THE RADAR RECEIVER 737 



The additional noise contribution under the condition where an impedance 

 is placed in the grid circuit is shown by the noise generator V g ■ This noise 

 source is related to the active grid loading. The resistance Rq represents 

 this effective loading which is due to transit time and tube lead inductance 

 effects and, therefore, has a value which is associated with the frequency of 

 operation and the particular design of tube employed. At very low^ fre- 

 quencies, i?G — ^ 00, the shot noise effects are entirely controlling, and the 

 optimum IF ampUfier input grid circuit condition for minimum noise figure 

 is that condition where the impedance of the signal source approaches an 

 infinite impedance. As the frequency of operation is increased, Rg assumes 

 a finite decreasing value and the optimum signal source impedance is given 

 by the relationship illustrated in Fig. 26. It should be noted that the 

 frequency values associated with the above characteristic are indicative of 

 the performance of the 6AK5 pentode, one of the most satisfactory of avail- 

 able tubes for this purpose. It should be observed that the input IF ampli- 

 fier stage noise figure is independent of the impedance of the signal source 

 providing that a perfect or lossless transformer can be employed to achieve 

 the optinum impedance transformation indicated. 



The realization of proper impedance transformation characteristic in the 

 radar IF amplifier input circuit is basically a network problem. The input 

 grid impedance which can be mamtained over the desired band of frequencies 

 is limited by the total capacitance present in the grid circuit. For narrow 

 IF band widths, particularly at the lowei frequencies, the realizable imped- 

 ance at the grid may be in excess of the active grid loading value and here 

 the noise performance indicated in Fig. 26 may be realized. However, for 

 wide IF band widths at normal radar IF midband values the maximum grid 

 circuit impedance which can be achieved under the limitation of the grid 

 circuit capacitance will be less than the value associated with the active grid 

 loading and the noise figure obtained will be somewhat higher than the opti- 

 mum shown. This restrictive design condition is referred to as "band width 

 limited." In modern military radar receivers this condition normally 

 obtains. 



To achieve the maximum input coupling network efficiency it is extremely 

 desirable to minimize all parasitic capacitances under control of the designer 

 and to employ the most effective network arrangement available. The 

 double-tuned transformer and autotransformer networks are commonly 

 employed for this purpose. Any loss in the impedance transforming net- 

 work will result in further degradation of the noise performance, since this 

 loss is effective in reducing the signal energy while having no effect on the 

 tube noise level. The magnitude of the impedance transformation ratio 

 required in a typical radar design case for optimum noise figure is approxi- 

 mately seven times, where a crystal converter of the type shown in Fig. 14 



