A 2-GHz Bandwidth, 0.25–1.7 ns True-Time-Delay Element Using a Variable-Order All-Pass Filter Architecture in 0.13 $\mu$ m CMOS

An all-pass filter architecture that can be generalized to high orders, and can be realized using active circuits is proposed. Using this, a compact true-time-delay element with a widely tunable delay and a large delay-bandwidth product (DBW) is demonstrated. This is useful for beamforming and equalization in the lower GHz range where the use of LC or transmission line-based solutions to realize large delays is infeasible. Coarse tuning of delay is realized by changing the filter's order while keeping the bandwidth constant and fine tuning is implemented by changing the filter's bandwidth utilizing the delay-bandwidth tradeoff. A test chip fabricated in 0.13 μm CMOS process demonstrates a delay tuning range of 250 ps-1.7ns, over a bandwidth of 2 GHz, while maintaining a magnitude deviation of ±0.7 dB. The filter achieves a DBW of 3.4 and a delay per unit area of 5.8 ns/mm². The filter has a worst case noise figure of 23 dB, and -40 dB intermodulation (IM3) distortion for 37 mVppd inputs. The chip occupies an active area of 0.6 mm², and dissipates 112 mW-364 mW of power between its minimum and maximum delay settings. Computed radiation pattern with four antennas spaced λ_fmax/2 apart shows ±90° beam steering off broadside.

I. Introduction

Delay lines are important building blocks of analog signal processing circuits such as beamforming systems [1], [2], and continuous-time equalizers [3]. Many of these applications process wideband signals and need true-time-delay elements (as opposed to emulating delays with phase shifts which is possible only for narrowband signals) which are widely tunable. [1, Fig. 1 (a)] shows a beamforming architecture where the signal received by each element of an antenna array is delayed and combined to achieve directional selectivity. The same can also be done after downconversion for signals with a lower bandwidth than the carrier as shown in [4, Fig. 1 (b)]. The spatial resolution of these beamforming systems improves as the number of antenna elements increases. But this also increases the required maximum delay (for an endfire beam) which is equal to the time taken by an electromagnetic wave to traverse the width of the array. Fig. 1(c) shows a continuous-time equalizer. Here again, the longer the impulse response of the channel, the longer must be the delay span of the equalizer. Since the signal is broadband, the group delay has to be uniform over the signal bandwidth. Therefore, there is a need to realize delay elements which maintain a large, flat group delay over a wide bandwidth. In other words, true-time-delay elements with a large DBW are necessary. Fig. 1.

Examples of usage of true-time-delay elements. (a) Beamforming by delaying and combining at RF [1]. (b) Beamforming at IF by delaying and combining post downconversion [4]. (c) Channel response equalization [3].