User's Manual

Shenzhen Neoway Technology Co., Ltd. Page 26 of 32
Figure 21 Using capacitors to couple audio outputs
As the description of TDD noise before, the GSM radio frequency is modulated at 217Hz. The
217Hz and its derivative frequency is well within audio band, therefore a TDD noise often affect the
audio performance through power and air. Some small capacitors between 27pF-100pF and ferrite
beads, placed on the audio path can attenuate TDD noise.
For Suppressing the TDD noise, differential audio interface is preferable. The PCB trace of audio
signal should be routed as differential line.
In particular, the microphone interface is a pair of small signal, peak-peak voltage <200mV, must
comply the rule of differential line. The microphone interface should be routed as short as possible.
6.8 Antenna Interface
A 50Ω antenna is required. VSWR < 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.
ESD protection is built in module. For special ESD protection, a ESD diode can be placed close to
the antenna. But ensure to use a low junction capacitance one. The junction capacitance should be
less than 0.5pF, otherwise the RF signal will be attenuated. RCLAMP0521P from Semtech, or
ESD5V3U1U from Infineon, can be used here. See Figure 22.
The trace between the antenna pad of module and the antenna connector, should have a 50Ω
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.