Analog Devices ADRV9005 Reference Manual page 306

Reference Manual ADRV9001
PRINTED CIRCUIT BOARD LAYOUT RECOMMENDATIONS
that signals can be routed between the openings without disturbing the isolation benefits provided by the array of apertures. A careful designer
notices various bends in the routing of differential paths. Develop and tune these routes through iterative electromagnetic simulation to minimize
magnetic field coupling between differential paths. Figure 292 shows these techniques.
Figure 292. RF I/O Isolation Structures
When utilizing the proposed isolating structures, it is important to place ground vias around the slots and apertures. Figure 293 shows the
methodology used on the ADRV9001 evaluation card. When using slots, place the ground vias at each end of the slots and along each side.
When using square apertures, place at least one single ground via next to each square. These vias are through-hole vias connecting the top to
the bottom layer and all layers in between. The function of these vias is to steer return current to the ground planes near the apertures.
Figure 293. Current Steering Vias Placed Near Isolation Slots and Square Apertures
For accurate slot spacing and square apertures layout, use simulation software when designing a PCB for an ADRV9001 based transceiver. As
a general rule, the spacing between square apertures should be no more than one-tenth of the shortest wavelength. Calculate the wavelength
Wave lengtℎ [m] =
calculated using Equation 1.
300
Frequency [MHz] × ε r
(1)
where:
ε is the dielectric constant of the isolator material.
r
For ISOLA I-speed material, ε = 3.56, and for FR4-408 HR material, ε = 3.77.
r r
Example: Given a maximum RF signal frequency of 6 GHz, for ISOLA I-speed material, using microstrip structures and ε = 3.56, the minimum
r
wavelength is approximately 26.4 mm. To fulfill the one-tenth of a wavelength rule, the square aperture spacing should be at a distance of 2.64
mm or closer.
Provide additional shielding by connecting VSSA balls under the device to form a shield around RF I/O ball pairs. This ground provides a
termination for stray electric fields. Figure 294 shows how this is done for Rx1. Do the same for each set of sensitive RF I/O ports. Use ground
vias along single-ended RF I/O traces. Optimal via spacing is one-tenth of a wavelength, but that spacing can vary somewhat due to practical
layout considerations.
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