1-51 



THE MODULATION OF RADAR SIGNALS 



23 



These modulations create information sidebands about the carrier and 

 subcarrier frequencies. The designer's problem is to determine how this 

 information may be extracted from the target return signal. 



Extraction of Target Intelligence from Radar Signals. One thing 

 is common to all the many techniques for extracting target information 

 from a radar return signal. This is the concept of taking a product between 

 the target return signal and another quantity which serves as the reference 

 for the particular piece of information being extracted from the target 

 return. Thus, the generic building block for a radar receiving system is a 

 product-taking device, as shown in Fig. 1-17. 



Fig. 1-17 Generic (Product) Building Block for a Radar Receiver. 



Conceptually, the simplest product-taking device is a network — or 

 filter — composed of linear impedances which can be characterized by a 

 transfer function F{jui). Each frequency component of the incoming signal 

 is multiplied by the vector transfer function of the network corresponding 

 to the frequency (see Fig. 1-18). The output product is a signal containing 



Input 



F(;co) 



Output 



[Output] =[f(; CO)] X [input] 

 Fi/co) - Fl(j} 



Fig. 1-18 Impedance Products. 



the same frequencies as the input; however, the amplitude and phase of 

 each frequency component may be changed with respect to the input. 

 In this type of product device, the references are the characteristics built 

 into the filter. 



The second type of product-taking device is the nonlinear impedance. 

 A simple example of such a device is shown in Fig. 1-19. The operation 

 of the device is such that positive inputs are faithfully reproduced at the 

 output while negative inputs are completely suppressed. Thus, for an 



