LISA-U1 series - System Integration Manual
3G.G2-HW-10002-3
Preliminary
Design-In
Page 92 of 125
Buried striplines exhibit better shielding to external and internally generated interferences. They are therefore
preferred for sensitive application. In case a stripline is implemented, carefully check that the via pad-stack
does not couple with other signals on the crossed and adjacent layers
Minimize the transmission line length; the insertion loss should be minimized as much as possible, in the
order of a few tenths of a dB
The transmission line should not have abrupt change to thickness and spacing to GND, but must be uniform
and routed as smoothly as possible
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
Avoid coupling with
VCC
routing and analog audio lines
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 micro strip, use the “Coplanar Waveguide” model for 50
Ω
characteristic impedance calculation
Don’t 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
line
Ensure no coupling occurs with other noisy or sensitive signals
Min. 500 um
Min.
250 um
Top layer
Buried metal layer
GND
plane
Microstrip
50 ohm
Figure 47: GND keep-out area on the top layer around the ANT pad and on the buried metal layer below the ANT pad
Any RF transmission line on PCB should be designed for 50
Ω
characteristic impedance.
Ensure no coupling occurs with other noisy or sensitive signals.