User manual

ECE 477 Final Report Spring 2004
9.0 PCB Layout Design Considerations
The wireless ordering device (WOrD), which allows customers to view limited menus,
requires two separate PCBs: one for the base unit and one for the remote unit. A serial port
connection with the host computer serves as the interface between the base station and the
computer providing control. This PCB is not size critical and only requires special RF
considerations. The remote device will act as a user interface between the user and the base
station. The PCB for this device must be made as small as possible to reduce size and weight for
the user. In addition, this device also requires special considerations for the RF section.
PCB Layout Considerations
The WOrD has two separate PCBs layouts. On the base side there was one main concern
was the RF section. First, the specification sheet
1
for the AT86RF211 recommends that there are
no traces or components underneath any of the traces. This helps to prevent noise from coupling
onto the RF lines on the top side of the board. Since this component is obviously sensitive to
noise, a separate ground pour was placed under the entire RF section. This is intended to
minimize the amount of noise on the ground plane and to isolate it from the noise created by the
digital components. The final major consideration in laying out the RF section is the necessity to
place all external components close to the RFIC. Since this design uses lumped element
components, this requires trace lengths to remain much less than a wavelength.
The digital system of the base unit does not have any special considerations, but several
precautions should be taken to eliminate excess noise on the digital lines
2
. First, the power lines
are decoupled near the microcontroller pins in order to reduce the most noise possible. Next, the
digital traces are routed such that there are a minimum number of lines crossing perpendicular to
one another and they are all separated from one another by the maximum distance. This should
prevent digital signal lines from excessively coupling to one another. Finally, a ground pour was
placed under the entire digital section. This is intended to reduce the trace distance between
components and ground.
Similar to the base station layout, the remote unit layout also required the same
considerations for the RF section and digital section. Just as above, the intent is to keep the RF
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