User`s guide

PICDEM Z Demonstration Kit User’s Guide

C.4 PCB LAYOUT

The PICDEM Z MRF24J40 2.4 GHz Daughter Card is fabricated as a four layer PCB.

The material is FR4 with signal traces in 0.5 oz copper. Figure C-4 through Figure C-9

show the individual layers from top to bottom. Figure C-10 shows the layer stack up.

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 C-3). The guidelines will explain

the requirements of these layers.

FIGURE C-3: 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 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 trans-

ceiver noise, particularly when this noise degrades performance. 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.

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.