The transducer is fixed over the mother's abdominal wall, with the help of a gel (or) oil. A Radio Frequency Oscillator ( RFO) is switched ON to drive the pulses and hence the transducer produces ultrasonic waves of 2 MHz. These ultrasonic waves are made to incident on the fetus.
The reflected ultrasonic waves from the fetus are received by the transducer and are amplified by Radio Frequency Amplifier (RFA). Both the incident and received signals are mixed by the mixer and is filtered to distinguish the various types of sound and finally the
Doppler shift (or) change in frequency is measured. The movement of heart can be viewed visually by CRO (or) can be heard by the loud speaker, after necessary amplification by Audio frequency (AF) Amplifier.