MHz Chirp Research Articles

Photonic generation of Barker encoded dual-nonlinear frequency modulated RADAR waveforms

Photonics enables the development of versatile waveform generators with carrier frequency tunability and broad bandwidths, allowing for high-range resolution for modern radar systems. Linear frequency-modulated waveforms are widely employed in various radar applications due to their large time-bandwidth products but are limited in functionality due to high sidelobe levels, resulting in a poor signal-to-noise ratio (SNR). On the other hand, non-linear frequency-modulated waveforms (NLFM) have a faster sweep rate at the edges than at the center, resulting in high SNRs, which can be boosted further through encoding techniques. A photonic approach for generating Barker-encoded dual-NLFM waveforms through a dual-drive Mach Zehnder modulator is proposed with improved range resolution, pulse compression ratio, and peak-to-sidelobe ratio. A theoretical analysis is performed, which is then modeled, and experimentally confirmed. Barker-encoded dual-NLFM waveforms are generated at 6 GHz with 500 MHz chirp bandwidth showing a maximum range resolution of 19.89 cm with PCR of 9803.

Multiband LFM waveform generation and band-selection using stimulated Brillouinscattering.

Modern radar systems are designed to simultaneously serve multiple applications such as ranging, surveillance, imaging, or warfare, which necessitates operation at multiple carrier frequencies. Linear frequency modulated (LFM) signals are inherently capable of pulse compression leading to enhanced range resolution and good signal-to-noise ratios; therefore, they are widely employed in various radar applications. In this paper, a photonic-based generation scheme for carrier frequency multiplication of LFM waveforms up to a factor of four through a single dual-drive Mach-Zehnder modulator is proposed and experimentally demonstrated. The technique is employed to produce multiband LFM having wide-bandwidth chirps (500MHz, 1GHz) as well as narrow bandwidth chirps (10, 20MHz) that are compatible with the intrinsic linewidth of stimulated Brillouin scattering (SBS). The frequency bands of the narrow bandwidth chirps are further selected through a frequency-agile Brillouin RF filter. The generated tupled chirped waveforms are at continuous multiples of the RF carrier frequency at 2, 4, 6, and 8GHz, respectively, with the first three multiples having 10MHz and the fourth multiple having 20MHz chirp bandwidth. This scheme is also experimentally verified for generating different tupled products and respective filtering through SBS at multiples of 4GHz up to 16GHz, thereby verifying the system's agility and flexibility.

CW and Modulated Input Second Harmonic Injection for Efficiency Enhancement in Broadband Power Amplifiers

The second-harmonic input impedance plays a critical role on the performance of GaN power amplifiers. In a recent paper, a drain efficiency enhancement for a continuous-mode power amplifiers was reported to be achieved using active broadband second-harmonic injection at the PA input. In this paper, the strategy for selecting the second-harmonic input impedance and the necessity for using active injection in such broadband PAs are discussed in detail. Additionally, the methodology for designing an embedded broadband diplexer in the input matching network is reported. Finally, the importance of the phase of the second-harmonic signal injected is demonstrated for both CW and modulated signals using both simulation and measurement, respectively. The effectiveness of the CW and modulated active second-harmonic injection methodology presented here are validated by previously reported measurements that demonstrated an average drain efficiency improvement of 9.4% from 1.3 to 2.4 GHz for CW signals and of 9.7% at 2 GHz for a frequency-modulated 30 MHz chirp radar signal.

A Novel Method to Detect Interface of Conductivity Changes in Magneto-Acousto-Electrical Tomography Using Chirp Signal Excitation Method

As a non-invasive and hybrid imaging modality, magneto-acoustic-electrical tomography (MAET) is extremely useful for the electrical conductivity measurement in vivo. Based on the Verasonics system and the MC600 displacement platform, we designed and implemented a novel MAET system with a chirp pulse stimulation (MAET-CPS) method for electrical conductivity measurement. In the system, a 2–3 MHz chirp signal was exploited for stimulating ultrasound power probe. Then, the interface positions of conductivity variation were obtained by digital demodulation of the excitation signal and the received voltage signal. Finally, five different homogeneous phantoms with same size were used to investigate the feasibility, accuracy, and repeatability of MAET-CPS. The results showed that: 1) when a chirp signal with pulse duration of $1000~\mu \text{s}$ was used to stimulate a homogeneous phantom with 0.5% NaCl, the reconstructed B-scan image of the conductivity distribution was highly consistent with the ultrasound B-scan imaging and physical size; 2) the signal-to-noise ratio of the system and the detection resolution of the interface of conductivity variations could be influenced by the linear frequency modulation period. The resolution obtained by using chirp signal with pulse duration of $1000~\mu \text{s}$ was better than that of $500~\mu \text{s}$ and $1500~\mu \text{s}$ ; and 3) the interfaces of conductivity variations of homogeneous phantoms with five different concentrations (0.4%, 0.5%, 0.6%, 0.7%, and 0.8%) were clearly presented and the measured thicknesses of each phantom showed good agreement with the thickness of the target sample. This paper has laid the foundation for the MAET-CPS modality in the phantom sample, and MAET-CPS is expected to become an alternative imaging method for early diagnosis and detection of biological cancerous tissues.

