Analog Devices ADRV9001 User Manual page 223

Preliminary Technical Data
through iterative electromagnetic simulation to minimize magnetic field coupling between differential paths. These techniques are
illustrated in Figure 205.
ISOLATION STRUCTURES
SLOTS
DIFFERENTIAL PATH
ROUTING
When utilizing the proposed isolating structures, it is important to place ground vias around the slots and apertures. Figure 206
illustrates the methodology used on the ADRV9001 evaluation card. When slots are used, ground vias should be placed at each end of the
slots and along each side. When square apertures are used, at least one single ground via should be placed adjacent to each square. These
vias should be 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.
For accurate slot spacing and square apertures layout, simulation software should be used when designing a PCB for an ADRV9001
based transceiver. As a general rule, spacing between square apertures should be no more than 1/10 of the shortest wavelength
supported. The wavelength can be calculated using Equation 1
=
Wave length m [ ]
Frequency MHz
where:
ε is the dielectric constant of the isolator material. For ISOLA I-speed material, ε
r
Example: given a maximum RF signal frequency of 6 GHz, for ISOLA I-speed material, using microstrip structures, and ε
minimum wavelength is approximately 26.4 mm To fulfil the 1/10 of a wavelength rule, square aperture spacing should be at a distance
of 2.64 mm or closer.
Additional shielding is provided by using connecting VSSA balls under the device to form a shield around RF IO ball pairs. This ground
provides a termination for stray electric fields. Figure 207 shows how this is done for Tx1. The same is done for each set of sensitive RF
Rx2B
Ext_LO2
Rx2A
Tx2
Figure 205. RF I/O Isolation Structures
Figure 206. Current Steering Vias Placed Near Isolation Slots and Square Apertures
300
ε
×
[ ]
r
Rx1B
= 3.56 and for FR4-408 HR material, ε
r
Rev. PrA | Page 223 of 253
ISOLATION STRUCTURES
SQUARE APERTURES
Rx1A
Tx1
= 3.77.
r
r
UG-1828
(1)
= 3.56, the