True Time Delay Research Articles

Continuous True-Time Delay Phase Shifter Using Distributed Inductive and Capacitive Miller Effect

A new true-time delay phase shifter concept is proposed exploiting the distributed Miller effect in coupled transmission lines. Simultaneous change in Miller capacitance and inductance controlled by a single analog voltage changes the propagation delay of a transmission line with constant input impedance and insertion loss. The true-time delay line feature of the proposed architecture accommodates a wide bandwidth signal without group delay distortion. A theoretical framework for understanding Miller inductance from voltage and current duality and the distributed Miller effect using coupled wave equations is presented. As a proof of concept, a four-stage tunable delay line with a high-speed phase modulation capability was fabricated in a 45-nm RF SOI CMOS process and occupies an active area of 0.28 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The measured IC demonstrates a broadband tunable true-time delay with a measured group delay tuning range of 18 ± 1 ps from 11 to 24 GHz. The measured phase shift ranges are 173° and 182° with insertion losses of 10.6 ± 0.7 dB and 11.6 ± 1 dB at 28 and 30 GHz, respectively. The measured dc power consumption is 22 mW from a 1.2-V supply.

Three‐bit 2–18 GHz all‐pass network‐based trombone phase delay line with improved insertion loss variation

A complementary metal–oxide–semiconductor (CMOS) trombone true-time delay (TTD) operating from 2 to 18 GHz is presented. The second-order all-pass network (APN) is used to construct the delay lines. The higher-order delay response of the APN allows for a better trade-off between the delay and bandwidth compared to the artificial transmission line structures. The all-pass characteristic of the APN further improves the matching performance of the TTD. In order to improve the insertion loss (IL) variations between different delay states, the source degeneration by resistor–capacitor structure is used for the switching amplifiers. Two 3-bit TTDs are fabricated in a standard 0.13 µm CMOS process, namely TTD 1 and TTD 2. TTD 1 is based on conventional switching amplifiers and TTD 2 adopts the proposed source degeneration technique. The measurement results show that the root-mean-square IL variation of TTD 1 and TTD 2 is better than 2 and 1.4 dB. The input and output return loss of the two TTDs is >12 dB. Delays of 270 and 265 ps are achieved by TTD 1 and TTD 2. The power consumption for TTD 1 is 2–6.2 mW and is 2–3.6 mW for TTD 2.

High-resolution phased array radar imaging by photonics-based broadband digital beamforming.

A photonics-based broadband phased array radar is demonstrated to realize high-resolution imaging based on digital beamforming. This photonics-based phased array radar can achieve a high range resolution enabled by a large operation bandwidth, and can realize squint-free beam steering by digital true time delay (TTD) compensation. In addition, the photonic dechirp processing applied in the receiver can alleviate the hardware requirements for data sampling and storage, and hence remarkably enhance the real-time signal processing capability. In a proof-of-concept experiment, target imaging by a photonics-based 1 × 4 phased array radar that has a bandwidth of 4 GHz (22-26 GHz) is demonstrated, of which the range and azimuth resolution is measured to be 3.85 cm and 2.68°, respectively. The proposed scheme provides good solution to overcoming the bandwidth limitation and implementing high-resolution imaging in a phased array radar.

Compensation Method for a Wideband Signal’s Squint Problem of an AESA SAR System Using a Cross-Range-Variant Antenna Gain Equalizer

In order to obtain a high-resolution image, a wideband signal is required in a synthetic aperture radar (SAR). However, in an active electronically steered array (AESA) SAR, consideration of the beam squint problem is indispensable especially when a forward directed AESA operates in a squint spotlight SAR mode. In this paper, we present an accurate compensation method for the squint problem without added hardware, such as true time delays. To clarify the beam squint effect, an antenna gain radiated to point targets located on the ground was derived and we confirmed that the antenna gain for the targets placed on the same cross-range axis has similar tendency within 1.5-dB difference. Based on the results, we proposed a cross-range-variant antenna gain equalizer. Through test simulations, we confirmed that the suggested method effectively compensated for the degraded resolution performance of about 14% for the AESA SAR.

