Chirp Bandwidth Research Articles

Noninvasive Acoustic Measurements in Cylindrical Shell Containers.

Acoustic time-of-flight (ToF) measurements enable noninvasive material characterization, acoustic imaging, and defect detection and are commonly used in industrial process control, biomedical devices, and national security. When characterizing a fluid contained in a cylinder or pipe, ToF measurements are hampered by guided waves, which propagate around the cylindrical shell walls and obscure the waves propagating through the interrogated fluid. We present a technique for overcoming this limitation based on a broadband linear chirp excitation and cross correlation detection. By using broadband excitation, we exploit the dispersion of the guided waves, wherein different frequencies propagate at different velocities, thus distorting the guided wave signal while leaving the bulk wave signal in the fluid unperturbed. We demonstrate the measurement technique experimentally and using numerical simulation. We characterize the technique performance in terms of measurement error, signal-to-noise-ratio, and resolution as a function of the linear chirp center frequency and bandwidth. We discuss the physical phenomena behind the guided bulk wave interactions and how to utilize these phenomena to optimize the measurements in the fluid.

Optical FMCW Signal Generation Using a Silicon Dual-Parallel Mach-Zehnder Modulator

We experimentally demonstrate optical frequency-modulated continuous-wave (FMCW) signal generation utilizing a silicon dual-parallel Mach-Zehnder modulator (DP-MZM). The sideband suppression ratio (SSR) and the sideband-to-carrier suppression ratio (SCSR) of the single-sideband suppressed carrier (SSB-SC) modulation are higher than 15 dB when RF frequency is lower than 20 GHz. With the generated triangular modulated FMCW signal, we demonstrate simultaneous ranging and velocimetry. The chirp bandwidth is 10 GHz and the chirp period is 1 μs, corresponding to a ranging resolution of 13 mm and a velocimetry resolution of 0.5 m/s, consistent with the theoretical calculation. These results show that wideband FMCW signal generation by a silicon DP-MZM is promising for high-resolution light detection and ranging.

A Spurless and Wideband Continuous-Time Electro-Optical Phase Locked Loop (CT-EOPLL) for High Performance LiDAR

Frequency-modulated continuous-wave (FMCW) LiDAR systems are drawing increasing interest due to their potential applications in autonomous driving, machine perception, rapid prototyping, and medical diagnostics. The nonlinearity of a laser’s input-output transfer function can degrade the performance of an FMCW LiDAR. However, traditional discrete-time electro-optical phase-locked loops (DT-EOPLLs) face an unfavorable trade-off between chirp bandwidth and Mach-Zehnder delay. We present an integrated continuous-time electro-optic phase-locked loop (CT-EOPLL) to address this problem. The proposed EOPLL is very wideband, with its loop bandwidth equal to its reference frequency. This feature enables it to relax the trade-off between chirp bandwidth and Mach-Zehnder (MZ) delay by 10× in dB scale, which consequently reduces the area and loss associated with the silicon photonic delay implementation. It also does not suffer from the challenging issue of spurs in wideband PLLs because it features image and harmonic spur suppression in the loop using single-sideband (SSB) and harmonic-reject (HR) mixing techniques. The electrical part of this EOPLL is implemented in 65nm CMOS technology, and its optical integrated circuit is fabricated using a silicon photonic process. Featuring more than 25dB of suppression of the highest spur, this EOPLL is utilized in a high precision LiDAR sensor that shows an RMS depth precision of 558μm at 2m distance, and a 9.4mm RMS depth resolution at ranges exceeding 3.3m.

Statistically efficient simultaneous amplitude measurement of multiple linear chirp signals

In this article, the amplitude measurement of multiple and simultaneously received linear chirp signals is investigated. The achievable scalability, that is, the maximum number of simultaneously transmitted superimposed signals that a receiver can characterize with a given target accuracy, is explored using simulations. The effect of chirp duration and bandwidth on the estimation accuracy and on the scalability is investigated. The effect of inaccuracies affecting the realized chirp duration and bandwidth is also discussed. It is shown that the scalability of the proposed approach is competitive with respect to multiple access methods, such as frequency-division multiple access or code-division multiple access. It is also shown that accurate amplitude measurements are achievable even when the collected signal is the superimposition of up to 20 chirp signals with amplitudes differing by three orders of magnitude.

