Neo_M680 GPRS Module Hardware User Guide
Copyright © Neoway Technology Co., Ltd
26
Figure 3-26
Reference design for earphone output
If an external amplifier is used to drive the speakers, coupling capacitors of 2.2 μF to 4.7 μF should be used
to block the DC voltage, as shown in Figure 3-27
Figure 3-27
Coupling capacitor interfacing
3.8
RF Interface
3.8.1 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 3-28. 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 C1 and leave L1, L2 un-installed.
Avoid any other traces crossing the antenna trace on neighboring layer.
GPRS Module
C2
C1
EAR_R
EAR_L
33pF
33pF
GPRS Module
RECN/SPKN/EAR_R
RECP/SPKP/EAR_L
C1
C2
User's
Circuit
