MRF24J40_08 Datasheet, PDF (134/152 Pages) Microchip Technology – IEEE 802.15.4™ 2.4 GHz RF Transceiver

MRF24J40
4.3 PCB Layout Design
The following guidelines are intended to aid users 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 4-5). The guidelines will explain the
requirements of these layers.
FIGURE 4-5:
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.
• Thorough decoupling on each power pin is
beneficial for reducing in-band transceiver noise,
particularly when this noise degrades perfor-
mance. Usually, low value caps (27-47 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.
DS39776B-page 132
Preliminary
© 2008 Microchip Technology Inc.