A K-Band FMCW Frequency Synthesizer Using Q-Boosted Switched Inductor VCO in SiGe BiCMOS for 77 GHz Radar Applications

In this article, a fractional-N phase-locked loop (PLL) with integrated chirp generation circuit block for a 76~81 GHz frequency-modulated continuous-wave (FMCW) radar system is presented. Thanks to the switched inductor voltage-controlled oscillator (VCO) topology, the linearity, phase noise, chirp bandwidth, and chirp rate of the FMCW synthesizer can be optimized for the short-range radar (SRR) and long-range radar (LRR) applications, with switch at ON/OFF states, respectively, according to different requirements and concerns. In this way, the proposed FMCW synthesizer shows improved phase noise for switch OFF-state, good for LRR applications, compared to the conventional single-varactor VCOs or cap-bank VCOs. The switch loss at ON-state is further decreased with the Q-boosting technique, which helps the FMCW synthesizer to simultaneously obtain a wide chirp bandwidth, steep modulation rates and good phase noise for SRR applications. The FMCW synthesizer is fabricated in 0.13 µm SiGe BiCMOS technology, occupies an area of 1.7 × 1.9 mm2, and consumes 330 mW from a 3.3 V voltage supply. Measured results show that the FMCW synthesizer can cover 25.3~27 GHz (with a frequency tripler to fully cover 76~81 GHz band), showing optimized phase noise, chirp bandwidth, linearity, and modulation rates performance. The measured K-band phase noise is −110.5 dBc/Hz for switch OFF-state, and −106 dBc/Hz for switch ON-state at 1 MHz offset. The normalized root mean square (RMS) frequency error is 518 kHz for chirp rate of ±14.6 MHz/μs and 1.44 MHz for chirp rate of ±39 MHz/μs for the 77 GHz band. Moreover, the integrated waveform generator offers fully programmability in chirp rate, duration and bandwidth, which supports multi-slope chirp generations. With a frequency tripler, the chip is well suited for the 76~81 GHz FMCW radar system.

A C-Band FMCW SAR Transmitter Based on Synthetic Bandwidth Technique

A $C$ -band frequency-modulated continuous wave (FMCW) transmitter based on synthetic bandwidth technique for synthetic aperture radar (SAR) imaging is presented. A 500-MHz narrowband chirp signal is up-converted to two adjacent carriers to obtain 1-GHz chirp at the $C$ -band. The adjacent carriers are obtained through fast switching subharmonic injection locking oscillator (SHILO) technique with 14-ns settling time. For narrowband chirp, a wideband fractional- $N$ phase locked loop (PLL) is used, covering 1.8–2.3 GHz and achieving low in-band phase noise of −115 to 120 dBc/Hz. The proposed technique achieves reconfigurable chirp period ranging from 0.3 to 6.3 ms. It can also be easily extended for wider chirp bandwidth by simply increasing the number of SHILO carriers. The chip consumes 84 mW, while occupying a total active area of 2.24 mm2.

A variable chirp rate stepped frequency linear frequency modulation waveform designed to approximate wideband non-linear radar waveforms

The non-linear frequency modulation (NLFM) waveform is one of the existing waveforms that can be used in high range resolution radar applications. However, a high sampling frequency and consequently an expensive ADC are required. To overcome this drawback while taking advantage of the features of the NLFM waveform, we suggest approximating the wideband NLFM waveform by a piecewise linear waveform and using it in a stepped frequency (SF) framework. Thus, a variable chirp rate SF-LFM waveform is proposed where SF is combined with a train of LFM pulses having different chirp rates, durations, and bandwidths. In this paper, these parameters are derived from a tangent-based NLFM waveform. At the receiver, a generalized version of the time domain (TD) algorithm is proposed to process the received echoes. Our purpose is to obtain the high range resolution profile (HRRP) whose properties are of the same magnitude orders as those obtained using a tangent-based NLFM waveform. These properties are the peak sidelobe ratio, the integrated sidelobe, and the range resolution. Toward this goal, a multi-objective optimization issue is addressed to deduce the parameters of the proposed waveform by using two types of approaches based on evolutionary algorithms. Their relevance is compared. Our analysis and simulations show that the proposed approaches attain the targeted performance goals with a smaller sampling frequency at the receiver.

G-Band Radar for Humidity and Cloud Remote Sensing

VIPR (vapor in-cloud profiling radar) is a tunable G-band radar designed for humidity and cloud remote sensing. VIPR uses all-solid-state components and operates in a frequency-modulated continuous-wave (FMCW) radar mode, offering a transmit power of 200-300 mW. Its typical chirp bandwidth of 10 MHz over a center-frequency tuning span of 167-174.8 GHz results in a nominal range resolution of 15 m. The radar's measured noise figure over the transmit band is between 7.4 and 10.4 dB, depending on its frequency and hardware configuration, and its calculated antenna gain is 58 dB. These parameters mean that with typical 1 ms chirp times, single-pulse cloud reflectivities as low as -26 dBZ are detectable with unity signal-to-noise at 5 km. Experimentally, radar returns from ice clouds above 10 km in height have been observed from the ground. VIPR's absolute sensitivity was validated using a spherical metal target in the radar antenna's far-field, and a G-band switch has been implemented in an RF calibration loop for periodic recalibration. The radar achieves high sensitivity with thermal noise limited detection both by virtue of its low-noise RF architecture and by using a quasioptical duplexing method that preserves ultrahigh transmit/receive isolation despite operation in an FMCW mode with a single primary antenna shared by the transmitter and receiver.

