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MRF24J40
DS39776A-page 52
Advance Information
2006 Microchip Technology Inc.
A.2
PCB Layout Design
The following guidelines are intended to aid users who
are not experienced in high-frequency PCB layout
design.
The printed circuit board is comprised of four basic FR4
layers: signal layout, RF ground, power line routing and
ground (see Figure A-9). The guidelines will explain the
requirements of these layers.
FIGURE A-9:
FOUR BASIC COPPER FR4 LAYERS
It is important to keep the original PCB thickness
since any change will affect antenna performance
(see total thickness of dielectric) or microstrip
lines characteristic impedance.
The first layer width of a 50
Ω
characteristic
impedance microstrip line is 12 mils.
Avoid having microstrip lines longer than 2.5 cm,
since that line might get very close to a quarter
wave length of the working frequency of the board
which is 3.0 cm, and start behaving as an
antenna.
Except for the antenna layout, avoid sharp
corners since they can act as an antenna. Round
corners will eliminate possible future EMI
problems.
Digital lines by definition are prone to be very
noisy when handling periodic waveforms and fast
clock/switching rates. Avoid laying out a RF signal
close to any digital lines.
A via filled ground patch underneath the IC
transceiver is mandatory.
A power supply must be distributed to each pin in
a star topology and low-ESR capacitors must be
placed at each pin for proper decoupling noise.
Decoupling each power pin is a tedious task,
especially when the noise is affecting the perfor-
mance of the transceiver in a specific bandwidth.
Usually, low value caps (15-27 pF) combined with
large value caps (100 nF) will cover a large
spectrum of frequency.
Passive components (inductors) must be in the
high-frequency category and the SRF (Self-
Resonant Frequency) should be at least two
times higher than the operating frequency.
Figure A-10 and Figure A-11 illustrate the ground and
power plane for the RF board.
Dielectric
ε
= 4.5, Thickness = 7 mils
Signal Layout, Thickness = 1.8 mils
RF Ground, Thickness = 1.2 mils
Dielectric
ε
= 4.5, Thickness = 19 mils
Power Line Routing, Thickness = 1.2 mils
Dielectric
ε
= 4.5, Thickness = 7 mils
Ground, Thickness = 1.8 mils
Note:
Care should be taken with all ground lines to prevent breakage.