Telit Wireless Solutions GM862-GPS - Manual (Page 28)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 0 - 27/04/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 28 of 55
TIP: definition of the nominal sensitivity of the microphone lines .
The nominal sensitivity of the microphone lines indicates the voltage level on the GM862-GPS 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 -
45dBV
rms
/Pa , will produce
the electrical equivalent signal :
that means :
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 1
st differential microphone path .In this case the maximum differential input
voltage to “Mic_MT” lines is 365mV
rms
(1,03V
pp
) corresponding to –8,76dBV.
Now we can calculate the maximum voltage gain of an external microphone amplifier G
A:
( ) [ ]
dBV G dB MicLevel
A
76 , 8 20 − = + +
[ ]
76 , 8 20 7 , 49 − = + + −
A
G
A
G − = + − 20 9 , 40
dB G
A
94 , 20 =
you can set G
A
= +20dB to use standard resistor values .
MicLevel = ( -45) + (-4.7) = -49.7 dB
Vrms
MicVoltage = 10 ( -49.7 / 20 ) = 3.3* 10 -3 V
rms