Navigation and Seamanship

3.1.8.3. Receivers

The function of the receiver is to amplify the weak electrical oscillations produced in the transducer by the echo so that they can be recorded or displayed. The weak electrical signals produced in the transducer when echoes are received must be amplified several thousand times before being passed on to the recorder. This amplification takes place in the receiver and the amount of amplification can be decided by the sensitivity control. In order to remove the echoes from very close range which are much stronger than those from a distance, the sensitivity of the receiver is automatically suppressed.

In certain modern echo sounders, the automatic sensitivity control is adjusted to cover the complete operational depth range of the echo sounder and to compensate for dispersion and absorption of sound within this range. The strength of a returning echo is thus independent of the depth from which it comes and dependent only on the echo target strength of the reflecting objects, such as fish or sea bottom. This type of depth adjusted automatic amplification is known as Time Varied Gain (TVG) .The level of amplification automatically adjusted with the depth from which the echo is coming to the transducer .