RF is short for Radio Frequency. In communication engineering, The term “RF-signals”  is
used to denote signals containing frequency information in the frequency bands used for radio
communication. The term RF has been adopted by the ultrasound industry, where it is used as
a standard notation for unprocessed data, where the frequency information is intact. In
ultrasound imaging, the received RF signal is the output of the beamformer:

Modern ultrasound probes (linear- and phased array transducers) consist of several
rectangular elements of piezoelectric material. The piezoelectric elements are capable of
converting varying pressure to electrical signals.

At receive, the analog signal from each individual probe element is first amplified (Analog
Gain) to ensure optimal use of the dynamic range of the Analog to Digital (A/D) converters.
The analog gain factor varies with depth to amplify signals from deep regions most (TGC,
Time Gain Compensation).

The sampled signals are delayed individually to focus the beam to a certain depth and
direction. The delayed signals are weighted to obtain the desired apodization and beam
profile. Finally, the weighted and delayed signals are summed in phase, and this is the RF
signal.

The sampling rate of the RF signal at the output of the beamformer is 20 MHz, and the
resolution is 20 bits. The sampling rate of the A/D converters is 40 MHz with a resolution of
12 bits. 7 bits are added by summing 128 channels (128=2^7) and the last bit is added when the sampling rate is reduced from 40 to 20 MHz.

It is also important to note that when acquiring RF-data on the System Five, the Analog Gain
and weights applied to the individual channels are not affected by the Gain knob and TGC
sliders on the front panel. But the overall gain is adjusted if transmit power is changed.

摘自：《Johan Kirkhorn: Introduction to IQ demodulation of RF-data》，版权归原作者所有。