Telit Wireless Solutions GE863-QUAD, GE863-PY manual That means

GE863-QUAD

GE863-PY

1vv0300715 Rev. 1 - 19/09/06

TIP: definition of the nominal sensitivity of the microphone lines .

The nominal sensitivity of the microphone lines indicates the voltage level on the GE863-QUAD / PY pins present during "normal spoken" conditions.

For a handset , the "normal spoken” conditions take place when the talker mouth is 7cm far from the microphone ; under these conditions the voice will produce an acoustic pressure of -4,7dBPa @1kHz on the microphone membrane .

TIP: electrical equivalent signal and operating voice levels .

At "normal spoken" conditions, a microphone having the suggested nominal sensitivity of - 45dBVrms/Pa , will produce

the electrical equivalent signal :

MicLevel = ( -45) + (-4.7) = -49.7 dBVrms

that means :

MicVoltage = 10 ( -49.7 / 20 ) = 3.3* 10 -3Vrms

During a call, this level varies according to the volume of the talker voice; usually the following rough thumb rule for the dynamic range may be used :

1)the talker is screaming . This is the strongest voice level condition: the signal increases by +20dB;

2)the talker is whispering. This is the lowest voice level condition: the voice level decreases by – 50dB.

These changes must be considered for designing the external microphone amplifier.

TIP: example of external microphone amplifier calculation .

Let’s suppose to use the 1stdifferential microphone path .In this case the maximum differential input voltage to “Mic_MT” lines is 365mVrms(1,03Vpp) corresponding to –8,76dBV.

Now we can calculate the maximum voltage gain of an external microphone amplifier GA :

[(MicLevel + 20dB)+ GA ]= −8,76dBV [− 49,7 + 20 + GA ]= −8,76

−40,9 + 20 = −GA

GA = 20,94dB you can set GA= +20dB to use standard resistor values .

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