User's Manual

Telit GE863-GPS
Hardware User guide
1vv0300714, Rev. ISSUE#0, - 21/02/06
Reproduction forbidden without Telit Communication written authorization – All Right reserved – Right of modification reserved page 40 of 76
607
606
605
604
R
R
R
R
Gain ==
The C636 and C637 capacitors are placed in order to cut off the gain at higher frequencies than
the transmitted GSM band, the cutoff frequency (-3dB) should be 8 kHz in order to have -1dB at
4kHz.
The cutoff frequency is given by the formula:
636*606*2
1
637*604*2
1
.
CRCR
freq
ππ
== [Hz]
For example:
- Let's assume you have a microphone with these characteristics:
- sensitivity -45 dB
Vrms/Pa
and you want to use it in the internal mic. audio path.
With the mic. having nominal sensitivity -45dB
Vrms/Pa
at the "normal spoken" conditions: -
4.7dB
Pa
at 7 cm from the mouth of the talker.
At that acoustic pressure the voltage output from the microphone is:
Mic Voltage Output (dB) = ( -45) + (-4.7) = -49.7 dB
Vrms
corresponding to:
Mic_Voltage_Output = 10
( -49.7 / 20 )
= 3.3* 10
-3
V
rms
in order to have a signal of 50 mV
rms
on the GE863-GPS internal mic. inputs then the buffer must
have a gain of:
Voltage @ GE863-GPS_intmic/ Mic_Voltage_Output = (50 * 10
-3
)/( 3.3 * 10
-3
) = 15
Hence in these conditions the signal level on the input pads of the internal mic. of the GE863-
GPS is 24 dB (15 times) higher than the microphone output and therefore the buffer has to gain
24 dB.
The corresponding values for the resistors on the buffer could be ( if we keep the input resistance
10kΩ )
R604 = R606 = gain* R603= gain* R605 = 15* 10 = 150 kΩ
The commercial values of 150kΩ & 10kΩ are then chosen.