The modulated signal is sent through a stationary band-pass filter 

 centered at 97,000 cps . The carrier frequency component is rejected by 

 the filter because it always generates signals with frequency greater 

 than 97,000 cps. All upper sidebands are rejected because they contain 

 sum frequencies which are always greater than 97,000 cps. The lower 

 sidebands are also rejected, with the exception of that one lower side- 

 band whose difference frequency is exactly 97,000 cps. The net result 

 of the filter's activity is the passage of a pure sinusoid whose ampli- 

 tude is proportional to the amplitude of that component of the modulating 

 (random) signal which is at that moment specified by the local oscillator. 



This discussion is idealized as well as somewhat over-simplified. 

 The filter, in fact, is of finite width, let us say 5 cps. Consequently, 

 it will pass all lower sideband components with frequencies between 

 96,997.5 and 97,002.5 cps. Under such circumstances, it is seen that 

 the analyzer will sort out not individual frequencies in the spectrum 

 but bands of frequencies, depending on the bandwidth of the filter 

 employed. 



Care must be exercised in the initial generation of carrier signals; 

 they must always be of frequency greater than that of the upper side of 

 the band-pass filter (in this case, greater than 97,002.5 cps). 



Since the SEADAC has two analyzer systems (Figure 1) , the carrier 



wave is modulated simultaneously (and separately) by two of the signals 



on the magnetic tape and everything that has just been discussed occurs 



independently and simultaneously, with respect to these two signals. For 



10 



