For most seakeeping events, the 0- to 250-cps range is adequate. It 

 may be necessary to go to the 0- to 2500-cps range for slamming and vibra- 

 tion data. 



The technique of separating the frequency components in the signal 

 to be analyzed is based on amplitude modulation. The mathematics involved 

 will not contribute to the continuity of the discussion and is given in 

 the Appendix. However, a brief discussion of what amplitude modulation 

 does will be helpful in understanding the analysis process. 



The local oscillator generates a continuously changing "carrier" 

 frequency which is 97,000 cps plus some frequency, let us say, in the 

 0- to 250-cps range. This frequency is modulated by (mixed with) all the 

 frequency components in the random signal being analyzed. The result of 

 this mixing is a new signal, which comprises all the signals which would 

 result from separately amplitude-modulating the carrier by each frequency 

 component of the random signal. Single component modulation produces a 

 signal that contains the carrier frequency, the sum of the carrier 

 frequencyj and the particular modulating frequency as well as the difference 

 of these two frequencies. The sum frequency is called the 'Sapper sideband" 

 and the difference frequency is called the "lower sideband." The amplitudes 

 of these two waves are proportional to the amplitude of the modulating 

 wave and is the quantity that must be measured. 



The result of modulating a multif requency signal then is a new 

 signal which comprises the carrier frequency and all the upper (sum fre- 

 quencies) and lower (difference frequencies) sidebands associated with the 

 components of the signal to be analyzed. 



