B3780_1 EPCOS [EPCOS], B3780_1 Datasheet - Page 4

• Manufacturer Part Number: B3780_1
• Description: SAW filter Short range devices
• Manufacturer: EPCOS [EPCOS]

Data sheet
Matching network to 50
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.
Please read cautions and warnings and
important notes at the end of this document.
SAW Components
SAW filter
(element values depend on pcb layout and equivalent circuit)
4
July 24, 2008
C
L
L
C
s2
s3
p1
p4
=
=
=
=
1.5 pF
1.5 pF
39 nH
39 nH
433.92 MHz
B3780