GE863 datasheet(33/69 Pages) ETC1

33 / 69 page

Telit GE863-QUAD / GE863-PY

Hardware User guide

1vv0300697, Rev. ISSUE#0, - 21/11/05

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

637

604

freq

[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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