The need for significant improvements in radar continues to be an ongoing concern for the Department of Defense (DoD). One of the most impressive developments addressing this need is a high-resolution RF beam-forming system using phased-array antennas.
While phased arrays are far from new, a new wafer-scale antenna demonstrates important benefits that can support low-power-density arrays for a variety of radar applications. In the military arena, uses include:
- ground-based midcourse defense radars
- sea-based radars
- future block ballistic-missile defense system radars
- space-based radar constellations with missions that include persistent near real-time tracking of Moving Targets Indications
- high-resolution terrain information
- synthetic aperture radar
High-resolution RF beam-forming technology also has terrific potential in commercial wireless applications, such as ad hoc communication, point-to-point and point-to-multipoint wireless connectivity, radiometry, passive-medical imaging, mobility aids for the visually impaired, surveillance, and collision-avoidance systems.
Designers can take advantage of a compact, reliable, efficient, low-cost semiconductor and ceramic material solution for V-band (50- to 75-GHz) radars that support affordable, full field-of-view (FOV) operation while decreasing the hardware, logistics, and operating costs of current systems.
This solution is realized in a single wafer-scale antenna module (WSAM) that holds all of the antenna components and functions (such as RF beam forming).1 The WSAM concept, which provides an agnostic solution for phased-array antennas, significantly enhances the signal-to-noise ratio (SNR) of the radar systems.
At X-band (8 to 12 GHz), the concept involves a 64-element array containing the antenna elements, as well as all transmitter (Tx) and receiver (Rx) circuits on a standard 8-in. silicon wafer (Fig. 1a). At the higher V-band frequencies, with much smaller antenna elements, the wafer can handle up to 1024 elements and related circuitry (Fig. 1b).
CHALLENGES OF WAFER-SCALE INTEGRATION
Wafer-scale integration isn't without its challenges. For one, the design must maintain a uniform phase and amplitude from the central feed point to all antenna elements. Yet at 60 GHz, line attenuation can be severe, varying from less than 10 dB for 1 mm to about 200 dB for 150 mm.2 Therefore, the wafer uses a balanced Htree and distributed amplification along the signal path to help compensate for line attenuation (Fig. 2).
The width of the transmission line, also called a coplanar waveguide (CPW), is another issue. A wider CPW means more parasitic coupling due to the capacitance and inductance of the transmission line and its surrounding environment. But the narrower the CPW, the higher the resistance caused by the skin effect and surrounding parasitics.
We chose a 4-µm line because parasitic attenuation is more dominant than the skin effect. Though that narrow a line will create a higher resistance, a wider line's larger capacitance and inductance would deteriorate the signal significantly.
DISTRIBUTED AMPLIFIER DESIGN
The distributed amplification scheme employs an innovative approach that's based on load-balancing amplification (LBA) between the matching circuits and the driver amplifier pair (Fig. 3). By changing the ratio of TL1/TL2 (the combined length of TL1 and TL2 equal a multiple of a quarter wavelength), a nominal stable gain of around 10 to 15 dB per LBA stage (including the transmission-line attenuation) can be obtained. The power consumption for the LBA unit is less than 40 mW with a 1.5-V supply for the standard silicon-based process.
Figure 4 illustrates a proposed ultra-wideband (UWB) beam-forming Tx/Rx unit. During transmission, the RF signal is routed and phase-shifted, amplified, and coupled to the transmit antenna. The advantage of such a beam-forming function is that it provides the enhanced range and coverage that's required. It does so by improving the SNR of the transmit channel for a point-to-point or point-to-multipoint broadcast or for a fine-resolution radar function.
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