User's Manual

SARA-G3 and SARA-U2 series - System Integration Manual
UBX-13000995 - R18 Advance Information Design-in
Page 148 of 206
Reference Description Part Number – Manufacturer
C1, C2, C3, C4,
C5, C6, C7, C8
10 μF Capacitor X5R 0603 5% 6.3 V
GRM188R60J106M – Murata
R1, R3
0 Ω Resistor 0402 5% 0.1 W
RC0402JR-070RL – Yageo Phycomp
R2, R4
Not populated
Table 55: Connection to an analog audio device
2.7.1.4 Guidelines for analog audio layout design
Accurate analog audio design is very important to obtain clear and high quality audio. The GSM signal burst has
a repetition rate of 217 Hz that lies in the audible range. A careful layout is required to reduce the risk of noise
from audio lines due to both VCC burst noise coupling and RF detection.
Guidelines for the uplink path, which is the most sensitive since the analog input signals are in the microVolts
range, are the following:
x Avoid coupling of any noisy signal to microphone lines: it is strongly recommended to route microphone
lines away from module VCC supply line, any switching regulator line, RF antenna lines, digital lines and any
other possible noise source.
x Avoid coupling between microphone and speaker / receiver lines.
x Optimize the mechanical design of the application device, the position, orientation and mechanical fixing
(for example, using rubber gaskets) of microphone and speaker parts in order to avoid echo interference
between uplink path and downlink path.
x Keep ground separation from microphone lines to other noisy signals. Use an intermediate ground layer or
vias wall for coplanar signals.
x Route microphone signal lines as a differential pair embedded in ground to reduce differential noise pick-up.
The balanced configuration will help reject the common mode noise.
x Route microphone reference as a signal line since the MIC_GND pin is internally connected to ground as a
sense line as the reference for the analog audio input.
x Cross other signals lines on adjacent layers with 90° crossing.
x Place bypass capacitor for RF very close to active microphone. The preferred microphone should be designed
for GSM applications which typically have internal built-in bypass capacitor for RF very close to active device.
If the integrated FET detects the RF burst, the resulting DC level will be in the pass-band of the audio
circuitry and cannot be filtered by any other device.
Guidelines for the downlink path are the following:
x The physical width of the audio output lines on the application board must be wide enough to minimize
series resistance since the lines are connected to low impedance speaker transducer.
x Avoid coupling of any noisy signal to speaker lines: it is recommended to route speaker lines away from
module VCC supply line, any switching regulator line, RF antenna lines, digital lines and any other possible
noise source.
x Avoid coupling between speaker / receiver and microphone lines.
x Optimize the mechanical design of the application device, the position, orientation and mechanical fixing
(for example, using rubber gaskets) of speaker and microphone parts in order to avoid echo interference
between downlink path and uplink path.
x Route speaker signal lines as a differential pair embedded in ground up to reduce differential noise pick-up.
The balanced configuration will help reject the common mode noise.
x Cross other signals lines on adjacent layers with 90° crossing.
x Place bypass capacitor for RF close to the speaker.