Page 609 



RADIO ACOUSTIC RANGING 



6542 



When the sono-radio buoy amplifier is to be adjusted to a desired gain, it is only 

 necessary to introduce a known input voltage and to adjust the gain control of the 

 amplifier until the ratio of output to input voltage equals the desired value. 



6542. Apparatus for Gain Measurements 



of 



Figure 135.— Circuit for measuring and adjusting sono-radio buoy sensitivity. 



The apparatus for making direct gain measurements or adjustments consists 

 an audio-frequency oscillator, an attenuator, and an a-c voltmeter. 



The audio-frequency vacuum-tube oscillator should be a low power type, of % watt 

 or less, with a frequency range from 50 to 1,000 cycles. It should be reasonably stable 

 in voltage amplitude and 



frequency, the latter being ^.ooou, V^" 



independent of load. The '^'^'^ ' ' ^ 



output of the oscillator 

 should be single side (one 

 side at ground potential), 

 and the output voltage 

 should be controllable. The 

 output impedance of the 

 oscillator should be some- 

 where near that of the input 

 impedance of the attenuator. 



The oscillator should be portable so that measurements can be made either in the radio 

 room or on deck. The circuit of a simple oscillator-attenuator is given in figure 135. 

 A number of inexpensive low power test oscillators are manufactured. 



The attenuator is an electric network whose ratio of input to output voltage can 

 be definitely controlled. This ratio is always less than unity. The input voltage is 

 measured by means of a low-range rectifier-type a-c voltmeter, and this input voltage 

 when reduced by the known attenuation factor of the attenuator becomes the output 

 voltage. The attenuation factor of the attenuator can be varied by known amounts, 

 resulting in numerous known ratios. The control dials or switches of the attenuator 

 should be calibrated in volts of output voltage for a fixed input voltage. If some other 

 units, such as decibels, are used, a conversion table should be employed to convert 

 these units into volts of output voltage. The attenuator should have a frequency range 

 from 50 to more than 1,000 cycles. The output impedance should nearly equal the 

 input impedance of the sono-radio buoy. The minimum controllable range of the 

 attenuator output voltage should be from 10 microvolts to 100 millivolts. A satis- 

 factory commercial attenuator, known as the Microvolter, is type 546-^ manufactured 

 by the General Radio Company of Cambridge, Massachusetts. 



An a-c voltmeter is used to measure the output voltage of the sono-radio buoy 

 amplifier. This meter may be a rectifier type, such as the Weston model 301, 0-50 volt 

 rectifier type with a resistance of 2,000 ohms per volt. The use of a vacuum-tube 

 voltmeter is scarcely warranted since most of the measurements are of a comparative 

 nature. 



6543. Procedure for Gain Measurements 



The equipment described in 6542 is arranged as follows when used for testing sono- 

 radio buoys: The output of the oscillator is connected to the input of the attenuator, 



