B3744 Datasheet, PDF (5/10 Pages) EPCOS – SAW filter Short range devices

SAW Components
B3744
SAW filter
868.30 MHz
Data sheet
Matching network to 50 Ω (element values depend on pcb layout and equivalent circuit)
C_p1 = 2.7 pF
L_s2 = 18 nH
L_s3 = 18 nH
C_p4 = 2.7 pF
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 DCC6E package, pinning 2,4 (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.
Please read cautions and warnings and
important notes at the end of this document.
5 December 14, 2012