102 



USRL TEST STATIONS 



Figure 38. Simplified circuit arrangement for determina- 

 tion of peak factors. 



same deflection on the oscilloscope as the test signal. 

 The difference between the 30A set readings on the 

 test and oscillator signals gives the difference in db 

 between the crest factor of the complex wave and that 

 of the sine wave. Since values are being determined in 

 volts and a sine wave has a crest factor of 1 .4 1 ( = 3 db), 

 this amount has to be added to the observed differ- 

 ence in db before reverting to the ratio that expresses 

 the crest factor of the complex wave. 



The investigation includes observations of the total 

 (broad-band) energy and of the energy distribution 

 with respect to frequency from 100 c to 150 kc with 

 the receiving amplifier output connected directly to 

 the broad-band recorder. The examination of energy 

 distribution with respect to frequency over the test 

 signal spectrum can be made with the normal facili- 

 ties of system 2, which has three acceptance bands in 

 the detector circuit, 10, 300, and 6,000 c. As these 

 bands become a continuously smaller percentage of 



the frequency as it increases, the 10-cycle band usually 

 is carried only to 20 c and the 6-kc band is not used 

 below 4 kc. When the recurrence rate of the signal is 

 too low, the recorder will follow the individual cycles 

 which involve the difficult)' of evaluating wide and 

 irregular traces. Suitable resistors are inserted in the 

 pen-drive motor circuit to reduce the response rate 

 and thereby minimize the recorder excursions. 



The energy in band widths other than those pro- 

 vided by the detector circuit may be obtained by the 

 use of the circuit arrangement shown in Figure 39. 

 The plan of this circuit is to transpose the signal fre- 

 quencies by heterodyning them so that the desired 

 signal band may be obtained with available filters. 

 The method is illustrated in Table 2, which gives the 

 value of the signal frequency at various stages, with 

 the circuit adjusted to pass only a band of frequencies 

 1 kc wide centered at 10 kc. 



Band widths commonly used in such investigations 

 include 5 ± 0.25 kc, 10 ± 0.5 kc, 20 ± 1 kc, 30 ± 1.5 kc, 

 and 40 ± 2 kc. It is obvious, however, that the system 

 may be adjusted for any band width and mid-band 

 frequency requirement within the limits indicated on 

 Figure 39. 



A determination of the rate of signal recurrence is 

 usually made by inspection of the broad-band re- 

 corder traces. For high values, the speed of paper 

 drive on the recorder should be at a maximum. 



The investigation of the operational stability or 

 life characteristics of expendable devices is made by 



Table 2. Value of signal frequency in kilocycles per second at various stages 

 when circuit is adjusted to pass all frequencies in the hand 10 ± 0.5 kc. 



The dial setting of the heterodyne oscillator is 12.5 kc to obtain 637.5 kc. This is the sum of the lowest signal frequency to be 

 passed and the half-band width of the filter in the detector circuit added to a fixed frequency. 



Frequency of oscillator No. 3 is set at the sum of upper cutoff frequency of the detector filter and the cutoff frequency of 

 the low-pass filter minus the desired band width. 



xxx indicates the filter that cuts off the frequencies outside the desired band. 



