Telit Wireless Solutions GM862-GPS Manual de usuario descargar pdf (Pagina 35)

GM862-GPS Hardware User Guide

1vv0300728 Rev. 0 - 27/04/06

The buffer gain is given by the formula:

607

606

605

604

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 3500Hz in order to have -1dB at 3kHz.

The cutoff frequency is given by the formula:

636*606*2

637*604*2

CRCR

freq

ππ

== [Hz]

TIP: example of calculation .

Let's assume you have a microphone with a sensitivity of -45 dBV

rms

/Pa and you want to use it in 1st

differential microphone path (“Mi c_ 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 :

corresponding to:

When the talker is screaming ,we will have a signal of 330 mV

rms

on the “Mic_ MT “ inputs for a buffer

gain G

=20 log (AmplifierOutput / MicVoltage) =20 log (330 * 10

-3

)/( 3.3 * 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* R603= 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

-12

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

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

Vrms

MicVoltage = 10

(

49.7 / 20 )

= 3.3* 10

rms

1 2 ... 30 31 32 33 34 35 36 37 38 39 40 ... 54 55

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Telit Wireless Solutions GM862-GPS Manual de usuario Pagina 35

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