SimCom SIM808, Проектирование аппаратного обеспечения

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SIM808_Hardware Design_V1.00 2014.03.27 38
4.8.3 Audio Electronic Characteristics
Table 15: Microphone input characteristics
Parameter Min Typ Max Unit
Mic biasing voltage 1.9 2.2 V
Working Current 2 mA
Input impedance(differential) 13 20 27 K Ω
Idle channel noise -67 dBm
Input level:-40dBm0 29 dB
SINAD
Input level:0dBm0 69 dB
Table 16: Audio output characteristics
parameter Conditions Min Typ Max Unit
Normal output R
L
=32 Ω receiver - 90 - mW
4.8.4 TDD
GSM signal could interfere audio by coupling or conducting. Coupling noise could be filtered by adding 33 pF
and 10pF capacitor over audio lines.33pF capacitor could eliminate noise from GSM900MHz, while 10pF
capacitor could eliminate noise from DCS1800MHz frequency. Coupling noise has great relatives with PCB
layout. Under some scenarios, TDD noise from GSM 900MHz frequency affects heavily, but some different
story is from GSM1800MHz fervency, so customer should develop this filter solution according to field test
result.
GSM antenna is the key coupling interfering source of TDD noise. Pay attention to the layout of audio lines
which should be far away from RF cable & antenna and VBAT pin. The bypass capacitor for filtering should be
placed near module and another group placed near to connector.
Conducting noise is mainly caused by the VBAT drop. If Audio PA was powered by VBAT directly, then there
will be some cheep noise from SPK output easily. So, it's better to put big capacitor and ferrite bead near audio
PA input.
TDD noise has something to do with GND signal surely. If GND signal issued not good, lots of
high-frequency noise will interfere MIC and speaker over bypass capacitor. So, take care of GND well during
PCB layout.