Ublox TOBY-L2 series, System Integration Manual

Page: 90 / 164

TOBY-L2 and MPCI-L2 series - System Integration Manual
UBX-13004618 - R28 Design-in Page 90 of 164
 the gap from the transmission line to the adjacent ground plane on the same layer of the
transmission line (e.g. 500
µ
m in Figure 46 , 400
µ
m in Figure 47 )
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 the 50

calculation.
Additionally to the 50

impedance, the following guidelines are recommended for transmission lines
design:
 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,
 Add GND keep-out (i.e. clearance, a void area) on buried metal layers below any pad of component
present on the RF transmission lines, if top-layer to buried layer dielectric thickness is below
200
µ
m, to reduce parasitic capacitance to ground,
 The transmission lines width and spacing to GND must be uniform and routed as smoothly as
possible: avoid abrupt changes of width and spacing to GND,

Add GND stitching vias around transmission lines, as described in Figure 48 ,
 Ensure solid metal connection of the adjacent metal layer on the PCB stack-up to main ground
layer, providing enough vias on the adjacent metal layer, as described in
Figure 48 ,
 Route RF transmission lines far from any noise source (as switching supplies and digital lines) and
from any sensitive circuit (as USB),
 Avoid stubs on the transmission lines,
 Avoid signal routing in parallel to transmission lines or crossing the transmission lines on buried
metal layer,
 Do not route microstrip lines below discrete component or other mechanics placed on top layer
An example of proper RF circuit design is reported in Figure 48 . In this case, the ANT1 and ANT2 pins
are directly connected to SMA connectors by means of proper 50  transmission lines, designed with
proper layout.
SMA Connector
Primary Antenna
SMA Connector
Secondary Antenna
TOBY-L2 series
Figure 48: Suggested circuit and layout for antenna RF circuits on application board