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Recommended Microphone Characteristics
•
The impedance of the microphone has to be around 2KΩ
•
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.
Recommended Speaker Characteristics
•
Type of speakers: Electro‐magnetic /10mW
•
Impedance: 8Ω for hands‐free
•
Impedance: 32Ω 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.
WA_DEV_W228_PTS_002
Figure 25. Capacitor Soldered in Parallel to the Microphone
Rev 001
Product Technical Specification &
Customer Design Guidelines
CM
Page 62 of 108
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