Integration Manual

LISA-U series - System Integration Manual
3G.G2-HW-10002-A3 Preliminary Design-In
Page 108 of 160
 Add GND keep-out (i.e. clearance) on buried metal layers below ANT and ANT_DIV pads and below any
other pad of component present on the RF line, if top-layer to buried layer dielectric thickness is below
200 µm, to reduce parasitic capacitance to ground (see Figure 55 for the description of the GND keep-out
area below ANT and ANT_DIV pads)
 The transmission line up to antenna connector or pad may be a micro strip or a stripline. In any case must be
designed to achieve 50 Ω characteristic impedance
 Microstrip lines are usually easier to implement and the reduced number of layer transitions up to antenna
connector simplifies the design and diminishes reflection losses. However, the electromagnetic field extends
to the free air interface above the stripline and may interact with other circuitry
 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 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 relative 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