Smart Machine Smart Decision
SIM800H_Hardware_Design_V1.01
38
2013-08-20
4.7.3.
Audio Electronic Characteristic
Table 15: Microphone input characteristics
Parameter
Min
Typ
Max
Unit
Microphone biasing voltage
1.9
2.2
V
Working current
2
mA
Input impedance(differential)
13
20
27
K
Ω
Idle channel noise
-67
dBm0
Input level:-40dBm0
29
dB
SINAD
Input level:0dBm0
69
dB
Table 16: Audio output characteristics
Parameter
Conditions
Min
Typ
Max
Unit
R
L
=32 Ohm receiver
-
90
-
mW
Normal output
R
L
=8 Ohm speaker
-
-
1080
mW
4.7.4.
TDD
GSM signal could interfere audio by coupling or conducting. Coupling noise could be filtered by adding 33pF
and 10pF capacitor over audio lines. 33pF capacitor could eliminate noise from GSM850/EGSM900MHz, while
10pF capacitor could eliminate noise from DCS1800/PCS1900Mhz frequency. Coupling noise should have
something to do with PCB layout. Under some scenarios, TDD noise from GSM850/EGSM900MHz frequency
affects heavily, but some different story is from DCS1800/PCS1900Mhz frequency, so customer should develop
this filter solution according to field test result.
GSM antenna is the key coupling interfering source of TDD noise. Thereat, pay attention to the layout of audio
lines which should be far away from RF cable and antenna and VBAT pin. The bypass capacitor for filtering
should be placed near module and another group need to be 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 speaker output easily. So it is 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 is not good, lots of
high-frequency noises will interfere microphone and speaker over bypass capacitor. So care of good GND during
PCB layout need to be taken.
4.8.
SIM Card Interface
The SIM interface complies with the GSM Phase 1 specification and the new GSM Phase 2+ specification for
FAST 64 kbps SIM card. Both 1.8V and 3.0V SIM card are supported. The SIM interface is powered from an
internal regulator in the module.
