TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R28
Design-in
Page 115 of 164
General guidelines for the uplink path (microphone), which is commonly the most sensitive, are the
following:
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.
Avoid coupling between microphone and speaker / receiver lines.
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.
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.
