
WA_DEV_W228_PTS_002 Rev
001
Page 62 of 108
Product Technical Specification &
Customer Design Guidelines
Recommended Microphone Characteristics
•
The
impedance
of
the
microphone
has
to
be
around
2K
Ω
•
Sensitivity
is
from
‐
40dB
to
‐
50dB
•
SNR
>
50dB
•
Frequency
response
is
compatible
with
GSM
specifications
To
suppress
TDMA
noise,
it
is
highly
recommended
to
use
microphones
with
two
internal
decoupling
capacitors:
•
CM1=56pF
(0402
package)
for
the
TDMA
noise
coming
from
the
demodulation
of
the
GSM850/EGSM900
frequency
signals
•
CM2=15pF
(0402
package)
for
the
TDMA
noise
coming
from
the
demodulation
of
the
DCS1800/PCS1900
frequency
signals
The
capacitors
have
to
be
soldered
in
parallel
to
the
microphone
as
shown
in
the
figure
below.
CM
Figure 25. Capacitor Soldered in Parallel to the Microphone
Recommended Speaker Characteristics
•
Type
of
speakers:
Electro
‐
magnetic
/10mW
•
Impedance:
8
Ω
for
hands
‐
free
•
Impedance:
32
Ω
for
heads
kit
•
Sensitivity:
110dB
SPL
min
•
Receiver
frequency
response
is
compatible
with
GSM
specifications.
Recommended Filtering Components
When
designing
a
GSM
application,
it
is
important
to
select
the
right
audio
filtering
components.
The
strongest
noise,
called
TDMA,
is
mainly
due
to
the
demodulation
of
the
GSM850,
EGSM900,
DCS1800
and
PCS1900
signals,
where
a
burst
is
produced
every
4.615ms;
and
the
frequency
of
the
TDMA
signal
is
equal
to
216.7Hz
plus
harmonics.
TDMA
noise
can
be
suppressed
by
filtering
the
RF
signal
using
the
appropriate
decoupling
components.
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