MRF24J40 Datasheet, PDF (54/66 Pages) Microchip Technology – IEEE 802.15.4™ 2.4 GHz RF Transceiver

MRF24J40
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
Signal Layout, Thickness = 1.8 mils
Dielectric ε = 4.5, Thickness = 7 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.
• 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 50Ω 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.
DS39776A-page 52
Advance Information
© 2006 Microchip Technology Inc.