Low loss wideband protection circuit for high frequency ultrasound

Because of the low electromechanical conversion efficiency of current high frequency ultrasound transducers, a large voltage is used on transmit, resulting in weak signals on receive. These signals need to be amplified, and the preamplifier needs protection against the large transmit voltages and immunity from pulser noise. These needs are currently met by using a diode expander on transmit and diode limiters on receive. Limiting may be implemented with either a 50-Ω terminated limiter, which is insensitive to cable lengths, but suffers a loss in S/N due to attenuation of the received signal, or by the use of tuned circuits using fixed cable lengths suited for narrowband operation. Neither of these methods can simultaneously achieve wideband operation and high sensitivity. In this paper, diode front-ends are compared to the use of wideband RF-transformers. At 20 MHz a 4-8 dB increase in S/N was achieved using wideband transformers compared to the standard diode limiters, given the same pulse duration of the received echo, with a measured insertion loss between 55 and 68 dB depending on configuration. At 45 MHz, a 2-8 dB increase in S/N was achieved using transformers, with a measured insertion loss between 52 and 66 dB, but with the cost of an increase in the pulse duration.