor c3), … , (c32n-1xor c32n) where c32n 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

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