Design of Lumped-Component Programmable Delay Elements for Ultra-Wideband Beamforming

We introduce a ladder filter-based programmable time-delay element for beamforming in ultra-wideband (UWB) systems. Such a lumped-element realization becomes possible by approximating e^-std as a ratio of polynomials (based on Taylor and Padé expansions). When compared with conventional methods based on the tapped delay-line architecture, the proposed technique achieves lower power dissipation, higher delay range and resolution, and better area efficiency. A prototype delay line designed for the 3.1-10.6 GHz UWB range achieves a delay range of 140 ps and a gain range of -30 dB to +10 dB. Fabricated in a 0.25 μm SiGe BiCMOS process, the delay element occupies an active area of 1 mm² and consumes 53 mW from a 2.5 V supply. A four-antenna beamforming system using the delay element can achieve a scanning range of ±61^° with 0.86 ^° resolution for an antenna spacing of 15 mm.

I. Introduction

Pulse-Based ultra-wideband (UWB) radio technologies operating in the unlicensed 3.1–10.6 GHz band are used in radar and imaging. Using a multi-antenna system with a variable delay in each path imparts spatial selectivity to the antenna array, thereby enabling beamforming. The basic idea behind such a wideband receiver is explained with the aid of Fig. 1(a), which shows a three-antenna example. The individual channel outputs are combined after passing through front-end low-noise amplifiers and variable time-delay circuits. Denoting the velocity of light, antenna spacing, and the time delay in the antenna paths by , , and , it is easy to see that signals arriving at an angle to the broadside of the array interfere constructively, resulting in the maximum “spatial” gain. Electronically varying the delay and the gain of each antenna path imparts tunable spatial selectivity to the antenna array. The maximum scan angle and steering resolution of the array are functions of the maximum delay and time step of the delay element respectively. A typical pulse shape used in such applications, which meets spectral emission mask requirements [1], is a high-order derivative of the Gaussian pulse. In this work, like in [2], we use a Gaussian monopulse, which is the first derivative of a Gaussian pulse¹

A Gaussian monopulse was chosen to specifically suit laboratory time domain measurement capabilities, though it does not fit the FCC spectral mask as it does not roll off fast enough at low frequencies. However, this has no bearing on the design principles presented in this work.

and is approximated by , where and denote the amplitude and delay, respectively, and is related to the width of the pulse, as shown in Fig. 1(a). Due to the broadband nature of the input pulse, it is important that the group delay of the signal chain in each antenna path is largely flat in the frequency range of interest. This is a design challenge unique to wideband systems—in contrast to narrowband beamformers, where delay can be approximated by phase shift. For an excellent review of the applications and design considerations for UWB array processing, as well as the challenges involved in the design of broadband beamformers, the reader is referred to [3]. As seen from that work, the key difficulty is the realization of a broadband tunable delay element, whose group delay is largely flat over the signal bandwidth.