Ultrasound Physics
Ultrasound Signal Conditioning
Echo signal needs go through a series of processing before enters ADC or brightness modulation to construct image, including filter, amplification, and dynamic range control.

An about four-order LC filter should provide enough sharp cutoff for the required bandwidth. It is important to have a band-pass filter before the main amplification.

Echo signal needs about 60-90dB gain in total and a 60dB fixed gain with a 30dB adjustable gain should be enough for most applications.

Dynamic range control can be achieved by logarithmic amplification in analog domain or a look-up-table in digital domain which is more flexible. The echo signal from bone or air may be 100dB stronger than the scattering from soft tissue. Dynamic range has to be compressed since the display usually only have about 30dB (256 grayscale).

Echo signal is mainly amplitude modulated, and it needs go through envelop detection before brightness modulation. Envelop can be conducted in analog domain with a simple rectifier and low pass filter or using Hilbert transform in digital domain. The Hilbert transform method has better performance since it detects the true amplitude of the analytic signal. However, the analog circuit will reduce the required ADC clock dramatically since the carrier frequency is removed.