Multichannel FPGA-based arbitrary waveform generator for medical ultrasound S. Ricci, L. Bassi, E. Boni, A. Dallai and P. Tortoli A compact and cost-effective method for generating simultaneous arbitrary waveforms over several channels is presented. This method exploits the sigma–delta technique combined with the LVDS circuit capability of sustaining bitstreams up to 1 Gbit=s. A 32-channel synthesiser has been successfully fitted in a single FPGA. Experimental results show that this technique is suitable for generating the excitation waveforms needed for medical ultrasound array probes.
Introduction: Medical ultrasound (US) array probes contain hundreds of piezoelectric elements. For each active element the transmitter is requested to produce a waveform with frequency ranging from 1 to 10 MHz and bandwidth up to some MHz, depending on the application. The need for an independent excitation of each element involves a trade-off between electronics complexity and attainable performance in the transmitter design. For example, in the standard excitation scheme each active element is stimulated by a single short pulse [1] or by a sequence of square waves. Although this method implies relatively low electronic efforts, the application of sophisticated signal processing techniques, such as pulse compression [2] or dynamic transmission focusing [3], is prevented. The sigma–delta modulation [4] has been recently proposed to implement a single-channel arbitrary wave generator (AWG) for US medical applications [5]. In that implementation a chirp signal of 2.5 MHz central frequency and 1.5 MHz relative bandwidth was converted in a 40 Mbit=s bitstream through a firstorder modulator. The bitstream was then improved through an advanced tuning technique and finally amplified with a bipolar pulser. Although the pulser reduces transmitter complexity, the AWG performance results are heavily limited by the inability to handle higher frequency bitstreams. In this Letter, a 32-channel compact AWG is presented. The proposed solution, based on a single state-of-the-art field programmable gate array (FPGA) device, is capable of handling bitstreams up to 1 Gbit=s for each channel.
constituted by a double data rate (DDR), synchronous dynamic random access memory (SDRAM) which communicates through a 64 bit bus clocked at 166 MHz, resulting in a 2656 MB=s peak bandwidth capability. Such a performance is compatible with the need to produce 600 Mbit=s continuous data flow over 32 channels (32 600 Mbit=s ¼ 2400 MB=s). A FIFO buffer is placed in the FPGA between the SDRAM controller and the parallel-to-serial converters (PTSCs), to overcome the data flow gaps that occur owing to internal SDRAM operations such as refresh or page change. The FIFO is clocked at 100 MHz to supply words of 192 bits, which are distributed over 32 PTSCs, 6 bits each. The Altera dedicated PTSCs are programmed for a serialiser factor of 6, producing the 600 Mbit=s bitstream from the 100 Mword=s input parallel data flow. The bitstreams leave the FPGA after being translated to the LVDS electrical standard. An array of second-order filters with 32 MHz cutoff frequency recover the desired analogue waveforms that are finally amplified and applied to the transducer. AWG test: The proposed generator has been connected to 32 consecutive elements of a commercial US linear probe with 8 MHz central frequency and 65% bandwidth. The AWG has been programmed to produce a chirp wave focused at 18 mm from the probe. This was achieved by driving each transducer element with a chirp delayed by a time related to the focal depth, and apodised by a Hanning window to reduce the secondary radiation lobes. Equation (1) describes the signal programmed for the ith element: si ðtÞ ¼ HðiÞ rðt ðtl ti ÞÞ wðt ðtl ti ÞÞ
ð1Þ
where r(t) is a 10 ms-long linear chirp sweeping between 7 and 11 MHz, w(t) is a Tukey window with 40% taper ratio and H(i) is the amplitude of the ith sample of a Hanning window. ti represents the time of flight between the ith element and the focus, tl is the delay corresponding to the two lateral elements. These chirp signals were processed through a second-order sigma–delta modulator to produce the 32 independent bitstreams that were uploaded in the RAM buffer of the AWG. No compensation for amplifier and transducer transfer function was considered in this experiment. The transducer was immersed in a water tank in front of a calibrated hydrophone (HGL-0400, Onda, Sunnyvale, CA), placed at the focal point by means of a precision mechanism.
Fig. 2 Pressure measured at 18 mm focal depth when single element and 32 elements excited by chirp waveform a Single element b 32 elements
Fig. 1 Architecture of 32-channel AWG
Circuit description: The 32-channel AWG has been fitted in a single EP2S15F672C5 device of the Stratix II FPGA family produced by Altera (San Jose, CA, USA) (Fig. 1). In this implementation, 600 Mbit=s bitstreams have been obtained. The bitstream of each channel is synthesised off-line, through a sigma–delta modulator, and stored in the memory connected to the FPGA. This buffer is
Fig. 2b shows the pressure produced by the focused acoustic wave and Fig. 2a shows the waveform detected by the hydrophone when the chirp was fired by a single element. Focusing produced a 20 dB gain, not far from the a theoretical value of about 24 dB estimated for an ideal transmitter connected to probe elements with ideal radiation fields. Fig. 3a shows the spectrum of the signal received by the hydrophone (unbroken line) compared with the theoretical chirp spectrum (broken line). The filtering effect of the transducer contributes to the attenuation of the higher frequencies. The acquired chirp was finally processed through a compression filter weighted by a 90 dB Chebyshev window. As shown in Fig. 3b the resulting compressed pulse (unbroken line) features a peak sidelobe level (about 38 dB) very close to the
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theoretical one (broken line). This result confirms the high quality of the transmitted chirp signal.
Acknowledgment: This work has been supported by the Italian Ministry of Education, University and Research (COFIN-PRIN 2005). # The Institution of Engineering and Technology 2007 5 October 2007 Electronics Letters online no: 20072859 doi: 10.1049/el:20072859 S. Ricci, L. Bassi, E. Boni, A. Dallai and P. Tortoli (Department of Electronics and Telecommunications, Universita` degli studi di Firenze, Via S. Marta 3, Firenze 50139, Italy) E-mail:
[email protected] References
Fig. 3 Spectrum and compressed pulse of focused chirp signal acquired by hydrophone (unbroken lines) compared with ideal behaviour (broken lines) a Spectrum b Compressed pulse
Conclusions: A compact, scalable multichannel AWG for US array probe drive has been developed. The sigma-delta technique has been efficiently implemented by exploiting, in particular, the high-speed LVDS capabilities of state-of-the-art FPGAs, so far mainly used for serial digital communications.
1 Brown, J.A., and Lockwood, G.R.: ‘A low-cost, high-performance pulse generator for ultrasound imaging’, IEEE Trans. Ultrason., Ferroelectr., Freq. Contr., 2002, 49, (6), pp. 848–851 2 Misaridis, T., and Jensen, J.A.: ‘Use of modulated excitation signals in ultrasound. Part I: Basic concepts and expected benefits,’, IEEE Trans. Ultrason., Ferroelectr., Freq. Contr., 2005, 52, (2), pp. 192–207 3 Zhou, S., and Hossack, J.A.: ‘Dynamic-transmit focusing using timedependent focal zone and center frequency’, IEEE Trans. Ultrason., Ferroelectr., Freq. Contr., 2003, 50, (2), pp. 142–152 4 Aziz, P.M., Sorensen, H.V., and van der Spiegel, J.: ‘An overview of sigma-delta converters’, IEEE Signal Process. Mag., 1996, 13, (1), pp. 61–84 5 Huang, S.-W., and Li, P.-C.: ‘Arbitrary waveform coded excitation using bipolar square wave pulsers in medical ultrasound’, IEEE Trans. Ultrason., Ferroelectr., Freq. Contr., 2006, 53, (1), pp. 106–116
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