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can obscure Doppler signals of interest, as shown in figure 5 or produce unwanted “tones” in the Doppler spectrums. In pulsed Doppler applications the ratio of the transmit signal power to the noise at 1kHz offset needs to be less than 140dBc/Hz. For CWD, the requirement is even more demanding at 155dBc/Hz or greater. There are many sources of this low-frequency noise, but the largest and most common is low-frequency power-supply noise which can cause a host of Doppler problems. It can introduce jitter in sensitive digital transmit and receive clocks which can, in turn, limit the receiver’s dynamic range or produce unwanted Doppler tones. It can also create lowfrequency to judiciously use analog IC solutions that are already highly integrated, are low power, and meet the required levels of performance. They must also anticipate and then perform the necessary detailed system-level design work to avoid the common noise-related problems inherent in these highly compact designs. The benefits of these new more portable systems are more than worth any design risks. We have already seen the positive impact that these systems have had on healthcare worldwide. There is no reason to believe that this trend will not continue —as long as more highly integrated analog IC solutions become available to the equipment designers of these highly compact medical systems. Powering your next design. Let us be your power expert. We understand that you don’t have the time to master every aspect of electronic design. As a leading manufacturer of power supplies we are here to collaborate with you to ensure your next project is a success. Dc-Dc Converters www.cui.com/PowerExpert Novum® Advanced Power Ac-Dc Power Supplies noise on VGA gain control signals that can modulate large received signals from stationary tissue and obliterate weak adjacent Doppler signals. Power-supply noise in the audio spectrum can only be effectively reduced by active regulation of the supplies. Traditionally, in larger cartbased systems, power-inefficient linear regulators were distributed liberally throughout the system to effectively control this source of noise. In more portable systems this type of solution is quite often not acceptable. As a result, designers must utilize distributed switching regulators to improve efficiency. Unfortunately, this type of regulation can introduce significant RF in-band conducted and radiated switching noise that is difficult to control, even with proper bypassing. Spectral Doppler is particularly sensitive to this type of noise as the discrete switching frequencies can cause tones in the Doppler spectral displays, a common artifact in these systems. One way to ensure that this type of noise is not visible is to ensure that the switching-regulator frequency is synchronized to the master clock of the system. In this way, switching noise can be more easily managed out of the Doppler bands of interest, and a high level of efficiency can be achieved. Considerable attention must be given to the use of switching regulation in these designs to keep power low and avoid difficult-to-solve Doppler artifacts. Where do we go from here? Design engineers agree that designing portable ultrasound systems presents significant challenges. Limited space, managing power within shrinking space constraints, and the demand for higher and higher levels of performance are presenting new and quite significant problems to overcome. Designers need www.electronics-eetimes.com Electronic Engineering Times Europe January 2014 23


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