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BOARD LAYOUT GUIDELINES
If you are familiar with RF you may be concerned about specialized layout
requirements. Fortunately, by carefully adhering to a few basic design and
layout rules transmitter integration is generally very straightforward.
Page 5 shows the suggested PCB footprint
for the HP-3 transmitter. A groundplane (as
large as possible) should be placed on a
lower layer of your PC board opposite the
transmitter. This groundplane can also be
critical to the performance of your antenna
which will be discussed later in the
manual.
The transmitter should be kept away from other components on your PCB,
especially high-frequency noise sources such as an oscillator or switching supply.
To mount a pinned version of the transmitter parallel to the PC board, bend it
over so that the plastic cover faces away from the board.
Do not route PCB traces directly under SMD packaged versions. The underside
of the module has numerous signal-bearing traces and vias which could short or
couple to traces on the product's circuit board.
The trace from the transmitter to the antenna should be kept as short as
possible. For runs greater than 1/4 inch use 50-ohm coax or a 50-ohm microstrip
transmission line as shown below. Handy software for calculating microstrip
lines is available on the Linx website (www.linxtechnologies.com).
Dielectric
Constant
Figure 16: Microstrip Formulas (Er = Dielectric constant of pc board material)
The typical output power of the HP-3 transmitter is
right at Part-15 limits. Sometimes, it is necessary
to slightly attenuate the output to compensate for
antenna gain. This is accomplished using a three-
resistor attenuation network as shown. While this
network is often referred to as a "T" pad the actual
resistor orientation is usually not critical. Use only
surface-mount type resistors grouped closely. The
series pads may be bridged if the network is not needed. Further details can be
found in application note #00150 - "Use and Design of T-Attenuation Pads".
Width/Height
(W/d)
4.8
1.8
4
2
2.55
3
Full groundplane on inner or lower board layer
Figure 15: Groundplane Treatment
Effective
Dielectric Characteristic
Constant
Impedance
3.59
50.0
3.07
51.0
2.12
48.0
MICROSTRIPS
RF IN
GROUNDPLANE
ON LOWER LAYER
Figure 17: T-pad Layout
TYPICAL LAYOUT
ANT.
R1
R1
R2
GROUND
Page 9
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