To improve performance, wireless communication and radar systems are increasingly demanding antenna architectures. Only those antennas that consume less power than conventional mechanically operated dish antennas can achieve many new applications. In addition to these requirements, there is a need to quickly relocate new threats or new users, transfer multiple data streams, and extend working life at an ultra-low cost. Some applications need to counteract the effects of the input blocking signal and reduce the probability of interception. Phased antenna designs that are sweeping across the industry provide solutions to these challenges. Advanced semiconductor technology has been used to address the shortcomings of phased array antennas in the past to ultimately reduce the size, weight and power of these solutions.
This article will briefly describe the advantages of existing antenna solutions and electronically controlled antennas. On this basis, this article will describe how the development of semiconductor technology can help achieve the goal of improving the electronically controlled antenna SWaP-C, and then illustrate how ADI technology can do this.
Radio subsystems that rely on antennas to transmit and receive signals have been in operation for more than 100 years. As accuracy, efficiency and higher-level metrics become 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 directionality is critical, and after years of optimization, many of these systems are well at relatively low cost. run. These dished antennas have a robotic arm for rotating the direction of the radiation, and they do have some drawbacks, including slow steering, large physical size, poor long-term reliability, and only one radiation pattern or data stream that meets the requirements. As a result, engineers have turned to advanced phased array antenna technology to improve these features and add new features.
Phased array antennas use an electric steering mechanism that offers many advantages over traditional mechanical steering antennas, such as low height/small size, better long-term reliability, fast steering, multiple beams, and more. With these advantages, phased arrays have been widely used in defense applications, satellite communications, and 5G telecommunications, including car networking.
Phased array technology
A phased array antenna is a collection of assembled antenna elements in which the radiation pattern of each element is structurally combined with the radiation pattern of an adjacent antenna to form an effective radiation pattern called the main lobe. The main lobe emits radiant energy at a desired location, and depending on the design, the antenna is responsible for destructively interfering with signals in the unwanted direction, forming an ineffective signal and side lobes. The antenna array is designed to maximize the energy of the main lobe radiation while reducing the energy of the sidelobe radiation to an acceptable level. The direction of radiation can be manipulated by varying the phase of the signal fed into each antenna element.
Figure 1 shows how the effective beam is controlled 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 an independent phase and amplitude setting to form the desired radiation pattern. Since there are no mechanical moving parts, it is easy to understand the properties of the beam steering in the phased array.
Figure 1. Basic theoretical diagram of phased array components.
IC-based semiconductor phase adjustment can be done in a few nanoseconds, so we can change the direction of the radiation pattern and respond quickly to new threats or users. Similarly, we can change the radiation beam into an effective zero to absorb the signal of the interferer, making the object appear invisible, as is the case with stealth aircraft. Repositioning the radiation pattern or changing to an effective zero, these changes can be done almost immediately, because we can use an IC-based device instead of a mechanical part to electrically change the phase setting. Another advantage of a phased array antenna over a mechanical antenna is that it can radiate multiple beams simultaneously, thereby tracking multiple targets or managing user data for multiple data streams. This is accomplished by digitally processing multiple data streams at baseband frequencies.
A typical implementation of the array uses patch antenna elements arranged in equally spaced rows and columns in a 4x4 design, meaning that there are a total of 16 components. Figure 2 shows a small 4 x 4 array in which the patch antenna is a radiator. In terrestrial radar systems, such antenna arrays can become very large and may have more than 100,000 components.
Figure 2. Radiation pattern display of a 4×4 component array.
The trade-off between array size and the power of each radiating element is considered in the design, which affects the beam directivity and effective radiated power. The performance of the antenna can be predicted by examining some common quality factors. Typically, antenna designers look at antenna gain, effective directional radiated power (EIRP), and Gt/Tn. There are some basic equations that can be used to describe these parameters as shown in the following equations. We can see that the antenna gain and EIRP are proportional to the number of components in the array. This can lead to large arrays that are common in terrestrial radar applications.
N = number of components
Ge = component gain
Gt = line gain
Pt = total transmitter power
Pe = power of each component
Tn = noise temperature
Another key aspect of phased array antenna design is the spacing of the antenna elements. Once we have determined the system goals by setting the number of components, the physical array diameter is highly dependent on the size limit of each cell component, which is less than about one-half wavelength, as this prevents the grating lobes. The grating lobes correspond to the energy radiated in the unwanted direction. This imposes stringent requirements on the electronics that enter the array and must be small, low power, and lightweight. Half-wavelength spacing is particularly challenging for design at higher frequencies because the length of each of the unit components becomes smaller. This pushes up the integration of higher frequency ICs, making packaging solutions more advanced and simplifying the increasingly difficult thermal management techniques.
When we build the entire antenna, the array design faces many challenges, including control circuit, power management, pulse circuits, thermal management, and environmental considerations. There is a huge push in the industry that has prompted us to move toward low-profile, low-profile arrays. A conventional circuit board structure uses a small PCB board on which electronic components are fed vertically to the back of the antenna PCB. In the past 20 years, this method has been continuously improved to continuously reduce the size of the board, thereby reducing the depth of the antenna. The next-generation design moves from this board structure to a flat-panel approach, where each IC has a high enough integration to be easily mounted on the back of the antenna board, greatly reducing the depth of the antenna, making them easier Loaded into portable applications or onboard applications.
In Figure 3, the left image shows the gold patch antenna component 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 can 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, antenna designs need new Semiconductor technology to help improve the viability of the solution.
Figure 3. Flat panel array with the antenna patch on top of the PCB and the IC on the back of the antenna PCB.
