As another example, the waveform generator 12 comprises any of the transmit beamformers disclosed in U. In alternative embodiments, the waveform generators 12 comprise transmit beamformers disclosed in U. The waveform generator [] 12 also includes control and timing circuits, such as digital or analog devices for controlling the transition between different amplitude levels.
For example, a bipolar transmitter responsive to a MHz timer or sampling clock frequency is provided, but other slower or faster clock frequencies may be used. In one embodiment, the waveform generator 12 generates waveforms with fewer than four amplitude levels e. Due to the analog nature some waveform generators 12 , amplitude level as used herein allows for transition and ringing around the desired amplitude level.
The waveform generator [] 12 generates a transmit waveform which is a function of one or both of pulse width modulation and coding. The transmit waveforms may be a function of other parameters such as amplitude and phase modulation. The transducer [] 14 comprises a single element, one-dimensional array, or multi-dimensional array of elements.
The elements are micro-electromechanical MUT or piezoelectric elements for transducing between acoustic and electrical energy. The transducer 14 receives waveforms generated by the waveform generator 12 and converts the electrical waveform to acoustic energy. The acoustic energy travels within a patient and reflects from tissues and fluids.
The transducer 14 receives the reflected acoustic energy and generates responsive electrical signals. The electrical signals generated by the transducer [] 14 for receiving acoustic energy are provided to a receive beamformer and filter or processor. The filter or processor apply the inverse of the coding used for the transmit waveform.
For example, a matched finite impulse response filter or other time or frequency domain filtering is provided. If the inverse coding is applied to in-phase and quadrature or base band information, a complex filter may be used. A real or non-complex filter may be provided for ratio frequency information. Resulting information is detected and processed for generating an ultrasound image, such as a B-mode or flow-image. In act [] 18 , a waveform is generated. The waveform is applied to the transducer in act Additional acts may be provided.
In one embodiment of act [] 18 , the waveform generated is a unipolar or bipolar waveform. The magnitude of the waveforms 22 , 24 as shown is a normalized magnitude. The bipolar waveforms 22 , 24 have fewer than four amplitude levels, such as using two or three amplitude levels. In yet other alternative embodiments, a greater number of amplitude levels are provided, such as five or more amplitude levels for more closely resembling a sinusoidal waveform.
The waveform generator in act [] 18 is responsive to coding. Coded excitation is implemented through one or combinations of two or more of phase, amplitude and frequency changes within the transmit waveform as a function of time or between sequential transmit waveforms.
Any one of various types of coding may be used, such as chirp or frequency swept coding, Golay coding, or Barker coding. For example, the Golay coding described in U. As another example, a chirp coding is provided. The chirp spans any of various frequency ranges, such as a range within MHz.
In one embodiment, a 1 to 10 MHz wide frequency linear sweep is provided by the chirp waveform. The chirp coding begins at the lower end of the frequency range of the chirp and ends at the higher frequency range, but chirps beginning with a high frequency component and ending with a low frequency component may be used. The frequency within the waveform varies linearly or in response to non-linear or other function.
A chirp waveform is described by the following equation:. The dashed waveform [] 24 of FIG. As in FIG. As a result, the width of the pulses 28 at lower numbered samples is greater than the width of the pulses 28 at higher numbered samples. While chirp coding is discussed above, other coding, such as Golay, Barker, phase, amplitude, or frequency coding may be provided. The chirp bipolar waveform 24 of FIG. Since bipolar waveforms are generated with few amplitude levels, pulse width modulation is used to reduce harmonic content without amplitude modulation, reducing the sidelobe levels.
Pulse width modulation changes the width of pulses throughout a transmit waveform. The width is modulated to provide a desired energy magnitude content of the waveform. For example, the pulse width modulation is responsive to an envelope function that rises gradually and falls gradually, such as a Gaussian, Hamming or other function. There may be various methods to perform pulse width modulation.
One is to make the bipolar rectangular pulse with the same area as the desired waveform pulse. The width may be determined using various functions. The window represents the desired amplitude envelope function, such as a Gaussian, Hamming or other function. The envelope [] 32 of the ideal waveform 30 of FIG. The envelope 32 rises gradually and falls gradually for minimizing harmonic content.
The solid line bipolar waveform 22 e. Spwm t of FIG. The width or time duration of each pulse 28 e. Pulses 28 at the beginning and end of the waveform are narrower than at the middle of the waveform. Likewise, the pulses 28 are narrower at the beginning and end of the waveform than the pulses would be without pulse width modulation.
Where the width or duration of a pulse 28 is reduced for pulse width modulation, the zero magnitude level or amplitude is generated to maintain a same frequency of the pulses. Since the width of each pulse [] 28 is also a function of the frequency associated with the pulse, chirp coded excitations also affect the width of pulse width modulated pulses For example, the pulses 28 associated with low frequency components the beginning of the transmit waveform as shown in FIG.
In alternative embodiments, the coded excitation does not alter the width of the pulses. The pulse width modulated bipolar chirp waveform 22 provides a more compact spectrum with fewer harmonics than the bipolar chirp waveform As a result, the pulse width modulated bipolar chirp waveform 22 provides reduced axial sidelobes after convolution on the receiver side. In act [] 20 , the generated transmit waveform is applied to the transducer 14 FIG.
The transducer 14 converts the applied electrical transmit waveform to an acoustic waveform. The frequency response characteristic of the transducer [] 14 may alter the waveform. The acoustic waveform generated at the face of the transducer is effectively a filtered version of the electrical transmit waveform applied to the transducer The frequency response of the transducer 14 may be accounted for in the generation of the transmit waveform or may not be considered.
