Telit Wireless Solutions GE863-PY Gain = RR604605 = RR607606, Buffer gain is given by the formula

GE863-QUAD

GE863-PY

1vv0300715 Rev. 1 - 19/09/06

The buffer gain is given by the formula:

Gain = RR604605 = RR607606

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 3500Hz in order to have -1dB at 3kHz. The cutoff frequency is given by the formula:

freq. =	1	=	1	[Hz]
	2π * R604*C637		2π * R606*C636

TIP: example of calculation .

Let's assume you have a microphone with a sensitivity of -45 dBVrms/Pa and you want to use it in 1st differential microphone path (“Mic_MT” inputs) in "normal spoken" conditions at acoustic pressure of -4.7dBPa.

As reported at page 33 , the electrical level output from the microphone will be :

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

corresponding to:

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

When the talker is screaming ,we will have a signal of 330 mVrms on the “Mic_MT “ inputs due to a 20dB higher Mic Level (see TIP 1) with a buffer gain GA :

GA =20 log (AmplifierOutput / MicVoltage) =20 log (330 * 10 -3)/( 33 * 10 -3) = 20 log 10=20dB

The corresponding values for the resistors on the buffer could be ( if we keep the input resistance 10kΩ )

R604 = R606 = gain* R607= gain* R605 = 10* 15 = 150 kΩ

The commercial values of 150kΩ & 15kΩ are then chosen.

As a consequence the values of the capacitors C636 and C637 shall be:

C636=C637= 1/ (2π*4000*R606)= 265 *10 -12F

A commercial value of 270pF gives a cutoff frequency of 3931Hz with an errorless than 1,8% .

Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 40 of 79