Optical Beamformer Based on Diffraction Order Multiplexing (DOM) of an Arrayed Waveguide Grating

A simplified beamforming architecture using a wavelength-selective optical true time delay (OTTD) module is proposed for phased array antenna (PAA) systems. Based on diffraction order multiplexing of an arrayed waveguide grating (AWG), the beamformer can realize multiple simultaneous time delays with only one OTTD module. We design, fabricate, and characterize a compact integrated optical beamformer using this structure for an eight-element X -band PAA system. The time delays generated by the fabricated beamformer are measured to be in the range from 0 to 278 ps in 5.7 ps increments of eight diffraction orders. The measured overall variation in the insertion loss of this device is less than 1.85 dB from 1527 to 1607 nm. The experimental results demonstrate that the proposed architecture can offer many advantages, such as multibeam capability, low insertion loss, a small footprint, and high resolution.

Computer-Aided Design of a Switchable True Time Delay (TTD) Line With Shunt Open-Stubs

This paper presents a new technique to design switchable true time delay (TTD) lines using periodic shunt open-stubs, shorter than a quarter wavelength at the highest frequency of interest. Structural parameters are obtained by minimizing the group delay variation and maximizing the return loss over the operating band. Advantages of the proposed design are low insertion loss, higher 1 dB compression point, and less number of switches compared to the conventional designs. To verify the proposed technique, 2-states and 8-states TTD lines are fabricated for L- and S-band applications. For the 8-states delay networks, measured insertion losses are smaller than 1.9 dB at 3 GHz. Return losses are more than 10 dB over 1–4 GHz band. Measured maximum time delay is 84 ps for the 8-states delay lines with a step size of 10 ps. All the TTDs operate as linear device at least up to 23 dBm.

Reconfigurable RF notch filter based on an integrated silicon optical true time delay line

The development of radio-frequency (RF) filters with bandwidth reconfigurability and frequency tunability in the MHz range is increasingly important to broadband wireless communication and radar systems. In this paper, we demonstrate a reconfigurable RF filter based on a silicon integrated optical true time delay line (OTTDL). The OTTDL is a four-path parallel structure with the reconfiguration enabled by thermo-optic Mach–Zehnder interferometer switches. By controlling the power splitting ratio of the switches, three different orders of finite impulse response filters with a variable bandwidth are realized. The bandwidth can be varied from 180.9 MHz to 662.0 MHz and the center frequency can be tuned in one free-spectral range up to 1 GHz.

Signal-to-Noise Ratio of True-Time Delay Beamformers Based on Microwave Photonic Links Using an Incoherent Broadband Optical Source

The signal-to-noise ratio (SNR) of true-time delay beamformers based on microwave photonic links using an incoherent broadband optical source (IBOS) is investigated. A comprehensive theory considering the bias angle of the Mach-Zehnder modulator employed in the microwave photonic link is presented to analyze the SNR. When the RF signal at the fundamental frequency is fed by the beamformer, the SNR is a function of the bias angle, the frequency of the applied RF signal, the modulation index, the system dispersion, and the shape of the IBOS. We find that by biasing the MZM near the minimum transmission point, the SNR at the fundamental frequency is maximized. On the other hand, by exactly biasing the MZM at the minimum transmission point, the RF signal at the double frequency can be fed by the beamformer with maximum power. The maximum SNR at the double frequency is a function of the frequency of the applied RF signal, the system dispersion, and the shape of the IBOS while its dependence on the modulation index is eliminated. In both cases, the relationship between the SNR and the frequency of the RF signal, the system dispersion, and the shape of the IBOS is also investigated. An experiment is carried out to verify the theoretical analysis, and the experiment results agree well with the calculation.