As a result of accounting for the transducer response, a more desired waveform is generated in the acoustic domain. The waveform generator 12 of FIG. For example, a bipolar or unipolar transmit waveform is generated so that the acoustic energy at the face of the transducer has less second harmonic energy than the electrical waveform applied to the transducer to generate the acoustic waveform. Frequency components of the electrical transmit waveform associated with greater filtering by the transducer 14 are emphasized to account for the greater filtering.
For example, a bipolar chirp coded excitation transmit waveform is generated for second harmonic imaging. Since the same transducer 14 is used for both transmit and receive processing, the fundamental transmit frequencies of the chirp coded excitation transmit waveform are positioned at a lower frequency portion of the bandwidth of the transducer In this portion of the bandwidth, the frequency response of the transducer 14 more greatly filters or removes information associated with lower frequencies.
To counteract the difference in transducer filtering as a function of frequency, the pulse width modulation is adjusted to generate the transmit waveform with greater amounts of energy. Less pulse pulses narrowing, i. The pulse width modulation is a function of the frequency response characteristic of the transducer. In addition to the pulse width modulation being a function of the envelope 32 of the desired waveform, the pulse width modulation also accounts for the frequency response of the transducer For a chirp coded excitation, the pulse widths are responsive to the pulse width modulation and associated envelope signal, the frequency associated with the chirp coding and the frequency response of the transducer For example, pulses 28 associated with higher frequencies of a chirp coded excitation are narrower in response to both less filtering of higher frequency signals by the transducer 14 and the gradually falling amplitude of the envelope function implemented by the pulse width modulation.
Characteristics of the transmit waveform, such as frequency, phasing, amplitude, pulse width or other characteristics, are adjusted to provide a desired amplitude spectrum. Any one or combination of two or more of techniques for accounting the frequency response of the transducer, or reducing sidelobe levels may be used. The electrical waveform applied to the transducer has an amplitude spectral function that varies as a function of the frequency response characteristic of the transducer [] The variance is approximately an inverse of the transducer frequency response within the bandwidth of the transmitted waveform to counteract the frequency filtering effects of the transducer The amplitude spectrum will have larger amplitudes for lower frequencies than for higher frequencies where the frequency response of the transducer more greatly filters lower frequency components.
In alternative embodiments, a coded excitation without pulse width modulation or non-coded excitation waveform with fewer than four amplitude levels is generated with characteristics altered to account for the frequency response of the transducer.
As the signal propagates to a focal region, additional filtering effectively provided by the tissue may be accounted for as well as the frequency response of the transducer [] Likewise, the spectral response of components of the transmit path for applying the electrical waveform to the transducer 14 , such as cabling, digital analog converter, or other components may be counteracted by altering the transmit waveform.
Pulse width modulated encoded excitation waveforms reduce the sidelobe levels generated by bipolar and unipolar transmit pulses. Reduced sidelobe levels may increase visibility of tissue structures, such as cysts, due to improvements in the signal-to-noise ratio.
The pulse width modulation reduces range or axial sidelobes due to harmonic information provided by square waves or waves with few amplitude levels. Some sidelobes may be evident due to processes other than the transmission of harmonic information.
With chirp coding, lower frequency components may have second harmonics at a same frequency as higher frequencies of the intended fundamental components of the chirp excitation. Such harmonics are not filtered from receive signals since filtering may remove useful information as well. Pulse width modulation reduces the amount of harmonic information from low frequency components introduced at a same frequency as high frequency fundamental components. Pulse width modulation of coded excitation waveforms provides a more controlled or desired transmit spectral design for imaging.
Network Security. Computer Network Quizes. Table of Contents. Save Article. Improve Article. Like Article. Previous What is Scrambling in Digital Electronics? Next Manchester Encoding in Computer Network. Recommended Articles. Article Contributed By :. Easy Normal Medium Hard Expert. Writing code in comment? With RZ coding, the waveform returns to a zero—volt level for a portion usually one—half of the bit interval. The waveforms for the line code may be further classified according to the rule that is used to assign voltage levels to represent the binary data.
This type of signaling is also called on—off keying OOK. The binary 0 is represented by a zero level. The term pseudoternary refers to the use of 3 encoded signal levels to represent two—level binary data. This is also called alternate mark inversion AMI signaling. Each binary 1 is represented by a positive half—bit period pulse followed by a negative half—bit period pulse. Similarly, a binary 0 is represented by a negative half—bit period pulse followed by a positive half—bit period pulse.
This type of signaling is also called split—phase encoding. For example, the unipolar NRZ line code has the advantage of using circuits that require only one power supply, but it has the disadvantage of requiring channels that are DC coupled i. However, the circuitry that produces the polar NRZ signal requires a negative voltage power supply as well as the positive voltage power supply.
The Manchester NRZ line code has the advantage of always having a 0 DC value, regardless of the data sequence, but it has twice the bandwidth of the unipolar NRZ or polar NRZ code because the pulses are half the width. When serial data is passed through many circuits along a communication channel, the waveform is often unintentionally inverted i. This result can occur in a twisted pair transmission line channel just by switching the 2 leads at a connection point when a polar line code is used. Note: such switching would not affect the data of a bipolar signal.
Each digit in an differential encoded sequence is obtained by comparing the present input bit with the past encoded bit. A binary 1 is encoded if the present input bit and past encoded bit are of opposite state.
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