Designing and optimizing an antenna is no easy task. When it comes to designing an industrial Internet of Things (IIoT) antenna, a reference design can be used as a starting point. The question is, what parameters can be modified to optimize the design when applying it to the final product design?
Designing and optimizing an antenna is no easy task. When it comes to designing an industrial Internet of Things (IIoT) antenna, a reference design can be used as a starting point. The question is, what parameters can be modified to optimize the design when applying it to the final product design?
People might think that designing and optimizing an antenna would be straightforward for experienced designers. After all, they are experts who have witnessed or personally experienced numerous failures and learned valuable lessons in their work. However, technical standards are upgrading rapidly. Young designers have no experience of making mistakes at all, yet they are required to design products for market launch in a shorter time and ensure the design is a one-time success. How can this be achieved? The solution is to use EDA software! Because to a certain extent, electronic design can be automatically realized with software.
This article discusses the Bluetooth antenna design for an industrial IoT product. The design goal is to integrate the Bluetooth antenna into a product without a display and use an application on a smartphone to implement various configurations of the device. In fact, this is also a commonly used design, and the design concept is equally applicable to various other products.
Through an online search, a Bluetooth antenna reference design from Cypress Semiconductor was found. However, its specification clearly states that the design is for reference only and cannot be used for actual product module design. Because it is designed solely for laboratory work, aiming to build a development platform for software engineers, enabling them to carry out software development synchronously before the PCB design is completed.
Since design data such as schematics, BOMs and PCB layouts are all provided in Cadence Allegro format, it can be used as a starting point for optimization. Nevertheless, the reference design uses a large connector in the laboratory.
Therefore, the design has two goals: antenna miniaturization and antenna performance optimization. Replacing the large connector with a rigid-flex PCB enables the miniaturization of the antenna structure. But what impact will this have on antenna performance? A series of hypothetical analyses were conducted to understand the effects of these changes and propose the correct strategies to achieve these two goals.
The import and setup of microwave design data are very simple. Ports are set automatically, and designers must input some parameters for the netlist. In addition, the actual material values of the selected PCB were entered, including appropriate thickness and dielectric constant values (FR-4: standard Isola 370HR, εr=4.0).
In the initial analysis, the focus was on the length of the folded inverted-F antenna (IFA) and the impedance matching network between the chip and the antenna.
Based on the antenna return loss at the design frequency of 2.45 GHz, the optimal antenna length was obtained through sweep frequency analysis of various antenna length combinations.
Through multiple analyses, combined with the discrete components available in the component library and the parameter values of the selected PCB materials, an automatic sweep frequency analysis was re-conducted. The results showed that the performance of the impedance matching network was improved. It is easy to find that the antenna performance is improved by 2 dB only through the above analysis and improvements. However, miniaturization was not considered at this time.
Sufficient grounding is required in antenna design, so antenna miniaturization is indeed a problem. Reducing the physical structure will result in a smaller ground plane on the PCB. In the reference design, the grounding system consists of two planes, several vias and an external cable connected to the ground.
Analysis of a small piece of PCB where the antenna and circuit are mounted revealed insufficient grounding in the rigid area, leading to the antenna performance failing to meet the expected indicators. Therefore, an additional grounding area needs to be provided in the flexible part of the PCB. Simulation can be completed very easily and quickly if it is used as a solid plane on the flexible part. However, if a hatched structure is adopted to achieve bending without cutting the copper foil, both the grid size and simulation time will increase significantly.
However, comparisons of various plane shapes and sizes show that the hypothetical analysis structure can be simplified without increasing errors, but the final simulation should be as accurate as possible.
In another sweep frequency analysis, the minimum distance between the antenna plastic housing and the antenna was analyzed. In this case, it was found that the antenna performance is no longer affected when the spacing is greater than 10 mm.
The next question is, how much impact will be generated when changing the shape of the flexible part by designing different shapes in 2D or folding and bending in 3D? After determining the mounting position in the product and the length and shape of the flexible part, the next question is how to mount the rigid part of the PCB.
Sweep frequency analysis of the screw positions showed that mounting the screws at the optimal mechanical positions also had a significant impact on antenna performance. To find a suitable alternative mounting method, a snap-fit solution is the first choice. This will require cutting the PCB.
Visualization of the current density in the PCB shows that when the corners or mounting holes are cut at 90°, the current at each corner is large, which will lead to EMI problems. Some additional optimization changes are needed to minimize EMI.
The final result is that the antenna size is reduced to 53% of the original size of the reference design, with a wider bandwidth and a 6 dB improvement in PCB antenna performance.
It can be concluded that antenna design becomes very demanding when miniaturizing the antenna, and attention must be paid to various mechanical mounting constraints at this time, such as mounting holes, spacing, housing materials and EMI issues. Finally, the design should be a one-time success, resulting in a shorter time-to-market compared to redesigning an additional prototype.
Collaborative Implementation from Design to Measurement Verification
Nordcad and FlowCAD jointly designed a state-of-the-art antenna amplifier and an inverted-F antenna module. Schematic entry and PCB layout were completed with Cadence's Allegro PCB Designer.
For RF simulation of PCB design data, designers loaded all components (including the antenna amplifier, antenna design and impedance matching network) onto Cadence's AWR Microwave Design Platform.
Impedance matching was finally achieved through component optimization. The physical design was measured using R&S's vector network analyzer, and the results were compared with the simulation results.
This example of professional collaboration and teamwork proves that the mutual collaboration between theory, simulation and measurement is excellent, enabling efficient and predictable RF design.
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