Feature: Communications design
By having wider beamwidth, subarrays are more tolerant to beam squint effects
Phase shifters and true time delays Instantaneous bandwidth (IBW) can be defined as the frequency band where no tuning is required to stay within the target performance criteria set by the system design. TTDs exhibit constant phase slope over frequency, which
means ESAs implemented with TTDs instead of PSs do not have the beam squint effect. As a result, TTD-based ESAs are more convenient for high IBW applications. PSs exhibit constant phase over their operating frequency
range; hence, a particular phase shifter setting throughout the system results in different beam-steering angles for different frequencies. As a result, PS-based arrays tend to have narrower IBW compared to TTD-based arrays. This phenomenon is called “beam squint” and is calculated is maximum
Combining the benefits of true time delays and phase shifters
By Bilgin Kiziltas, Field Applications Engineer, Analog Devices
H
ere we will discuss the strengths and weaknesses of two electronic beamforming techniques: phase shifters (PSs) and true time delays (TTDs). When combined to form a hybrid beamforming architecture, these two methods offer a simpler design, with better
size, weight, power and cost. Electronically scanned arrays (ESAs) utilise PSs, TTDs
or a mix of both to point the summed beam in the desired direction within an array’s steering limits.
24 September 2022
www.electronicsworld.co.uk
with Equation 1, where Δθ is peak squint angle, θ0 beam angle, f0
frequency. (1) Using Equation 1 we can calculate that Δθ at worst case – that
is, at the low frequency edge (carrier at 3GHz and instantaneous signal at 2.9GHz), which is around 1.15° for a ±30° beam- steering angle system for 3GHz signal with 100MHz IBW. Changing the beam-steering angle to ±60° and IBW to 200MHz results in around 8.11° beam squint at worst. Thus, it is evident that TTDs are a better choice, even in radar applications. Arguably, phase shifter dominance in ESAs can be attributed to their design simplicity and cost advantage over TTDs.
TTDs vs PSs If we had a TTD that meets the system requirements, how could it be reasonable to use PSs in the same signal chain? To investigate, we will examine a 32 × 32 square ESA with d = λ/2 lattice spacing (d) between antenna elements desired to operate between 8GHz and 12GHz with ±60° scanning angle, with EIRP criteria assumed to be met for all scenarios; see Figure 1. In this example, the system beamwidth in both azimuth and
elevation would be ≅ 3.17° at boresight (θ = 0°) and ≅ 6.35° at the max scan angle (θ = 60°) by the half power beamwidth approximation formula for a uniform linear array given in Equation 2, where N is the number of elements on one axis and θB
is the beamwidth in degrees on the same axis. is carrier frequency and f is instantaneous signal
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