Telit Wireless Solutions GE863-PY Pb balls, Hardware User'S Manual

Page: 35 / 71

Telit GE863-QUAD / GE863-PY
Hardware User guide
1vv0300715, Rev. ISSUE#0, - 21/02/06
Reproduction forbidden without Telit Communication written authorization – All Right reserved – Right of modification reserved
page 35 of 71
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 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 dB
Vrms/Pa
and you want to use it in the internal mic. audio path.
With the mic. having nominal sensitivity -45dB
Vrms/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 dB
Vrms
corresponding to:
Mic_Voltage_Output = 10
( -49.7 / 20 )
= 3.3* 10
-3
V
rms
in order to have a signal of 50 mV
rms
on the GE863-QUAD/PY internal mic. inputs then the
buffer must have a gain of:
Voltage @ GE863-QUAD/PY_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/PY 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
10k
Ω )
R604 = R606 = gain* R603= gain* R605 = 15* 10 = 150 k Ω
The commercial values of 150k
Ω & 10k Ω are then chosen.