478 AUTOMATIC TRACKING CIRCUITS 



(3) Mechanical stresses increase with the square of the lobing rate. 

 Thus, to avoid excessive wear and vibration the lowest possible lobing 

 frequency should be employed. 



(4) The lower limit on lobing frequency is dictated by stability consider- 

 ations. 



For these reasons, the lobing frequency is usually between 30 and 100 cps; 

 it may even be variable to provide a defense against countermeasures. As 

 previously discussed, the design of the AGC loop, as well as the track loop, 

 is influenced by the choice of the lobing frequency. 



The carrier frequency and a band of frequencies 10 cps above and below 

 it must be transmitted by the receiver over the full range of input power, 

 or possible lobing frequency variation, without an attenuation greater than 

 1 db or with a phase shift greater than 10°. This is necessary because the 

 target information needed to provide accurate tracking line motion may 

 have significant frequencies up to 1 cps, and adequate stability and accu- 

 racy in each control channel can be obtained in practice only if frequencies 

 up to 10 times this significant frequency are provided within the specified 

 tolerances. This is discussed in more detail in Paragraph 9-8. 



The receiver transmits the space error information in terms of a voltage 

 with respect to milliradians of target angle from the boresight line. This 

 scale factor should be controlled within ± 1 db as stated above to prevent am- 

 plitude modulation and false error information, although its nominal design 

 value is not critical. For practical realization of the angle track amplifiers, 

 however, it would be desirable to have a scale factor compatible with the 

 practical threshold levels and amplification limits in the control system am- 

 plifiers. In the actual mechanization, it is necessary to distribute the track- 

 ing loop gain appropriately between a-c amplifiers, d-c amplifiers, and inter- 

 vening compensating networks. Proper location of the compensating net- 

 works will do much to lessen saturation problems during transient operation. 



Glint Noise Inputs. The magnitude of the glint noise at the antenna 

 drive angle track input will be essentially. the same as that appearing at the 

 receiver input. It is unmodified by the receiver because glint noise is 

 actually angular noise which affects the magnitude and phase angle of the 

 carrier frequency. Since the carrier frequency contains the true angular 

 error information as well as glint noise, it must be transmitted through the 

 receiver unmodified. Only after the angular information is extracted from 

 the carrier frequency can the glint noise be modified, to a limited extent, 

 without seriously affecting the target information. In Chapter 4, the 

 anticipated target glint noise is described. It is assumed here to have a 

 spectral density of the form 



J^(l + ^)"" (9-1) 



