EQUIPMENT AT MOUNTAIN LAKES 



105 



i PROJECTOR 

 I AND COUPLING 



Y0R0PH0NE AND 

 PREAMPLIFIER 



NOTE: S0LI0 LINES REPRESENT PERMANENT 

 CONNECTIONS.DOTTEO LINES OPTIONAL 

 CONNECTIONS 



Figure 11. Block diagram of high-frequency system. 



It consists essentially of a high-frequency thermocou- 

 ple calibrated on direct current and matched to 72 

 ohms. When properly used, this instrument will meas- 

 ure the power dissipated in a 72-ohm load. 



Projector Coupling Network. The purpose of the 

 projector coupling network is to match the imped- 

 ance of a projector to that of the driver and the 

 associated transmission line. Arrangements are also 

 provided for measuring the current to the projector. 



The most common network is a transformer that is 

 designed for standard instruments over this range of 

 frequencies. The majority of the projectors encount- 

 ered have the same general characteristics as the 

 standards and so operate satisfactorily with this trans- 

 former. If a closer match is required, various resist- 

 ances may be added to bring the impedance to the 

 desired value. 



The transformer for high-frequency standards has 

 a nominal impedance ratio of 72:1,000 ohms and is 

 designed especially for a capacitive (crystal) load. 

 Other ratios used are 72:20, 72:72, 72:250, and 

 72:2,000. 



Hydrophone Coupling Circuit. For high-imped- 

 ance hydrophones the usual coupling circuit is a 

 high-impedance input amplifier. Since it is necessary 

 to match the coupling circuit to a low (72-ohm) im- 

 pedance transmission line, it is convenient to look 

 upon these coupling circuits as impedance trans- 

 formers. One of the best electronic circuits for this 

 purpose (an electron tube is necessary because of the 

 high input impedance requirements) is a cathode- 

 follower type circuit. These circuits used in connec- 

 tion with underwater acoustic devices are known as 

 preamplifiers. The single-stage preamplifiers used in 

 the high-frequency system have an input impedance 

 of about 25 megohms in parallel with about 5 fi/xL 

 The output impedance is 72 ohms balanced or un- 

 balanced and the gain is —20 db fiat to within 0.25 

 db over the entire range. One preamplifier is de- 

 signed particularly for the high-frequency standards, 

 while the others are portable for use in current meas- 

 urements and with other hydrophones. 



It should be noted that the abnormally high values 

 of grid resistors are not necessary as at the lower fre- 



