Telit GE863-QUAD / GE863-PY
Hardware User guide
1vv0300697, Rev. ISSUE#0, - 21/11/05
Reproduction forbidden without Telit Communication written authorization – All Right reserved – Right of modification reserved
page 33 of 69
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
.
C
R
C
R
freq
π
π
=
=
[Hz]
For example:
- Let's assume you have a microphone with these characteristics:
- sensitivity -45 dBVrms/Pa
and you want to use it in the internal mic. audio path.
With the mic. having nominal sensitivity
-45dBVrms/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 dBVrms
corresponding to:
Mic_Voltage_Output = 10 ( -49.7 / 20 ) = 3.3* 10 -3 Vrms
in order to have a signal of 50 mVrms on the GE863-QUAD internal mic. inputs then the buffer
must have a gain of:
Voltage @ GE863-QUAD_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-
QUAD 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.
As a consequence the values of the capacitors C636 and C637 shall be:
C636=C637= 1/ (2π*8000*R606)= 132 *10 -12 F
A commercial value of 100pF is then chosen.
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