4

Sep 05, 2002

SAW Components

B3574

868,30 MHz

Low Loss Filter

Data Sheet

Matching network to 50

Ω (element values depend on pcb layout and equivalent circuit)

Minimising the crosstalk

For a good ultimate rejection a low crosstalk is necessary. Low crosstalk can be realised with

a good RF layout. The major crosstalk mechanism is caused by the “ground-loop” problem.

Grounding loops are created if input-and output transducer GND are connected on the top-side

of the PCB and fed to the system grounding plane by a common via hole. To avoid the common

ground path, the ground pin of the input- and output transducer are fed to the system ground

plane (bottom PCB plane) by their own via hole. The transducers’ grounding pins should be

isolated from the upper grounding plane.

A common GND inductivity of 0.5nH degrades the ultimate rejection (crosstalk) by 20dB.

The optimised PCB layout, including matching network for transformation to 50 Ohm, is shown

here. In this PCB layout the grounding loops are minimised to realise good ultimate rejection.

Optimised PCB layout for SAW filters in QCC8C package, pinning 1,5 (top side, scale 1:1)

The bottom side is a copper plane (system ground area). The input and output grounding pins

are isolated and connected to the common ground by separated via holes.

For good contact of the upper grounding area with the lower side it is necessary to place

enough via holes.