Broad-bandwidth high-temporal-contrast carrier-envelope-phase-stabilized laser seed for 100 PW lasers

We demonstrate in this Letter the generation of carrier-envelope-phase (CEP)-stabilized laser pulses at 910 nm with simultaneously high-temporal-contrast, broad spectral bandwidths and few-cycle pulse durations. Through combining the techniques of cascaded optical parametric amplification (OPA) and second-harmonic generation (SHG) in the laser setup, a pulse temporal contrast as high as ${ \gt }{{10}^{12}}$>1012 has been obtained at the laser output. During the OPA and SHG processes, both the pulse chirp and gain bandwidth are perfectly optimized, leading to the generation of 170 µJ pulses with ${ \gt }{200}\;{\rm nm}$>200nm bandwidth and ${\sim}{15}\;{\rm fs}$∼15fs pulse duration. Moreover, the CEP of the laser is stabilized passively to a noise level of less than 340 mrad. This high-quality pulsed light source, as the seed laser of the deuterated potassium dihydrogen phosphate (DKDP)-based 100 PW system, will be integrated into the Station of Extreme Light facility in the near future.

Effective Light Beam Modulation by Chirp IDT on a Suspended LiNbO3 Membrane for 3D Holographic Displays.

An acousto-optic (AO) holographic display unit based on a suspended waveguide membrane was developed. The AO unit consists of a wide bandwidth chirp interdigital transducer (IDT) on a 20 µm thick suspended crystalline 128° YX LiNbO3 membrane, a light blocker with a 20 µm hole near the entrance, and an active lens near the exit. The 20 µm thickness of the floating membrane significantly enhanced surface acoustic wave (SAW) confinement. The light blocker was installed in front of the AO unit to enhance the coupling efficiency of the incident light to the waveguide membrane and to remove perturbations to the photodetector during measurement at the exit region. The active lens was vertically attached to the waveguide sidewall to collect the diffracted beam without loss and to modulate the focal length in free space through the applied voltage. As SAWs were radiated from the IDT, a Bragg grating with periodic refractive indexes was formed along the waveguide membrane. The grating diffracted incident light. The deflection angle and phase, and the intensity of the light beam were controlled by the SAW frequency and input power, respectively. The maximum diffraction efficiency achieved was approximately 90% for a 400 MHz SAW. COMSOL simulation and coupling of mode modeling were performed to optimize design parameters and predict device performance.

Fully Integrated Dual-Mode X-Band Radar Transceiver Using Configurable Receiver and Local Oscillator

In this paper, a fully-integrated dual-mode X-band radar transceiver that supports Doppler radar and frequency modulated continuous wave (FMCW) radar is proposed. To remove large off-chip DC-blocking capacitors in the Doppler mode, the double-conversion technique and local oscillator (LO) chopping technique are introduced. These techniques remove the DC offset and makes the transceiver more robust to the TX to RX leakage. In addition, they also improve the noise figure (NF) by filtering out 1/f noise of the analog baseband. The FMCW radar is realized using a direct-conversion receiver and a chirp generator. The chirp generator consists of a frequency sweep generator (FSG) and a fractional-N phase-locked loop (PLL). All the analog baseband components including a programable gain amplifiers (PGA) and 1/R 4 range compensation filters are integrated on the chip as well as a bandgap and low-dropout regulators. The operating frequency is from 9.7 to 12.3 GHz, which consumes 216 mW in Doppler mode and 201.6 mW in FMCW mode from a 1.2-V supply voltage. The maximum chirp bandwidth is 750 MHz and the maximum receiver gain is 76 dB with 6-dB gain step. The chip size of the transceiver is 7.68 mm 2 including all the pads.

