Electronic Design - Phased array beamforming IC simplifies antenna design

The radio subsystem that relies on antennas to send and receive signals has been in operation for more than 100 years. As accuracy, efficiency, and higher-level indicators become more and more important, these electronic systems will continue to improve and improve. In the past few years, dish antennas have been widely used for transmitting (Tx) and receiving (Rx) signals, where directivity is very important, and after years of optimization, many of these systems can be used well at a relatively low cost. run. These dish antennas have a robotic arm for rotating the radiation direction. They do have some shortcomings, including slow turning, large physical size, poor long-term reliability, and only one radiation pattern or data stream that meets the requirements. Therefore, engineers have turned to advanced phased array antenna technology to improve these features and add new functions. Phased array antenna adopts electric steering mechanism, which has many advantages compared with traditional mechanical steering antenna, such as low height/small size, higher long-term reliability, fast steering , multi-beam, etc. With these advantages, phased array antennas have been widely used in military, satellite communications, Internet of Vehicles, 5G communications and other fields.

Phased Array Technology

A phased array antenna is a collection of antenna elements assembled together, where the radiation pattern of each element is structurally combined with the radiation pattern of the adjacent antenna to form an effective radiation pattern called the main lobe. The main lobe emits radiated energy at the desired position, and according to the design, the antenna is responsible for destructively interfering with signals in useless directions, forming invalid signals and side lobes. The antenna array is designed to maximize the energy radiated by the main lobe while reducing the energy radiated by the side lobe to an acceptable level. The radiation direction can be manipulated by changing the phase of the signal fed into each antenna element. Figure 1 shows how to control the effective beam in the target direction of the linear array by adjusting the phase of the signal in each antenna. As a result, each antenna in the array has independent phase and amplitude settings to form the desired radiation pattern. Since there are no mechanical moving parts, it is easy to understand the properties of the rapid beam steering in the phased array . IC-based semiconductor phase adjustment can be completed within a few nanoseconds, so that we can change the direction of the radiation pattern and respond quickly to new threats or users. Similarly, we can change from a radiation beam to an effective null point to absorb the interference signal, making the object appear invisible, as is the case with invisible aircraft. Repositioning the radiation pattern or changing to the effective zero point, these changes can be done almost immediately, because we can use IC-based devices instead of mechanical parts to electrically change the phase setting. Another advantage of a phased array antenna over a mechanical antenna is that it can radiate multiple beams at the same time, so it can track multiple targets or manage user data for multiple data streams. This is achieved by digital signal processing of multiple data streams at baseband frequencies.

A typical implementation of the array uses patch antenna elements arranged in equal intervals in rows and columns, which adopts a 4*4 design, which means there are 16 elements in total. Figure 2 shows a small 4*4 array in which the patch antenna is a radiator. In ground-based radar systems, such antenna arrays can become very large, possibly with more than 100,000 elements.

In the design, the trade-off relationship between the size of the array and the power of each radiating element should be considered. These elements will affect the directivity of the beam and the effective radiated power. The performance of the antenna can be predicted by examining some common quality factors. Usually, antenna designers will look at antenna gain, effective isotropic radiated power (EIRP), and Gt/Tn. There are some basic equations that can be used to describe the parameters shown in the following equations. We can see that the antenna gain and EIRP are proportional to the number of elements in the array.

Among them: N = number of elements; Ge = element gain; Gt = antenna gain; Pt = total transmitter power; Pe = power of each element; Tn = noise temperature.

Another key aspect of phased array antenna design is the spacing of antenna elements. Once we determine the system goal by setting the number of components, the physical array diameter largely depends on the size limit of each unit component, which is less than about one-half of the wavelength, because it can prevent grating lobes. The grating lobes are equivalent to the energy radiated in useless directions. This places strict requirements on the electronic devices that enter the array, which must be small in size, low in power, and light in weight. Half-wavelength spacing is particularly challenging for design at higher frequencies, because the length of each unit component becomes smaller. This pushes up the integration of higher frequency ICs, prompts packaging solutions to become more advanced, and simplifies the increasingly difficult thermal management technology.

When we build the entire antenna, the array design faces many challenges, including control circuit routing, power management, pulse circuits, heat dissipation management, environmental considerations, etc. There is a huge driving force in the industry that urges us to move towards small and light low-profile arrays. The traditional circuit board structure uses a small PCB board, on which the electronic components are fed vertically into the back of the antenna PCB. In the past 20 years, this method has been continuously improved to continuously reduce the size of the circuit board, thereby reducing the depth of the antenna. The next-generation design shifts from this board structure to a flat-panel method, where each IC has a sufficiently high integration level that can be simply mounted on the back of the antenna board, greatly reducing the depth of the antenna and making them easier Load it into a portable application or an onboard application. In Figure 3, the left image shows the golden patch antenna element on the top of the PCB, and the right image shows the antenna analog front end on the bottom of the PCB. This is only a subset of the antenna, where a frequency conversion stage may occur at one end of the antenna; it is also a distribution network that is responsible for routing from a single RF input to the entire array. Obviously, more integrated ICs significantly reduce the challenges in antenna design, and as antennas become smaller and smaller, more and more electronic components are integrated into smaller and smaller spaces, and antenna design requires new Semiconductor technology to help improve the feasibility of the solution.
The antenna patch on the top of the PCB, and the IC is located on the back of the antenna PCB.
Digital beamforming and analog beamforming
Most phased array antennas designed in the past few years have used analog beamforming technology, in which the phase adjustment is performed at RF or IF frequencies, and the entire antenna uses a set of data converters. People are paying more and more attention to digital beamforming, where each antenna element has a set of data converters, and the phase adjustment is done digitally in the FPGA or some data converters. Digital beamforming has many benefits, from the ability to easily transmit multiple beams to even instantly changing the number of beams. This superior flexibility is extremely attractive in many applications, and it also plays a role in promoting its popularization. Continuous improvements in data converters have reduced power consumption and expanded to higher frequencies. RF sampling in the L-band and S-band makes this technology applicable to radar systems. When considering both analog and digital beamforming options, many factors need to be considered, but the analysis usually depends on the number of beams required, power consumption, and cost targets. The digital beamforming method usually has high power consumption because each component is equipped with a data converter, but it is extremely flexible and convenient in forming multiple beams. Data converters also require a higher dynamic range, because beamforming that rejects blocking can only be done after digitization.