Abstract-Linear frequency modulated (LFM) excitation combined with pulse compression provides an increase in signal to noise ratio (SNR) at the receiver. LFM signals are of longer duration than pulsed signals of the same bandwidth. Consequently, in many practical situations, maintaining temporal separation between echoes is not possible. Where analysis is performed on individual LFM signals, a separation technique is required. Time windowing is unable to separate signals overlapping in time. Frequency domain filtering is unable to separate signals with overlapping spectra. This paper describes a method to separate time overlapping LFM signals through the application of the fractional Fourier transform (FrFT), a transform operating in both time and frequency domains. A short introduction to the FrFT, its operation and calculation are presented. The proposed signal separation method is illustrated by application to a simulated ultrasound signal, created by the summation of multiple time overlapping LFM signals and the component signals recovered with ±0.6% spectral error. The results of an experimental investigation are presented where the proposed separation method is applied to time overlapping LFM signals, created by the transmission of a LFM signal through a stainless steel plate and water-filled pipe.

A novel switched excitation method for linear frequency modulated excitation of ultrasonic transducers in pulse compression systems is presented that is simple to realise, yet provides reduced signal sidelobes at the output of the matched filter shows improvements in signal to sidelobe noise power in the order of 7 to 8 dB.The reported quinary switched method for linear frequency modulated excitation provides improved performance compared to pesudo chirp excitation without the need for high performance excitation amplifiers.

Abstract-Coarse time quantization of delay profiles within ultrasound array systems can produce undesirable sidelobes in the radiated beam profile. The severity of these sidelobes is dependent upon the magnitude of phase quantization error -the deviation from ideal delay profiles to the achievable quantized case. This paper describes a method to improve inter channel delay accuracy without increasing system clock frequency by utilising embedded Phase-Locked Loop (PLL) components within commercial Field Programmable Gate Arrays (FPGAs). Precise delays are achieved by shifting the relative phases of embedded PLL output clocks in 208 ps steps. The described architecture can achieve the necessary inter element timing resolution required for driving ultrasound arrays up to 50 MHz. The applicability of the proposed method at higher frequencies is demonstrated by means of extrapolating experimental results obtained using a 5 MHz array transducer. Results indicate an increase in Transmit Dynamic Range (TDR) when using accurate delay profiles generated by the embedded PLL method described, as opposed to using delay profiles quantized to the system clock.

Abstract-A method of output pressure control for ultrasound transducers using switched excitation is described. The method generates width-modulated, square-wave pulse sequences that are suitable for driving ultrasound transducers using MOSFET devices or similar. Sequences are encoded using an optimized level-shifted, carrier-comparison, pulse-width modulation (PWM) strategy derived from existing PWM theory, and modified specifically for ultrasound applications. These modifications are: a reduction in carrier frequency so that the least amount of pulses are generated and minimal switching is necessary; alteration of a linear carrier form to follow a trigonometric relationship in accordance with the expected fundamental output; and application of frequency modulation to the carrier when generating frequency modulated, amplitude tapered signals.The PWM method permits control of output pressure for arbitrary waveform sequences at diagnostic frequencies (approximately 5 MHz) when sampled at 100 MHz, and is applicable to pulse shaping and array apodization. Arbitrary waveform generation capability is demonstrated in simulation using convolution with a transducer's impulse response, and experimentally with hydrophone measurement. Benefits in coded imaging are demonstrated when compared with fixed-width square-wave (pseudo-chirp) excitation in coded imaging, including reduction in image artifacts and peak sidelobe levels for two cases, showing 10 dB and 8 dB reduction in peak sidelobe level experimentally, compared to 11 dB and 7 dB reduction in simulation with the Field II package. In all cases the experimental observations correlate strongly with simulated data.

Switched-mode operation allows the miniaturization of excitation circuitry but suffers from high harmonic distortion. This paper presents a method of phase-inversion-based selective harmonic elimination (PI-SHE) and the use of multiple switching levels. PI-SHE is shown to enable multiples of any selected harmonic to be eliminated through controlled timing of the transition between different excitation voltage levels. Multiples of the third harmonic are shown to be eliminated in three-level tone waveforms. In addition, multiples of the fifth harmonic are shown to be eliminated using five-level tone waveforms. A method of calculating the expected amplitude of each harmonic is presented. The application of PI-SHE in linear frequency-modulated (LFM) excitation is proposed. A heuristic derivation of the spectral properties of multilevel switched LFM waveforms is presented. The performance of the proposed PI-SHE method is confirmed through experimental measurement of the harmonics present in an ultrasound wave using two, three, and five levels for both tone and LFM excitation. The proposed method of controlling harmonics through the use of multilevel switched excitation is especially suitable for applications in which portability, high channel counts, and precise harmonic control are required.