Low sidelobe level and high time resolution for metallic ultrasonic testing with linear-chirp-Golay coded excitation

Linear-chirp-Golay (LCG)-coded excitation combined with pulse compression is proposed in this paper to improve the time resolution and suppress sidelobe in ultrasonic testing. The LCG-coded excitation is binary complementary pair Golay signal with linear-chirp signal applied on every sub pulse. Compared with conventional excitation which is a common ultrasonic testing method using a brief narrow pulse as exciting signal, the performances of LCG-coded excitation, in terms of time resolution improvement and sidelobe suppression, are studied via numerical and experimental investigations. The numerical simulations are implemented using Matlab K-wave toolbox. It is seen from the simulation results that time resolution of LCG excitation is 35.5% higher and peak sidelobe level (PSL) is 57.6 dB lower than linear-chirp excitation with 2.4 MHz chirp bandwidth and 3 μs time duration. In the B-scan experiment, time resolution of LCG excitation is higher and PSL is lower than conventional brief pulse excitation and chirp excitation. In terms of time resolution, LCG-coded signal has better performance than chirp signal. Moreover, the impact of chirp bandwidth on LCG-coded signal is less than that on chirp signal. In addition, the sidelobe of LCG-coded signal is lower than that of chirp signal with pulse compression.

Increasing the sonoporation efficiency of targeted polydisperse microbubble populations using chirp excitation

The therapeutic use of microbubbles for targeted drug or gene delivery is a highly active area of research. Phospholipid- encapsulated microbubbles typically have a polydisperse size distribution over the 1 to 10 μm range and can be functionalized for molecular targeting and loaded with drugcarrying liposomes. Sonoporation through the generation of shear stress on the cell membrane by microbubble oscillations is one mechanism that results in pore formation in the cell membrane and can improve drug delivery. A microbubble oscillating at its resonant frequency would generate maximum shear stress on a membrane. However, because of the polydisperse nature of phospholipid microbubbles, a range of resonant frequencies would exist in a single population. In this study, the use of linear chirp excitations was compared with equivalent duration and acoustic pressure tone excitations when measuring the sonoporation efficiency of targeted microbubbles on human colorectal cancer cells. A 3 to 7 MHz chirp had the greatest sonoporation efficiency of 26.9 ± 5.6%, compared with 16.4 ± 1.1% for the 1.32 to 3.08 MHz chirp. The equivalent 2.2- and 5-MHz tone excitations have efficiencies of 12.8 ± 2.1% and 15.6 ± 1.1%, respectively, which were all above the efficiency of 4.1 ± 3.1% from the control exposure.

Chirp excitation technique to enhance microbubble displacement induced by ultrasound radiation force

Ultrasound radiation force has been proposed to increase the targeting efficiency in ultrasonic molecular imaging and drug delivery. A chirp excitation technique is proposed to increase the radiation force induced microbubble displacement and might potentially be used for enhancing the targeting efficiency of microbubble clouds. In this study, a modified Rayleigh-Plesset equation is used to estimate the radius-time behavior of insonified microbubbles, and the translation of insonified microbubbles is calculated by using the particle trajectory equation. Simulations demonstrate that the chirp excitation is superior to the sinusoidal one in displacing microbubbles with a wide-size distribution, and that the performance is dependent on the parameters of the chirp signal such as the center frequency and frequency range. For Gaussian size distributed microbubble clouds with mean diameter of 3.5 microm and variance of 1, a 2.25 MHz chirp with frequency range of 1.5 MHz induces about 59.59% more microbubbles over a distance of 10 microm during 200 micros insonification, compared to a 2.25 MHz sinusoidal excitation with equal acoustic pressure.

Frequency chirp in optical reflection modulators

The chirping behaviour of reflective, resonant electroabsorptive optical modulators is analysed for the first time and an equivalent to the chirp parameter for these devices derived. Using the example of a Franz-Keldysh Effect based device the frequency chirp behaviour of a matched resonator is investigated and compared to the non resonant case. The chirp is found to display oscillatory behaviour which is explained by the interaction of the electric field induced changes in complex refractive index with the cavity. The effect of a front face reflectivity lower than that for matching is shown to be poorer modulation and chirping performance. If the front face reflectivity is greater than that needed to match then negative chirp is possible but the insertion loss is greater than 8 dB. By fine tuning the length of the device it is shown that this can be reduced to 3 dB producing a contrast ratio of 4.5 dB and −500 MHz chirp for 400 ps, 15 V pulses.