MICRF620Z datasheet(20/21 Pages) MICREL

20 / 21 page

Micrel, Inc.

MICRF620/MICRF620Z

July 2006

M9999-120205

Layout Considerations

Except for the antenna input/output signal, only digital and

low frequency signals need to interface with the module.

There is therefore no need of years of RF expertise to do a

successful layout, as long as the following few points are

being followed:

•

Proper ground is needed. If the PCB is 2-layer, the

bottom layer should be kept only for ground. Avoid

signal traces that split the ground plane. For a 4-

layer PCB, it is recommended to keep the second

layer only for ground.

•

A ground via should be placed close to all the

ground pins. The bottom ground pad should be

penetrated with 4-16 ground vias.

•

The antenna has an impedance of ~50 ohm. The

antenna trace should be kept to 50 ohm to avoid

signal reflection and loss of performance. Any

transmission line calculator can be used to find the

needed trace width given a board build up. Ex: A

trace width of 44 mil (1.12 mm) gives 50

impedance on a FR4 board (dielectric cons=4.4)

with copper thickness of 35

µm and height (layer 1-

layer 2 spacing) of 0.61 mm.

•

RF circuitry is sensitive to voltage supply and

therefore caution should be taken when choosing

power circuitry. To achieve the best performance,

low noise LDO’s with high PSSR should be

chosen. What is present on the voltage supply will

be directly modulated to the RF spectrum causing

degradation and regulatory issues. To make sure

you have the right selection, please contact local

sales for the latest Micrel offerings in power

management and guidance. To avoid “pickup”

from other circuitry on the VDD lines, it is

recommended to route the VDD in a star

configuration with decoupling at each circuitry and

at the common connection point (see above

layout). If there are noisy circuitry in the design, it

is strongly recommended to use a separate power

supply and/or place low value resistors (10ohms),

inductors in series with the power supply line into

these circuitry.

•

Digital high speed logic or noisy circuitry

should/must be at a safe distance from RF

circuitry or RF VDD as this might/will cause

degradation of sensitivity and create spurious

emissions. Example of such circuitry is LCD

display, charge pumps, RS232, clock / data bus

etc.

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