Service manual

Chapter 4: System Overview 27
The number of elements driven is determined by element sensitivity off axis and depth of field considerations. The
waveform is selected to drive the transducer at a certain center frequency, bandwidth, and power and is optimized
for the given mode.
The receive section consists of a transmit/receive switch to protect the receiver from the transmit voltage, a vari-
able gain receiver to amplify and condition the return echoes, an A/D to digitize the data, a delay block to focus
the return signals and a weight block to scale the return echoes for each channel. All the signals are then summed
together to generate the beam-formed receive data. The analog gain varies with depth to compensate for signal
attenuation through the medium. The delays and weights are independent for each channel. The delay and
weight for the receive channel can typically be changed dynamically to keep the receive beam in continuous
focus. The delay is simply set by the time of flight in the medium from the point of interest to the element, which
starts at skin-line and proceeds to the deepest depth of interest.
The control section drives the data to the various data path elements on a line by line basis, controls the timing of
the transmit and receive sections and controls the tagged information and timing of the data to the rest of the
system.