3.6 RF Interface
3.6.1 RF Design and PCB Layout
A 50 Ω antenna is required. VSWR ranges from 1.1 to 1.5. The antenna should be well matched to achieve
best performance. It should be installed far away from high speed logic circuits, DC/DC power, or any other
strong disturbing sources.
For multiple-layer PCB, the trace between the antenna pad of module and the antenna connector, should
have a 50 Ω characteristic impedance, and be as short as possible. The trace should be surrounded by ground
copper. Place plenty of via holes to connect this ground copper to main ground plane, at the copper edge.
If the trace between the module and connector has to be longer, or built-in antenna is used, a π-type
matching circuit should be needed, as shown in Figure 3-19. The types and values of C1, L1, and L2 should
be verified by testing using network analyzer instrument. If the characteristic impedance is well matched,
and VSWR requirement is met, just use a 0 Ω resistor for L1 and leave C1, L2 un-installed.
Avoid any other traces crossing the antenna trace on neighboring layer.
Figure 3-19 Reference design for antenna interface
On two-layer boards which cannot control resistance properly, the RF route should be as short and smooth
as possible and at a width of 1mm; the RF is 1 mm away from the ground.
Figure 3-20 shows a two-layer board application. The RF is connected to GSC RF connector through traces
on PCB, which is connected to the antenna via cable.
Remove the copper from the area of 2 mm in diameter around the RF testing point. Dig ground holes as
many as possible. Separate this area from the copper-removing area of the 21st pin by ground.
Ensure complete ground around the 20
th
pin in case that its signal is affected by other high-speed signals
(SIM signal, e.g.).
GPRS
Module
ANT
C1
GPRS_ANT
L2
L1
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