INTRODUCTION TO SONAR 



three basic functions: transmission, reception, 

 and presentation, A block diagram of a repre- 

 sentative sonar system is seen in figure 6-5. 

 The diagram shows only the system's major 

 units and main signal paths. 



Transmission 



Transmission of the sound pulse is initiated 

 in the control-indicator, which contains the neces- 

 sary keying circuits. Pulse length is also deter- 

 mined at the control-indicator by the sonar 

 operator. The keying pulse from the control- 

 indicator triggers the transmitter oscillator in 

 the receiver-scanning system assembly and actu- 

 ates the transmit-receive switch in the audio- 

 frequency (a-f) amplifier. In the receiver-scanner 

 the pulse is modulated to the equipmi^nt operating 

 frequency, amplified, and delivered to the sonar 

 transmitter, where it is further amplified to 

 the power level required for transmission. 



The output of the sonar transmitter is fed 

 to the transmit-receive switch in the a-f ampli- 

 fier, then to the transducer transfer switch 

 where selection is made between NORMAL or 

 MCC operation. The signal then goes to the 

 transducer where it is converted to acoustical 

 energy and propagated into the water. The shape 

 of the transmitted signal resembles that of a 

 hollow cylinder (or sphere, depending on the 

 shape of the transducer), which expands in 

 diameter as it travels outward. The thickness 

 of the cylinder walls depends on the pulse 

 length selected by the operator. (See fig, 6-6.) 



Reception 



If the transmitted sound wave strikes an 

 object having sufficient reflective characteristics, 

 a small portion of the signal is returned to the 

 transducer. In the transducer, the acoustic signal 

 is converted to an electrical signal by the 

 piezoelectric effect of the receiving staves. 

 Each of the 48 staves has its own preamplifier, 

 located in the a-f amplifier. After preamplifica- 

 tion the signal is sent to the video and audio 

 scanning switches in the receiver-scanning switch 

 assembly. 



The video scanning switch rotates contin- 

 uously, thereby sampling noise signals and echoes 

 from all bearings. The audio scanning switch 

 is positioned to any bearing selected by the sonar 

 operator. Signals from the scanning switches 

 are fed to the receiver section of the receiver- 

 scanner where they are converted to suitable 

 frequencies and amplified for presentation. 



PULSE LENGTH 



SOUND CYLINDER 



71.52 

 Figure 6-6. — Pulse length determines thickness 

 of sound cylinder. 



Presentation 



For the returning echo to be of any value, 

 it must be presented in such a manner that 

 the sonar operator can interpret the informa- 

 tion it represents. The echo is presented to the 

 operator both visibly and audibly. 



The manually positioned audio scanning switch 

 feeds the signal to the audio portion of the re- 

 ceiver, which is of the superheterodyne type. 

 Incorporated in the receiver audio circuits is a 

 control for eliminating the effect of own ship's 

 speed on the reverberation pitch. This control 

 is called the own doppler nullifier (ODN). It 

 removes own ship doppler effect from target 

 echoes, greatly aiding the operator in target 

 classification. From the receiver the audio signal 

 is sent to a headset position at the control- 

 indicator, and also can be fed to loudspeakers. 



After the output of the video scanning switch 

 is processed in the video portion of the receiver, 

 it is displayed at the control-indicator on a 

 cathode ray tube. The sweep on the CRT is a 

 spiral scan, which is synchronized with the video 

 scanning switch. (See fig. 6-7.) 



One method of obtaining a spiral scan is to 

 rotate the field of the deflection coils around 

 the CRT, simultaneously applying a sawtooth 

 voltage to the coils to cause displacement of 

 the sweep with each succeeding rotation of the 

 coil. For clarity of explanation, the coil yoke 

 is mechanically connected to the video scanning 

 switch. As the video scanner and deflection 

 coil rotate, the sawtooth voltage causes the 

 sweep to spiral outward from the center of the 



96 



