A digital beamformer for high-frequency annular arrays

Digital transmit and receive beamformers for a 45-MHz, 7-element annular array are described. The transmit beamformer produces 0- to 80-Vpp monocycle pulses with a timing error of less than +/- 125 ps. Up to four adjustable transmit focal zones can be selected. The dynamic receive beamformer uses a variable frequency sampling technique in which the frequency of analog-to-digital conversion on each channel is adjusted as the signals are received. The variable frequency clock signals required to trigger analog-to-digital conversion are obtained using a pair of high-frequency field-programmable gate arrays and a precision quartz oscillator. The gate arrays are also used to sum the digitized signals. A maximum receive beamformer timing error of less than 900 ps was obtained on each channel. The performance of the combined transmit and receive beamformer was tested by imaging wire phantoms. Images of CD-1 mice were also generated. The system produced images with a dynamic range of 60 dB.