Digital Beam Synthesis and Analog Beam Synthesis
Most phased array antennas designed in the past few years have used analog beamforming techniques in which the phase adjustment is performed at RF or IF frequencies and the entire antenna uses a set of data converters. There is increasing interest in digital beamforming, where each antenna element has a set of data converters and phase adjustments are done digitally in an FPGA or some data converter. Digital beamforming has many advantages, from the ability to easily transmit multiple beams, and even instantly change the number of beams. This superior flexibility is extremely attractive in many applications and is also driving its popularity. Continuous improvements in data converters reduce power consumption and extend to higher frequencies, and RF sampling in the L-band and S-band systems allows this technology to be used in radar systems.
There are a number of factors to consider when considering both analog and digital beamforming options, but the analysis typically depends on the number of beams required, power consumption, and cost targets. The digital beamforming method has a high power consumption because each component is combined 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. Analog beamforming can support multiple beams, but each beam requires an additional phase adjustment channel. For example, to form a 100-beam system, the number of RF phase shifters of a 1-beam system needs to be multiplied by 100, so the cost considerations of the data converter and phase adjustment IC may vary depending on the number of beams.
Similarly, for analog beamforming methods that can utilize passive phase shifters, their power consumption is typically low, but as the number of beams increases, if additional gain stages are needed to drive the distribution network, power consumption will also increase. A common compromise is a hybrid beamforming approach in which there is an analog beamforming sub-array followed by some digital combination of sub-array signals. This is an increasingly popular area in the industry and will continue to grow in the years to come.
A standard pulse radar system transmits a signal that can be reflected from an object, and the radar waits for a return pulse to map the field of view of the antenna. In the past few years, this antenna front-end solution has used discrete components, which are likely to use gallium arsenide technology. The I C components used as building blocks for these phased array antennas are shown in FIG.
Figure 4. Example of a typical RF front end for a phased array antenna.
They include a phase shifter for adjusting the phase of each antenna element (final control antenna), an attenuator that can make the beam tapered, a power amplifier for transmitting signals, and a low noise amplifier for receiving signals. There is also a switch for switching between transmission and reception. In the past embodiments, each of these ICs may be placed in a 5mm x 5mm package, and more advanced solutions may implement this functionality with an integrated monolithic single-channel GaAs IC.
The popularity of phased array antennas in recent years is inseparable from the advancement of semiconductor technology. Advanced nodes in SiGe BiCMOS, SOI (Silicon On Insulator) and bulk CMOS integrate the combined digital circuitry for steering in the array and the RF signal path for phase and amplitude adjustment into a single IC. Today, we have been able to implement multi-channel beamforming ICs that can adjust gain and phase in a 4-channel configuration and support up to 32 channels for millimeter-wave designs.
In some low power examples, silicon-based I C is likely to provide a single-chip solution for all of the above functions. In high-power applications, GaN-based power amplifiers significantly increase power density to accommodate the needs of phased-array antenna unit components, which are traditionally based on traveling wave tube (TWT)-based power amplifiers or based on Low power GaAs power amplifier servo.
In airborne applications, we have seen an increasing trend in flat panel architectures because of the power-added efficiency (PAE) advantages of GaN technology. GaN also enables large ground-based radars to move from TWT-driven dish antennas to phased array-based antenna technology. We are currently able to use a single GaN IC that can deliver more than 100 watts of power with a PAE of over 50%. Combining this PAE level with the low duty cycle of the radar application determines the size, weight and cost of the antenna array.
In addition to the pure power capability of GaN, an additional benefit compared to existing GaAs IC solutions is the reduced size. Comparing X-band 6 W to 8 W GaAs power amplifiers with GaN-based solutions can reduce footprint by 50% or more. This reduction in footprint is of significant significance when these electronic components are assembled into the unit components of a phased array antenna.
Analog Phase Insulator IC from ADI
ADI has developed integrated analog beamforming ICs that support a range of applications including radar, satellite communications, and 5G communications. The ADAR1000X-/Ku-band beamforming IC is a 4-channel device that covers the band from 8 GHz to 16 GHz and operates in Time Division Duplex (TDD) mode with its transmitter and receiver integrated into one IC. The device is ideal for X-band radar applications and Ku-band satellite communications, where the IC can be configured to operate in transceiver mode or receiver only mode. Housed in a 7 mm x 7 mm QFN surface mount package, this 4-channel IC can be easily integrated into a flat panel array, consuming only 240 mW/channel in transmit mode and 160 mW/channel in receive mode. The transceiver and receiver channels are directly available and can be used externally with the Front End Module (FEM) from AD I.
Figure 5 shows gain and phase control with full 360° phase coverage, which enables phase steps of less than 2.8° and gain control better than 31 dB. The ADAR1000 integrates on-chip memory to store up to 121 beam states, one of which contains all phase and gain settings for the entire IC. The transmitter provides approximately 19 dB of gain and 15 dBm of saturated power with a receive gain of approximately 14 dB. Another key indicator is the phase change of the gain control, which is approximately 3° in the 20 dB range. Similarly, the phase control gain varies by approximately 0.25 dB over the entire 360° phase coverage, mitigating calibration challenges.
Figure 5. ADAR1000 Tx gain/return loss and phase/gain control at frequency = 11.5 GHz.
Developed for analog phased array applications or hybrid array architectures, the hybrid array architecture combines some digital beamforming techniques with analog beamforming. Analog Devices offers complete solutions from antenna to location, including data converters, frequency conversion, analog beamforming ICs, and front-end modules. The combined chipset enables Analog Devices to combine multiple functions and optimize the IC to easily implement antenna designs for customers.
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