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Understanding True Time Delay for Phased Arrays

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by James Cheng, Senior Product Line Manager, Qorvo

Phased array antennas use phase shifters, true time delay, or a combination to point the summed beam more accurately toward the desired direction within an array’s steered angle. This article reviews both methods and how wider bandwidth antenna arrays are driving the use of true time delay in their system design.

Phased arrays change the shape and direction of the radiation pattern without physically moving the antenna. The antennas are uniquely placed into a larger array using individual elements—summing them up to provide more gain performance and directing the signal within the array’s steering angle limits, as shown in Figure 1.

Figure 1: Phased arrays steer a signal electronically, eliminating the need to physically reposition the antenna. They use multiple smaller antennas in a precise arrangement, controlling the signal sent to each antenna element to increase signal strength and direct its transmission.

In today’s phased array systems, the bandwidth is increasing to expand their utility and flexibility. Wider bandwidth introduces system challenges that affect the phase shifting of the beam. Because of this trend, many AESA systems require true time delay to eliminate beam squint in larger bandwidth circumstances.

Phased Arrays Defined

Phased array size is inversely proportional to the operational frequency, so the higher the frequency, the smaller the antenna element spacing. The opposite is true for lower frequency applications.

So, how is the beam steering enabled? Traditionally, for narrowband arrays, the desired signal delay is converted at a given frequency using phase shifters, with each antenna element fed with different phase shifters. Therefore, the beam direction of the array can be steered by changing the phase shift between each element to form a beam at the angle of interest.

Figure 2: In a two-element antenna array, phase shifters control beam direction and improve efficiency by adjusting the signal timing between the elements

For example, assume two antenna elements separated by distance “d,” as shown in Figure 2. The phase shifts between these two elements, altering the beam direction. The beam can be steered using a phase shifter at the alternative antenna element to alter its direction and improve antenna efficiency.

Figure 3: The image demonstrates how waveform steering in an antenna array shapes the field pattern: a focused main lobe with minimized side lobes and measured phase angle data

In Figure 3, the steering of a waveform in the antenna array creates a main lobe at a given angle and minimizes the side lobes. Note the measured data of phase angle and field pattern of these lobes.

Figure 4: Two common phased array antenna systems differ in their module configuration: Passive Electronically Steered Arrays (PESA) use a single shared transmit/receive module. In contrast, Active Electronically Steered Arrays (AESA) use dedicated modules for each element.

Two types of phased array antenna systems are commonly used (Figure 4). The Passive Electronically Steered Array (PESA) uses a single transmit/receive module shared across all antenna elements. The Active Electronically Steered Array (AESA) uses phased array antennas with transmit/receive modules dedicated to each element.

In AESAs, there are separate transmitters controlled by microcontrollers for each antenna element. These AESAs are more advanced than PESAs and can transmit several radio waves at various frequencies simultaneously in different directions. As higher performance and higher resolution systems are developed, the bandwidth requirements for the waveform increase. This presents a problem for AESAs that traditionally steer the beam using phase shifters because the beam will squint as a function of frequency. Beam squint can be calculated using the following equation:

For these wide instantaneous bandwidth waveforms and narrow beam widths, beam squint can be enough to steer the beam off target, resulting in poor signal quality and reduced accuracy and resolution.

Phase Shifters Versus True Time Delay

AESAs use phase shifters or a time delay circuit or a mix of both to point the signal beam in the desired direction within the array’s steering angle limits. As shown in Figure 5, phase shifters are used for directing the beams in phased array antennas and help improve efficiency in narrowband systems. Phase shifters have wide market dominance and provide a fixed insertion phase difference between the two states. They are typically used in applications with lower bandwidth because wideband phase shifting is more difficult and often comes with a penalty of increased insertion loss and phase accuracy across the operational frequency bandwidth.

Figure 5: Phase shifters direct beams in phased array antennas, improving efficiency in narrowband systems, but can cause beam distortion (“squint”) over wider frequencies due to the lack of true time delay

The two states are only slightly different in time delay, differing in path length by less than a wavelength. Phase shifters steer the beam at each antenna element but do not provide true time delay. Without this true time delay, the beam distorts, or “squints,” over a more extensive frequency range. With newer, broader bandwidth array systems, beam squint has become more of an issue. True time delay units are used to mitigate this squinting effect.

Time delay units provide many wavelengths of phase shifting, and the phase shift is exactly proportional to the frequency. This allows the group delay difference between the two states to create a flat phase over the entire frequency bandwidth. The time delay unit provides a noticeable benefit of squint reduction over the bandwidth frequency, improving radar image resolution over the wider bandwidth.

True Time Delay MMICs

Time delay can be accomplished in many ways, e.g., coax, optical, microstrip, and strip-line. Electronic methods using MMICs are more popular due to their compact size and cost-benefit. Typical multi-bit time delay units include switches, time delay elements and equalizers to form both reference path and time delay pathways, as shown in (Figure 6).

Figure 6: The time delay unit provides a noticeable benefit of squint reduction over the bandwidth frequency, improving radar image resolution over the wider bandwidth

Overall delay range and delay steps can be generated by switching different path combinations. A reference line and delay element are typically created using different lengths of transmission lines. As the line length increases, so does the insertion loss and frequency. An equalizer is typically used to improve the overall time delay flatness over the frequency range.

Recent semiconductor developments and advances in modeling have helped to enable physically smaller delay circuits, which are helpful in high frequency array applications. Additionally, different semiconductor technologies like CMOS, GaAs and MEMS can help optimize performance requirements for some applications.

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