A Self-Calibrated 16-GHz Subsampling-PLL-Based Fast-Chirp FMCW Modulator With 1.5-GHz Bandwidth

This article presents a 16-GHz frequency-modulated continuous-waveform (FMCW) modulator for radar applications in 28-nm CMOS. A two-point modulation technique is applied to reach a fast-chirp operation. A digital-to-time converter (DTC)-based subsampling phase-locked loop (PLL) is implemented to eliminate the power and noise from the frequency divider. Compared with the conventional delta-sigma division ratio-modulated PLL, this article presents an FM modulator with a finer loop-time resolution that lowers the quantization noise and improves the chirp FM accuracy. A voltage-driven analog varactor enables a programmable chirp bandwidth up to 1.5 GHz. The frequency-digital to analog converter (FDAC) nonlinearity is pre-distorted by an on-chip calibration engine. This FMCW modulator chirps 1.5-GHz bandwidth within 10 μs. The corresponding root-mean-square (rms) frequency error is 230 kHz, which is less than 0.015% of the chirp bandwidth.

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Ultrasonic chirp-coded excitation for highly attenuating material testing

This paper deals with ultrasonic chirp-coded excitation for highly attenuating material testing. We reformulate the expression of gain in signal-to-noise ratio (GSNR) between the compressed signal and short-pulse signal, and propose a method which only needs to adjust the center frequency and bandwidth of chirp. The performance of the chirp-coded method is analyzed and the experimental results match well with the simulation in GSNR and axial resolution except for the main-to-side lobe ratio. Specifically, rubber with an attenuation coefficient of 1.4 Np/(MHz cm) is detected using the transmitting and receiving transducer with a center frequency of 800 kHz. The proposed chirp-coded method has a 4.3 dB GSNR increase to the conventional chirp-coded method, and their axial resolution is approximate to 2.7 μs, which is close to that of the short-pulse method using a square wave with a 6.25 ns pulse width.

Air-coupled ultrasonic vertical reflection method using pulse compression and various window functions: feasibility study

Nondestructive inspection using the air-coupled ultrasonic vertical reflection method has not thus far been realized because of the large mismatching of acoustic impedances between the test objects and air; this study aims to realize the air-coupled ultrasonic vertical reflection method. In order to separate and detect the faint defect-reflected wave from the surface-reflected wave, we examine the use of ultrasounds using linear frequency modulation (chirp signal) with window functions and the pulse compression technique. As a first step, we investigate the required conditions of the excited chirp signal to detect the signal received after transmittance into the test objects. We find that chirp signals with larger bandwidths and appropriate window functions are required for successful defect inspections. In addition, we investigate the effectiveness of using multiple probes and merging chirp signals with different frequency ranges in order to enhance the bandwidth of the excited chirp.

A High-Resolution 220-GHz Ultra-Wideband Fully Integrated ISAR Imaging System

In this paper, an ultra-wideband fully integrated imaging radar at sub-terahertz (sub-THz) frequencies is presented, which demonstrates a fine lateral resolution without using any focal lens/mirror. We have achieved a lateral resolution of 2 mm for an object at 23-cm distance as well as a range resolution of 2.7 mm. To achieve the decent range resolution, in a frequency modulation continuous wave radar configuration, a state-of-the-art chirp bandwidth (BW) of 62.4 GHz at a center frequency of 221.1 GHz is generated and efficiently radiated. We have presented a design technique for the optimal design of the passive embedding around the core transistor to maximize the tuning BW of the voltage controlled oscillator. At the receiver side, to maximize the intermediate frequency level, a subharmonic mixer is utilized, which is designed for the lowest conversion loss. Finally, to obtain the fine lateral resolution, we have implemented near-field beamforming algorithm based on the inverse synthetic aperture radar (ISAR) systems. The synthesized beamwidth is less than 0.5°; hence, high-resolution images are reconstructed. The system is fabricated in a 55-nm BiCMOS process. To the best of our knowledge, this is the first imaging radar at THz/sub-THz frequencies, which utilizes ISAR to achieve a high lateral resolution while the radar system is fully integrated.

Multifrequency Subpulse SAR: Exploiting Chirp Bandwidth for an Increased Coverage

Spatial resolution and swath width are fundamental quality parameters for spaceborne synthetic aperture radar (SAR) products. They have driven the research on new spaceborne SAR system concepts in the last decades, and a new generation of SAR systems is emerging. Main feature of these future systems is the use of multiple digital channels and new SAR processing techniques. A further characteristic is that they can resort to a large radar signal bandwidth. In this letter, a novel SAR operational mode is presented, denoted as multifrequency subpulse (MFSP). The MFSP exploits the available radar signal bandwidth to increase the imaged swath extension, without the emergence of range ambiguities. The proposed approach is explained both theoretically and by a design example, based on the future German X-band SAR system, HRWS. Moreover, the achievable SAR imaging quality is investigated.

