A Compact Wideband Inductorless Cascadable Tunable True Time Delay

We present a compact inductorless cascadable gigahertz-frequency true time delay (TTD) circuit implemented as a gm-C all-pass filter (APF) cell having poles and zeros prescribed by the Padé approximation. The proposed APF cell not only overcomes the large chip areas required by lumped-LC and transmission-line delay circuits, but it also maintains wide bandwidths by eliminating the need for dc-blocking capacitors or buffers, which are often necessary when cascading active APFs. In this article, we develop design equations and propose techniques to enhance bandwidth, improve phase linearity, and minimize delay variation across the bandwidth. Results were obtained using a 65-nm CMOS delay circuit containing four cascaded APF cells closely agreed with those of the postlayout simulations, achieving a delay tuning range of 300–500 ps over a 0.1–4-GHz bandwidth. Furthermore, the delay can be tuned to 550 ps, causing the bandwidth to drop to 3.55 GHz while still maintaining the delay-bandwidth product (DBW) of 1.95. The filter exhibited a worst case input of 1-dB compression point (IP1dB) of −21 dBm, while occupying an active area of 0.04 mm2 and dissipating 6.2 mW/cell from a 1.3-V supply. The APF demonstrated a delay-per-area metric at least 1.6 times higher than what was realized with prior gm-C APFs while also operating over a wider bandwidth, exhibiting a high DBW of 1.95, and maintaining a 3.3 dB gain.