Optimum PC Board Layout
The following diagram details some recommended steps to take when configuring a PC board to extract the optimum performance form the SAW device. One concept is paramount. The input and the output port must be isolated from one another to the greatest extent possible. In some extreme applications, the PC card can be mounted to a metal septum or compartment to form isolated cavities for each port and its accompanying impedance matching elements.
Figure 4. Optimum layout for a SAW package
A ground plane should be present on both the top and bottom of the PC board.
Plated-through via holes connecting the top and bottom ground planes should be distributed as shown. Note how the two vertical rows of via holes are offset from one another to increase the isolation.
A plated-through slot, isolating the input port from the output port, should be added to minimize RF leakage through the dielectric layer of the PC board.
The case of the SAW device must be in direct contact with the ground plane and should not be offset from the board. The case ground pads should be soldered to the ground plane.
If a device is going to be matched externally, there are several important techniques that can be used to minimize the crosstalk between the matching components.
- Place the matching components on the input port perpendicular to those on the output port. This is especially important for inductors.
- Place the input and output matching components as far apart from one another as reasonably feasible.
- If possible, mount the input and output matching components on the topside and underside, respectively, of the PC board.
- If necessary, RF-shielded matching components can be obtained from various manufacturers to minimize crosstalk.
- To maximize performance, a metal shield or septum can be placed between the input matching components and the output matching components to prevent electromagnetic energy from radiating to the other die through the freespace. This shield is often incorporated as part of the PC board housing or framework