Recent Trends and Advances of Silicon-Based Integrated Microwave Photonics

Multitude applications of photonic devices and technologies for the generation and manipulation of arbitrary and random microwave waveforms, at unprecedented processing speeds, have been proposed in the literature over the past three decades. This class of photonic applications for microwave engineering is known as microwave photonics (MWP). The vast capabilities of MWP have allowed the realization of key functionalities which are either highly complex or simply not possible in the microwave domain alone. Recently, this growing field has adopted the integrated photonics technologies to develop microwave photonic systems with enhanced robustness as well as with a significant reduction of size, cost, weight, and power consumption. In particular, silicon photonics technology is of great interest for this aim as it offers outstanding possibilities for integration of highly-complex active and passive photonic devices, permitting monolithic integration of MWP with high-speed silicon electronics. In this article, we present a review of recent work on MWP functions developed on the silicon platform. We particularly focus on newly reported designs for signal modulation, arbitrary waveform generation, filtering, true-time delay, phase shifting, beam steering, and frequency measurement.

Microwave Photonic Devices Based on Liquid Crystal on Silicon Technology

This paper reviews the recent developments in microwave photonic devices based on liquid crystal on silicon (LCOS) technology. The operation principle, functions and important specifications of an LCOS based optical processor are described. Three microwave photonic devices, which are microwave photonic notch filters, phase shifters and couplers, reported in the past five years are focused on in this paper. In addition, a new multi-function signal processing structure based on amplitude and phase control functions in conjunction with a power splitting function in a commercial LCOS based optical processor is presented. It has the ability to realize multiple time -shifting operations and multiple frequency-independent phase shifting operations at the same time and control multiple RF signal amplitudes, in a single unit. The results for the new multi-function microwave photonic signal processor demonstrate multiple tunable true time delay and phase shifting operations with less than 3 dB amplitude variation over a very wide frequency range of 10 to 40 GHz.

A novel tunable true-time delay line/phase shifter based on distributed Schottky structure

A novel tunable true-time delay line/phase shifter based on distributed Schottky structure

5GHz CMOS All-Pass Filter-Based True Time Delay Cell

Analog CMOS time-delay cells realized by passive components, e.g., lumped LC delay lines, are inefficient in terms of area for multi-GHz frequencies. All-pass filters considered as active circuits can, therefore, be the best candidates to approximate time delays. This paper proposes a broadband first-order voltage-mode all-pass filter as a true-time-delay cell. The proposed true-time-delay cell is capable of tuning delay, demonstrating its potential capability to be used in different systems, e.g., RF beam-formers. The proposed filter achieves a flat group delay of over 60 ps with a pole/zero pair located at 5 GHz. This proposed circuit consumes only 10 mW power from a 1.8-V supply. To demonstrate the performance of the proposed all-pass filter, simulation results are conducted by using Virtuoso Cadence in a standard TSMC 180-nm CMOS process.

Optical true time delay unit with wide range and high resolution for phased array beamforming

An optical true time delay (OTTD) unit based on uniform fiber Bragg gratings, photonic crystal fiber and wavelength converter array is proposed for the phased array antennas (PAAs) beamforming system. The PAAs system based on the proposed OTTD unit can provide the scanning range from − 69.7° to + 69.7° and resolution of 1° with greatly reduced number and length of the time delay lines. The PAAs beamforming system equipped with this OTTD unit is compact, and has low bandwidth requirement on the tunable laser source. Theoretical analysis and numerical simulations are presented to demonstrate the feasibility of the proposed system.

Integrated On-Chip Bragg Time-Delay System for Thermo-Optical Control of a Microwave Antenna

This theoretical modeling-and-simulation paper presents designs and projected performance of ∼1550-nm silicon-on-insulator beamsteering of a microwave phased array antenna by the use of true time delays. The tunable on-chip optical delay line is a cascade connection of waveguide Bragg grating resonators (WBGRs) separated by a piece of straight waveguide. The notch in the reflectivity spectrum is translated along the wavelength axis by means of a low-power TO heater stripe atop the grating, inducing a time delay that depends upon the line position of the WBGR affected by TO switching. The filter resonator is a new in-guide array of identical Bragg structures, each one comprising N closely coupled phase-shifted Bragg-grating resonators. The length of each grating cavity in an N group is chosen according to the Butterworth filter technique to provide one resonant spectral profile with 40 GHz optical bandwidth. Finally, we examined the performances of the beamformer system operating in the X and Ku bands, respectively. The investigation demonstrated that steering angles up to 48° are feasible by assuming a minimum steering angle of 8° and a minimum WBGR spacing of about 354 and 236 μ m for X and Ku bands, respectively.

