UC864-G Hardware User Guide
DRAFT
- 29/10/07
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Rights Reserved
page 45 of 66
The buffer gain is given by the formula:
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 3500Hz in order to have -1dB at 3kHz.
The cutoff frequency is given by the formula:
636
*
606
*
2
1
637
*
604
*
2
1
.
C
R
C
R
freq
π
π
=
=
[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 (“
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:
corresponding to:
When the talker is screaming ,we will have a signal of 330 mV
rms
on the
“
Mic_MT
“
inputs due to a
20dB higher Mic Level (see TIP 1) with a buffer gain G
A
:
G
A
=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
10k
Ω
)
R604 = R606 = gain* R607= gain* R605 = 10* 15 = 150 k
Ω
The commercial values of 150k
Ω
& 15k
Ω
are then chosen.
As a consequence the values of the capacitors C636 and C637 shall be:
C636=C637= 1/ (2
π
*4000*R606)= 265 *10
-12
F
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
-3
V
rms
