Aerospace, Military & Defence
Physical size allocations for RF Electronics in digital beam-forming phased arrays
By Peter Delos, Analog Devices P
hased array radars and active electronically scanned arrays (AESA) have been in use and deployed
within the aerospace and defense market for well over a decade. This period began with primarily analog beam-forming systems, with continual migration to higher levels of digital beam-forming. System engineering objectives continually desire near elemental digital beam- forming implementations for maximum flexibility and programmability. There are many challenges associated with the migration to near elemental digital beam-forming. The challenges range from calibration, digital control, distribution of clocks, LO, power, processing the volume of data, and the physical size constraints of the electronics. The prolific advancement of RF ICs for the wireless industry continues to enable the ability for higher levels of integration in RF designs and now practical
implementations of every element digital beam-forming arrays is becoming a reality. In this article, we focus on the physical
size requirements for the electronics. The physical size requirements as a function of the operating frequency are discussed, and practical implementation methods are reviewed.
Antenna element spacing vs. frequency
First, consider the antenna element spacing as a function of frequency. To avoid grating lobes, an element spacing of /2 or less is required, where is the operating frequency wavelength.
Polarisation diversity is also becoming
a desired system objective. This feature provides the ability to program a variety of antenna polarisations including horizontal, vertical, or both left and right hand circular polarisation. The antenna element implementation to achieve this feature is a radiating element with two ports, where each port radiates with orthogonal polarisations. By controlling the relative phase and amplitude of each port, the varied polarisations are created. Although a significant benefit for the system, this feature unfortunately doubles the number of antenna ports required and complicates the supporting electronics. Figure 1 shows the element spacing
vs. frequency, assuming there is a /2 antenna element spacing implementation. With these physical size constraints outlined, the RF subsystems behind the antenna can be evaluated to assess implementations required to meet the electronic channel spacing vs. frequency.
Waveform generator and receiver channel spacing Figure 2 shows an evaluation board for one of the Analog Devices transceiver products. This board contains two
Figure 1. Element spacing vs. frequency Table 1. /2 spacing for select frequencies 20 March 2019 Components in Electronics
transceivers. Each transceiver contains two transmit and receive channels (see Figure 3) and, thus, four complete waveform generators and receivers are implemented. The board also includes a clock IC and several other I/O features for evaluation of the parts.
Although the board was not intended for the highest level of integration possible, the board provides insight into practical size limits for the waveform generator and receiver section. It is quickly apparent from the board that the
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