LISA-U2 series - System integration manual
UBX-13001118 - R27
Design-In
Page 123 of 183
C1-Public
•
The transmission line must be routed in a section of the PCB where minimal interference from
noise sources can be expected
•
Route RF transmission line far from other sensitive circuits as it is a source of electromagnetic
interference
•
Ensure solid metal connection of the adjacent metal layer on the PCB stack-up to main ground
layer
•
Add GND vias around transmission line
•
Ensure no other signals are routed parallel to transmission line, or that other signals cross on
adjacent metal layer
•
If the distance between the transmission line and the adjacent GND area (on the same layer) does
not exceed 5 times the track width of the microstrip, use the “Coplanar Waveguide” model for
50
Ω
characteristic impedance calculation
•
Do not route microstrip line below discrete component or other mechanics placed on top layer
•
When terminating transmission line on antenna connector (or antenna pad) it is very important to
strictly follow the connector manufacturer’s recommended layout
•
GND layer under RF connectors and close to buried vias should be cut out in order to remove stray
capacitance and thus keep the RF line 50
Ω
. In most cases the large active pad of the integrated
antenna or antenna connector needs to have a GND keep-out (i.e. clearance) at least on first inner
layer to reduce parasitic capacitance to ground. Note that the layout recommendation is not
always available from connector manufacturer: e.g. the classical SMA Pin-Through-Hole needs to
have GND cleared on all the layers around the central pin up to annular pads of the four GND posts.
Check 50
Ω
impedance of
ANT
and
ANT_DIV
lines
•
Ensure no coupling occurs with other noisy or sensitive signals
•
The antenna for the Rx diversity should be carefully separated from the main Tx/Rx antenna to
ensure that uncorrelated signals are received at each antenna, because signal improvement is
dependent on the cross correlation and the signal strength levels between the two received
signals. The distance between the two antennas should be greater than half a wavelength of the
lowest used frequency (i.e. distance greater than ~20 cm, for 2G/3G low bands) to distinguish
between different multipath channels, for proper spatial diversity implementation
☞
Any RF transmission line on PCB should be designed for 50
Ω
characteristic impedance.
☞
Ensure no coupling occurs with other noisy or sensitive signals.
2.2.1.2
Main DC supply connection
The DC supply of LISA-U2 modules is very important for the overall performance and functionality of
the integrated product. For detailed description, check the design guidelines in section
. Some
main characteristics are:
•
VCC
pins are internally connected, but it is recommended to use all the available pins in order to
minimize the power loss due to series resistance
•
VCC
connection may carry a maximum burst current in the order of 2.5 A. Therefore, it is typically
implemented as a wide PCB line with short routing from DC supply (DC-DC regulator, battery pack,
etc)
•
The module automatically initiates an emergency shutdown if supply voltage drops below
hardware threshold. In addition, reduced supply voltage can set a worst case operation point for
RF circuitry that may behave incorrectly. It follows that each voltage drop in the DC supply track
will restrict the operating margin at the main DC source output. Therefore, the PCB connection
must exhibit a minimum or zero voltage drop. Avoid any series component with Equivalent Series
Resistance (ESR) greater than a few milliohms
