5

August 10, 2010

B3786

313.85 / 315.00 MHz

SAW Components

SAW RF filter

Data sheet

Please read

cautions and warnings and

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 QCC8B package, pinning 2,5 (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.