The Most Common Trade-Offs of RF Filters

As you know, RF filters are used extensively in both military and commercial applications. However, in filter manufacturing, custom filter designs can differ quite significantly between the two. Military in particular have more constraints and performance requirements which means that there’s often a trade-off that has to be considered when designing new components.

Size, weight, power, and cost are the primary factors that often play off against one another. In research and development, it’s referred to as the SWaP-C concept. When designing communication and electronics systems, having a powerful RF filter may be limited by size or weight constraints. Similarly, designing filters that are to be used in thousands of devices will have cost constraints.

In this article, we look at how filter engineers work around your constraints and what the parameters are for the best trade-off. First, we consider the importance and influence of each of the four main factors of SWaP-C.

Trade-off Considerations for Custom RF Filters

In an ideal world, RF engineers would be able to design custom microwave filters that are small, powerful, lightweight, and cost-effective. Unfortunately, small often translates into less powerful handling. Similarly, focusing on lowering the cost usually expands the size or requires less stringent requirements.

Cost:

During the design phase, engineers will prioritize the application requirements for the components and try to balance this with specific applications. Making this work within a set budget can sometimes be challenging. It’s one of the reasons that cost is perhaps the biggest defining factor when it comes to trade-offs within custom designs.

With every custom design alteration there is a cost implication. A more complex design will typically be more expensive as it will require higher skilled labor. The requirement for high reliability materials and testing may also add to the costs. As each project has budget limitations, engineers need to carefully consider different combinations of components that will work together to deliver RF filters to your specifications and budget, while still meeting mission critical requirements.

Ultimately, the custom filter schematic often starts and ends with a cost and performance trade-off analysis. As different adaptations are made they always need to take into consideration the overall cost implications. That’s why our team at Q Microwave will always work closely with you to understand your application and provide guidance on the cost and performance trade-offs.

Size:

In almost all types of electronics, the leading trend is towards smaller components that enable small form factors and increase mobility. Consider smartphones and how indispensable they have become as part of modern life. A smartphone is much more than just a communication device. People transact, check emails, do research, and capture images and information to share on social media.

To support these ever-expanding applications, RF filters need to be more accurate while also being small enough to allow for greater mobility. This translates into requirements for smaller, more compact components – often called microelectronics.

There’s a similar requirement in military applications,  with an ever-evolving battlespace being able to provide Communication, Signal Intelligence, Electronic Warfare and other capabilities within a smaller form factor such as unmanned aerial systems (UAS) provide considerable advantages.

Being smaller than other types of military aircraft, components in systems within drones and similar craft have to be compact. They shouldn’t add unnecessary weight, or take up too much space in the overall design, while providing the necessary capabilities to achieve their objective. 

Weight:

In almost all applications, especially aerial vehicles, reducing the filter’s weight is critical. The lighter the weight, the greater the operational efficiency that can be achieved. This impacts the agility and range of the vehicle.

The challenge is that when a vehicle needs to be able to operate over a certain distance, it needs to be able to carry sufficient fuel or power for that range. This is where the trade-off occurs. Larger power sources or fuel capacity adds extra weight. As engineers often can’t compromise on this, they then look to how they can leverage weight efficiencies with other components such as RF filters.

Power:

Optimizing power output can contribute to the range and efficiency of the device or vehicle. The lower the insertion loss of the RF filter the less power, or gain, required by surrounding components, and thus the smaller the battery unit required. It will also last longer before needing to be recharged.

There is also the consideration of power handling. Depending on the system it may need to be able to survive high-energy RF spectrum attacks and be able to continue to perform its intended function. As previously mentioned as there is a trend to smaller applications many times this means that the filter, or component, may not be able to survive these high energy situations which could compromise the entire system. Power handling and efficiency is therefore an important consideration. 

Learn about the latest innovations in bandpass frequency and filters!

What are the Biggest Trade-offs Engineers Need to Consider?

When deciding on which RF filter will be best for a specific application, one of the biggest trade-offs is the performance required versus the size of the component. In a small filter, such as Surface Acoustic Wave (SAW) or Bulk Acoustic Wave (BAW) filters, fewer materials are required which is an advantage in terms of size. However, with fewer sections, like in a SAW filter, there's a trade-off between response time and selectivity, impacting overall performance.

Engineers need to decide what their application’s priority is. Do you prioritize refining the accuracy of bandwidth filtering or keeping the component small? For example, if a custom bandpass filter requirement is to pass a 500 MHz frequency but reject anything above 600 MHz, what’s the needed rejection cutoff? 60 dB or 30 dB? Keep in mind that the larger the rejection and closer in to the desired passing signal, the more sections of the filter will be required. This increases the size.

Another trade-off with size is the reliability. Many projects will often have specific RF packaging requirements. However, limiting the size while wanting higher performance isn’t always feasible. When compacting an RF filter, the proximity of components is reduced. This requires careful design to ensure that high power levels do not lead to issues like component arcing, especially in high-power applications.

A third trade-off relates to the amount of signal a filter can process relative to its size. In miniaturizing filters, engineers often use materials with higher dielectric constants. While this approach aids in size reduction, it can sometimes lead to increased insertion loss, a factor that needs careful consideration during design.

Additionally, working on a smaller filter is challenging. Smaller components are harder to adjust and tune accurately. 

Consider Your Trade-off Priorities For Your Next Custom Filter

Ultimately, key priorities will be determined by either the industry or the application. In the military, for example, bandpass filters need to be effective even in dense electromagnetic spectrums and over long distances. But signals tend to lose power the further they travel. Power capacity and signal accuracy are therefore higher priorities than size.

In commercial applications such as 5G cell phone towers, the filters also have to handle high volumes of traffic, but distance is less of a factor as towers can be located closer together. In commercial applications, engineers may be able to make a trade-off in terms of power and save on costs.

As illustrated, there are many considerations that go into deciding the design of a custom RF filter. Typically it’s a process of experimentation. We trade off one priority against the other until a compromise can be reached that will deliver the best performance for your specific application.

Q Microwave is a leader in designing custom RF filters. To find out more about our manufacturing process and how we determine priorities, explore this article.