Microresonator Frequency Combs for Integrated Microwave Photonics

Microresonator-based Kerr frequency combs (microcombs) have many outstanding features, including chip-level integration, high repetition rate, and ultrabroad bandwidth. These merits make microcombs very promising for integrated microwave photonic applications. Here, we provide a brief review on recent progress of microcomb generation and its application in microwave photonics. Both coherent and incoherent examples are discussed, including low-phase-noise microwave and optical frequency synthesis, programmable photonic microwave signal processing, true time delay beamforming, and channelized receiver. An outlook on the future research direction and key problems is also given.

Optical Fiber Displacement Sensor Based on Microwave Photonics Interferometry.

An optical fiber displacement sensor based on the microwave photonics interferometric (MWPI) method is proposed and experimented, which provides an ideal solution for large range displacement measurement with high resolution. The sensor used a Michelson microwave photonics interferometer to sense the displacement with one sensing arm and a length-adjusted reference arm. The displacement variation would change the period of the microwave response function of the interferometer. According to the principle that the phase difference in one free spectral range (FSR) of the microwave response function is 360°, the displacement can be retrieved by the microwave response function by means of a vector network analyzer (VNA). A programmable path-switching true time delay line was used in the reference arm to decrease the microwave bandwidth. The measurement results show that the displacement sensing range is larger than 3 m and the measurement resolution is 31 μm. Finally, the measurement stability is tested, and the factors affecting the measurement resolution of this method and the main source of errors are investigated in detail.

Few-mode fiber true time delay lines for distributed radiofrequency signal processing.

We report, for the first time to our knowledge, distributed radiofrequency signal processing built upon true time delay operation on a step-index few-mode fiber. Two 3-sample configurations with different time delay properties are implemented over the same 60-meter 4-LP-mode fiber link. The inscription of a long period grating at a specific fiber position converts part of the LP01 mode into the LP02, permitting sample time delay engineering. Delay line performance is experimentally demonstrated when applied to radiofrequency signal filtering, example of fiber-distributed processing functionality exhibiting one order or magnitude gain in terms of compactness.

A fully integrated 4-channel beamformer based on TTD phased array in 0.18μm CMOS

A fully integrated 4-channel beamformer based on TTD phased array is proposed in this paper. This architecture is especially suitable for MIMO systems which can increase throughput by improving multiplexing gain. A path-sharing true time delay (TTD) unit by using analog CMOS filter circuits is presented to realize flat time delay over 0.35–1 GHz. Compared to traditional MIMO architectures, highly multiplexed TTD reduces chip area and power consumption. The proposed beamformer can provide maximum scan angle of up to ±45°for four antennas with 10.5 cm spacing. The measurement results show that maximum delay is almost consistent with simulated results. Input signals from the same direction which reach different antennas, are delayed a different fixed time difference through different TTD units and have same phase at the output, realizing signal enhancement. The whole beamformer chip consumes 130 mA from a 1.8 V DC power supply, and has a total chip area of 4.65 mm × 4.30 mm.

CMOS true-time delay IC for wideband phased-array antenna

CMOS true-time delay IC for wideband phased-array antenna

A 24-GHz Source-Degenerated Tunable Delay Shifter With Negative Group Delay Compensation

A true time delay shifter is fabricated in TSMC 65-nm CMOS low power technology for 24-GHz vehicle radar application. Its mechanism of source degenerated negative group delay compensation is proposed for flat group delay generation and tuning in a medium-wide 1.3-GHz instantaneous bandwidth, with simultaneously a reduced S21 of −6 ~ −9 dB (measured S21 excluding simulated extra insertion loss of packaging modeling and printed circuit board traces) compared to typical switched-line delay shifters. For 10-ps relative delay range, the variation is about 1–2 ps (max to 6 ps). For 17-ps range, the variation becomes 8–13 ps due to measurement issues. It consumes 16.3 mW at 1.2-V supply and 0.11-mm2 chip area. This topology is demonstrated to be a suitable and potential choice of delay shifter for medium-wideband time array system.