u-blox NORA-W2 Series, Hardware Integration Manual

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NORA-W2 series - Hardware integration manual
UBX-22005177 - R01 Design-in Page 15 of 61
C1 – Public
Table 4 specifies additional requirements for implementing a dual antenna design.
Item Requirements Remarks
Isolation
(in-band) S21 > 30 dB recommended The antenna-to-antenna isolation is the S21 parameter
between the two antennas in the band of operation.
Lower isolation might be acceptable depending on use-
case scenario and performance requirements.
Isolation
(out-of-band) S21 > 35 dB recommended
S21 > 30 dB acceptable Out-of-band isolation is evaluated in the band of the
aggressor. This ensures that the transmitting signal from
the other radio is sufficiently attenuated by the receiving
antenna. It also avoids any saturation and intermodulation
effect on the receiver port.
Envelope Correlation
Coefficient (ECC) ECC < 0.1 recommended
ECC < 0.5 acceptable The ECC parameter correlates the far field parameters
between antennas in the same system. A low ECC
parameter is fundamental in improving the performance of
MIMO-based systems.
Table 4: Summary of Wi-Fi/Bluetooth coexistence requirements
⚠
When operating dual antennas in the same 2.4 GHz band, sufficient isolation is critical for
attaining an optimal throughput performance in Wi-Fi/Bluetooth coexistence mode.
Select antennas that provide:
• Optimal return loss (or VSWR) over all the operating frequencies.
• Optimal efficiency figure over all the operating frequencies.
•
An appropriate gain that does not exceed the regulatory limits specified in some regulatory country
authorities like the FCC in the United States.
A useful approach for the antenna micro-strip design is to place a U.FL connector close to the
embedded PCB or chip antenna. The U.FL connector only needs to be mounted on units used for
verification.
3.2.2 RF transmission line design
RF transmission lines, such as those that connect from RF_ANT pins to their related antenna
connectors or antenna, must be designed with a characteristic impedance of 50  .
Figure 3 shows the design options for implementing a transmission line, namely:
• Microstrip – track separated with dielectric material and coupled to a single ground plane.
• Coplanar microstrip – track separated with dielectric material and coupled to both the ground
plane and side conductor. This in the most common transmission line implementation.
• Stripline – track separated by dielectric material and sandwiched between two parallel ground
planes.
The parameters shown in the cross-sectional area of each trace design include:
• Width (W) – shows the width of the copper layer on the top layer
• Distance (S) – shows the distance between the top copper layer and the two adjacent GND planes.
• Dielectric substrate thickness (H) – shows the distance between the GND reference on the bottom
plane and the copper layer on the top layer.
• Thickness of the copper layer (T) – can also be represented by “Base Copper Weight”, which is
commonly used as the parameter for PCB stack-up.
Dielectric constant
(
ε
r
) defines the ratio between the electric permeability of the material against the
electric permeability of free space
☞
The width of a 50 Ω microstrip depends on mainly “
ε
r
” and “H” , which must be calculated for each
PCB layer stack-up.