When selecting a filter implementation, one factor that is common across all frequencies is optimizing the size of the filter given the application and the required performance. At mmWave frequencies this can be prove to be a particularly interesting problem, given the change in the physical dimensions of the system as one moves from say 600MHz to 38GHz.
As 5G innovation forges on, radio systems continue to emerge. Each system has a range of requirements, including specific RF filter performance needs, and it’s up to the 5G FR2 Ecosystem of suppliers to meet that demand. In response, Knowles Precision Devices (KPD) supports a wide variety of 5G radio applications.
To choose the right filter for your application, you'll need to evaluate filter type, identify the specific filter technology that best suits your application, and ensure the filter meets your required specifications. This post is designed to serve as a quick reference on the common terms that are used to discuss filter type, technology, and specifications.
To start, there are four key Filter behaviors that sort them into types: Low Pass, High Pass, Band Pass, and Band Stop.
The quality factor (Q) of a resonator is expressed as the ratio of stored versus lost energy per oscillation cycle. Overall losses through a resonator increase as Q factor drops and will increase more rapidly with frequency for lower values of resonator Q. However, truly understanding how Q factor is determined is a bit more intricate. Let’s take a closer look.
The Knowles Precision Devices DLI brand of microstrip bandpass filters offers innovative filter topologies yielding excellent performance in a small footprint when fabricated on high DK temp stable ceramic substrate materials. Since our customers use our surface-mount filters in a wide range of applications, we’ve designed our devices so they can be used on most RF PCB materials. To ensure our customers achieve optimal performance and are successful with our filters, we have compiled a number of best practices and resources, which are outlined below.
Many microwave applications, such as repeaters, and electronic warfare equipment, require increased spectral resolution. This means these devices only need to look at a narrow slice of a given band. Filters that are optimized for the whole band, such us our planar microstrip devices, are too broadband for these applications. Likewise, traditional high Q filters, such as waveguide devices, are often too large to consider using in these types of applications.
With more than 2,000 satellites currently orbiting the Earth, and that number expected to quintuple in the next 10 years, the demand for space-ready components is exponentially increasing (Figure 1). At the same time, the technology needed to control and transmit satellite data has changed from mechanically controlled parabolic or dish technology to active electronically steered arrays (AESAs).
An Ideal Filter
The Ideal Filter would have unit gain (0dB) in its pass band and a gain of zero (-infinity dB) in its stop band. Between pass band and stop band there would be no indecision and would transition from 0dB to -infinity dB asymptotically. It would pass only the required frequencies without adding or subtracting anything from the signal and like a very discrete and fastidious butler we would not see it - just its perfect management of the frequencies in its care.
The US MIL-STD-461 specification manages electromagnetic interference emissions by setting limits on the levels that can be emitted from electrical equipment. This specification also sets regulation to control equipment susceptibility to external noise sources and establishes guidelines for properly measuring the relevant equipment features.
EMI filtering plays an important role in reducing noise that could interfere with other devices; in medical or defense applications, for example, false alarms due to external interference could be detrimental. Here, we will continue our EMI filtering exploration with application and installation considerations. For earlier reading, review EMI filtering basics and filter performance.