7-20] PROBLEMS IN MEASUREMENT OF RECEIVER CHARACTERISTICS 393 



obtained from a standard signal generator which can provide the same 

 modulation characteristics as the radar target. In a noncoherent radar the 

 sensitivity is measured at various ranges. At minimum range the sensitivity- 

 is usually reduced owing to the attenuation characteristics of the TR tube. 

 It is sometimes convenient to define the sensitivity of a radar by an A scope 

 measurement. In these cases a ^angeniiai signal measurement is made. For 

 a tangential signal the signal-to-noise ratio is approximately +4 db. 



Measurement of the sensitivity of a tracking receiver requires that the 

 transfer function of the loop be determined at various input signal power 

 levels. The minimum signal power required to produce the full dynamic 

 tracking capability of the loop is determined. Measurement involves the 

 insertion of a fixed power level RF signal having the modulation character- 

 istics of the radar signal, and measurement of the transfer function of the 

 particular tracking loop for this fixed input signal level. More detail on the 

 means for measuring the transfer functions of the regulatory and tracking 

 systems will appear in the following two chapters. 



In making sensitivity measurements, accuracy is sometimes limited by 

 signal leakage from the standard signal generator. Frequently it is neces- 

 sary to put additional shielding around the generator, and connect a second 

 precision attenuator in the line between signal generator and receiver. 



When measuring the sensitivity of very narrow band receivers such as are 

 employed for doppler radar applications, it is usual to modulate the STALO 

 (stable local oscillator) signal to obtain a signal source of adequate fre- 

 quency stability that will remain within the narrow predetection filters. 

 If the long-term stability of the STALO is reasonably good, a standard 

 signal source which is crystal controlled may be used, provided the pre- 

 detection bandwidth is not less than about 10~* times the RF signal input 

 frequency. 



