u-blox LARA-R2 series System Integration Manual page 117


Keep ground separation from microphone lines to other noisy signals. Use an intermediate ground layer or
vias wall for coplanar signals.

In case of external audio device providing differential microphone input, 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.

Cross other signals lines on adjacent layers with 90° crossing.

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.
General guidelines for the downlink path (speaker / receiver) are the following:

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.

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.

Avoid coupling between speaker / receiver and microphone lines.

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.

In case of external audio device providing differential speaker / receiver output, 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.

Cross other signals lines on adjacent layers with 90° crossing.

Place bypass capacitor for RF close to the speaker.
UBX-16010573 - R02
LARA-R2 series - System Integration Manual
Objective Specification Design-in
Page 117 of 148