Making Fully Digital Beamforming for Radar and Electronic Warfare Applications a Reality
While the basic physics of beamforming were first identified more than a century ago, it wasn’t until the past two decades that advances in supporting technology enabled the rapid expansion of the types of applications that can take advantage of this technology.
As early adopters of beamforming technology in the 1960s, aerospace and defense organizations have a lot of experience using the initial large-scale active electronically scanned arrays (AESAs) for military radar tracking applications. As a result of the innovations led by military organizations for several decades, commercial applications such as weather detection and radio broadcasting were able to benefit from the use of beamforming technologies as well.
But today, the operational frequencies of the targets of interest are increasing. As a result, the wavelengths of these signals are shorter, which means the density of the array is increasing since antenna spacing needs to be set at one-half the wavelength. For example, at 25 GHz, the wavelength in free space is approximately 12 mm (0.47"), leading to half-wave spacing for antennas of 6 mm (0.24"). And the denser the array, the more important it is to be able to steer beams to avoid interference, especially when transmitting signals.
This article provides a brief overview of how the technology used inside arrays, such as passive components, has evolved to allow for the possibility of fully digital beamforming today and how RF designers can overcome the challenges associated with designing a fully digital array that can be used for military applications.
A Brief Overview of Beamforming Options
Over the years, beamforming technology has evolved significantly. Configurations have changed from traditional large analog applications to a hybrid digital/analog approach and to the introduction of fully digital arrays in the past decade. While digital beamforming is opening a new range of possibilities for phased array technology, it doesn’t mean all phased array applications should make the shift to digital. Instead, there are multiple considerations to make when thinking about the best beamforming technology for your application including the number of beams required, power dissipation, and budget.
This table shows how wavelength and dielectric constant are inversely proportional.
The diagram in Figure 1 provides a simplified overview of the key differences among these three beamforming approaches by showing the RF path to the single amplifier and ADC pairs. More specifically, with analog beam-forming, one ADC is used for the entire array. This means that adjustments to the array with a simple software rewrite for all the antenna elements is limited to the functionality of what that single ADC can do. Multiple beams can be supported but a phase adjustment channel is needed for each additional beam, which will typically increase costs and power consumption.
Analog beamforming is generally a good fit for low-cost, low-power, low-beam-count systems. On the opposite end of the spectrum is fully digital beamforming, which has a dedicated ADC for every antenna element. This means multiple beams can be simultaneously acquired and transmitted, improving dynamic range. This option typically uses more power and costs more to configure than an analog option.
A common compromise today that offers more flexibility than an analog option but reduces costs versus a fully digital option is a hybrid beamforming approach that uses subarrays of analog beamforming, followed by a digital combination of the subarray signals; for example, there may be one ADC in place for every four or 16 elements. This hybrid approach is popular for applications where digital beamforming is desired but fully digital beamforming may not be practical due to size, power, or cost constraints.
Fully Digital Beamforming – A Good Fit for Emerging Military Applications
As the signals the military is interested in monitoring are evolving to higher operational frequencies, there are a number of benefits that can be achieved by using fully digital beamforming for some applications. First, multiple simultaneous beams can be detected and transmitted since beams can be split in multiple directions at the same time. Plus, if you want to change the direction of antennas in the array when sending or receiving signals, that can be done much faster since each antenna is individually controlled through software.
Additionally, dynamic range is improved as more ADCs are added to an application and when the ADC is moved closer to the antenna element, like it is in a fully digital array. For example, if an array has 256 antenna elements and each element has its own ADC, the dynamic range is 256 times greater than it would be in the same analog application that has one ADC for all 256 elements. Therefore, detection is more flexible and powerful, meaning more signals can be detected.
Finally, since the job of each antenna element is software-defined, each element can be individually controlled and tuned, which means one system can be used for multiple missions. When one array can serve as a multi-function device with antennas pointing in different directions, a radar system can monitor for signals coming from multiple directions. Therefore, the limited space in some environments, like on a ship, can be used more efficiently while also achieving more comprehensive radar coverage.
While these benefits make it clear that digital beamforming is an excellent option for military applications, the physical size constraints for the electronics and passive components needed behind every radiating element — especially for high-frequency applications — make these systems challenging but not impossible to design. Let’s take a quick look at how to overcome these challenges today.
Addressing Size Constraints in Arrays Using Fully Digital Beamforming
Implementing fully digital beamforming means you need to integrate small form factor yet high-performance passives such as filters, capacitors, and resistors. Recent advances in substrates — such as the development of GaN — have made it possible to make these systems much smaller, more affordable, and more lightweight. The table shows the relationship between dielectric constant and wavelength at 25 GHz for three common dielectric materials, as well as three custom substrates developed by Knowles Precision Devices (PG, CF, and CG).
In addition to the substrate innovation enabling development of much smaller passive components, parts such as amplifiers and ADCs have rapidly become more powerful and inexpensive as the commercial world has pushed these technologies with the expansion of LG, LTE, and now 5G. Therefore, unlike the past when the commercial world leaned on the innovations of military and aerospace entities, the military can now take advantage of these commercial technologies.
With the evolution of high K dielectrics and by selecting to work with a company that has the knowledge to seamlessly integrate multiple passive components into a single circuit, the miniaturization of passive RF components to enable fully digital beamforming is possible. Thus, military applications can use array technology to develop multi-function devices that can save space while also receiving Ka-band signals.
This article was written by Peter Matthews, Senior Technical Marketing Manager at Knowles Precision Devices, Cazenovia, NY. For more information, visit here .
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