A 23-GHz Low-Phase-Noise Digital Bang–Bang PLL for Fast Triangular and Sawtooth Chirp Modulation

Frequency-modulated continuous-wave (FMCW) radars with high resolution require the generation of low-phase-noise, low-spurs, and highly linear chirp signals with large peak-to-peak value (chirp bandwidth) and a short period of the modulation signal [1]. In radar systems, the spot phase noise of the chirp generator is converted to the intermediate frequency of the receiver making it difficult to detect two close targets, while spurs cause the detection of false targets. For those reasons, medium-range radar applications in the 77-to-81GHz band typically specify spot phase noise lower than −90dBc/Hz at 1MHz offset and spur level below −50dBc. Unlike triangular chirps, saw-tooth chirps allow for a reduced dead time for range detection. However, any practical modulator needs a finite time (idle time) to make a large frequency jump at the end of the saw-tooth, and this limits the duty cycle of the saw-tooth. For instance, a fast saw-tooth chirp with 200kHz rate and 95% duty cycle leaves the idle time of only 250ns. Fractional-N PLLs can be used as chirp modulators. Unfortunately, low phase noise and spur levels require a narrow PLL bandwidth, while short idle time demands for a wide one. The two-point injection of the modulation signal, both from the modulus control of the divider and the tuning input of the voltage-controlled oscillator (VCO), is a known method to simultaneously achieve a narrow PLL bandwidth and fast modulation. However, even in that scheme, a frequency modulation error is mainly limited by gain mismatch between the two injection paths and by the linearity of the VCO [2]. In this work, a 20-to-24GHz digital bang-bang PLL, which uses the two-point modulation scheme to generate triangular and saw-tooth chirp signals, is presented. Unlike previous works [1-4], this architecture is able to generate fast saw-tooth chirps with the slope up to 173MHz/js, the idle time below 200ns, and the rms frequency error of better than 0.06%. The gain mismatch between the two modulation paths are automatically calibrated by a digital algorithm [5], and the input of the digitally controlled oscillator (DCO) is pre-distorted via an automatic background correction scheme, which compensates for the DCO nonlinearity.

A 77-GHz Mixed-Mode FMCW Signal Generator Based on Bang-Bang Phase Detector

A 77-GHz frequency-modulated continuous-wave (FMCW) signal generator is presented for automobile millimeter-wave (mm-wave) radar applications. The reconfigurable chirp is generated by a mixed-mode synthesizer operating at 38.5 GHz, in which the frequency doubling scheme is used to improve the chirp bandwidth and simplify the design. The bang-bang phase detector (BBPD) is employed for phase detection in the synthesizer, avoiding complicated linear time-to-digital converter (TDC) as well as reducing design complexity and power consumption. A 1-bit third-order single-loop $\Delta \Sigma$ modulator (SLDSM3), combining with the hybrid finite-impulse-response (FIR) filtering technique, significantly suppresses the BBPD induced quantization noise. A type-III slope estimator with a switchable polarity is embedded in the digital loop filter (DLF) to improve the linearity around the chirp turning-around points (TAPs). Two infinite-impulse-response (IIR) filtering stages smoothen the generated chirp waveform by reducing the instant variation of the DLF’s output. Implemented in 65-nm CMOS, the proposed FMCW signal generator consumes 43.1-mW power and occupies 2-mm2 die area, including testing pads. The measured phase noise from the 38.5-GHz carrier is −87.7 dBc/Hz at 1-MHz offset. The measured root-mean-square (rms) frequency errors of the generated triangle chirps over the 1-ms period are 189 and 336 kHz, with 0.914- and 1.827-GHz chirp bandwidth, respectively.

Reconfigurable photonic generation of broadband chirped waveforms using a single CW laser and low-frequency electronics

Broadband radio-frequency chirped waveforms (RFCWs) with dynamically tunable parameters are of fundamental interest to many practical applications. Recently, photonic-assisted solutions have been demonstrated to overcome the bandwidth and flexibility constraints of electronic RFCW generation techniques. However, state-of-the-art photonic techniques involve broadband mode-locked lasers, complex dual laser systems, or fast electronics, increasing significantly the complexity and cost of the resulting platforms. Here we demonstrate a novel concept for photonic generation of broadband RFCWs using a simple architecture, involving a single CW laser, a recirculating frequency-shifting loop, and standard low-frequency electronics. All the chirp waveform parameters, namely sign and value of the chirp rate, central frequency and bandwidth, duration and repetition rate, are easily reconfigurable. We report the generation of mutually coherent RF chirps, with bandwidth above 28 GHz, and time-bandwidth product exceeding 1000, limited by the available detection bandwidth. The capabilities of this simple platform fulfill the stringent requirements for real-world applications.