User's Manual

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 38 of 71
As shown in the balanced buffered mic. the gain adjustments can be done by changing R719 -
R720 and R708 - R711 and as a consequence the capacitors C726 and C727.
The bandwidth (-3dB) is given by the approximated formula (considering C725 >> C726) :
727*711*2
1
726*719*2
1
.
CRCR
freq
ππ
== [Hz]
The buffer bandwidth at -3dB shall be 8kHz.
Note that the biasing of the operational amplifier is given for the inverting amplifier by the series
divider R714-R715. The 100nF capacitor C719 is needed to filter the noise that could be coupled
to that divider. For the not inverting operational the biasing is given by a different divider R715-
R717 with the capacitor C720 and through a series resistor R718 of 470KΩ.
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 external mic. audio path.
With the mic. having nominal sensitivity -45dB
Vrms/Pa
at the "normal spoken" conditions: -
4.7dB
Pa
but at 50 cm from the mouth of the talker an additional 20 dB loss shall be considered.
At that acoustic pressure the voltage output from the microphone is:
Mic Voltage Output (dB) = ( -45) + (-4.7) -20 = -69.7 dB
Vrms
corresponding to:
Mic_Voltage_Output = 10
( -69.7 / 20 )
= 3.3* 10
-4
V
rms
in order to have a signal of 1 mV
rms
(10 dB lower than the nominal input for the GE863-
QUAD/PY external mic. path) on the GE863-QUAD/PY internal mic. inputs then the buffer must
have a gain of:
Voltage @ GE863-QUAD/PY_intmic/ Mic_Voltage_Output = (1 * 10
-3
)/( 3.3 * 10
-4
) = 3
Hence in these conditions the signal level on the input pads of the internal mic. of the GE863-
QUAD/PY is 10 dB (3 times) higher than the microphone output and therefore the buffer has to
gain 10 dB.
To calculate the resistor values it must be kept in mind that balancing the line will double the
signal and hence already add +6 dB, therefore the buffer must gain only 1.5 times.
The corresponding values for the resistors on the buffer could be ( if we keep the input resistance
10kΩ )