14-24] SYSTEM CONSIDERATIONS 797 



The signal-to-noise ratio of the system (a measure of system sensitivity) 

 is inversely proportional to NEP and the square root of bandwidth: 



S/N cc (NEP)-i(A/)-i/2. (14-41) 



Neglecting any other degradation, particularly of the recording system, 

 system sensitivity may well be orders of magnitude less than detector 

 sensitivity. For example, if the scan rate and resolution demands are such 

 as to require 1-Mc bandwidth, then 



S/N oc (NEP)-i(10«)-i/2 = (NEP)-i(10^)-i (14-42) 



and if the cell NEP is lO'^^ watt, then 



S/N oc 10" X 10-3 = 108 (14-43) 



whereas it would have been 10", given a normalized 1-cycle bandwidth. 

 Thus the price of bandwidth is system sensitivity. A compromise in 

 resolution, scan rate, and system sensitivity must be decided upon in the 

 light of the particular requirements of each system. 



Among the considerations for the electronic circuitry into which the 

 detector operates, the most important is that of noise. The preamplifier 

 noise level must be below that of the detector. Matching the detector to its 

 load may not be possible because of the extremely high impedance shown 

 by some cells (up to 150 megohms for lead telluride). Such high impedances 

 are extremely conducive to noise pickup. In the case of cells such as the 

 1.5-megohm P-type gold-doped germanium, matching is not desirable from 

 the bandwidth point of view. A cascode preamplifier is ideally suited to 

 this cell. Low noise and elimination of the Miller effect are features of this 

 circuit. 



Numerous techniques have been used for recording and displaying 

 infrared mapping information. Whatever the method, care must be taken 

 to assure full use of the resolution capabilities of the remainder of the 

 system. The resolution of the recorder should not provide the limiting 

 resolution. If it does, sensitivity is being scarificed. Furthermore, the 

 dynamic range of the recorder should match that of the foregoing system 

 as much as possible. 



As mentioned earlier, the scan rate must be variable in accordance with 

 the velocity-to-height ratio. Identical v/h information must be given to the 

 display system in order that it may be in synchronism with the scanning 

 element. 



Generally some provision is made for correction of errors caused by roll, 

 pitch, and yaw. Roll is usually a problem even if pitch and yaw may be 

 neglected. A stabilized platform may be used for mounting of the system. 

 An alternate scheme is to provide the correction electronically. This would 

 be done in the indicator and may be the more convenient, especially when 

 size and weight are critical factors. 



