366 THE RADAR RECEIVER 



Groups of two-poles (staggered pairs) are frequently used. Although the 

 restriction of dynamic range is not too severe, such designs are nevertheless 

 inferior to synchronous stages. 



When an amplifier is built with single-pole coupling circuits, the overall 

 frequency response exhibits geometric symmetry with respect to the IF 

 center frequency. With two-pole coupling provided by the magnetically 

 coupled double-tuned transformer, the response is more nearly arith- 

 metically symmetrical. With two-poles having ^ ratios of about 3 to 1 

 the usual amplifier requirements (bandwidth between 1.0 and 10.0 Mc) 

 can be realized with adequate stability margin and fewer components than 

 with single poles. 



One of the main difficulties encountered in the design of radar IF am- 

 plifiers is accurate control of the stability margin. Pole shifting (regenera- 

 tion) can occur under strong signal conditions and results in poor transient 

 characteristics or modulation distortion on desired signals. 



The principal source of feedback in an IF amplifier is the grid-to-plate 

 capacitance of the tube and circuit. Other feedback paths are: 



1. Coupling between input and output leads 



2. Coupling due to the chassis acting as a waveguide beyond cutoff 

 frequency 



3. Grid-to-cathode feedback 



4. Inadequate decoupling circuits resulting from self-inductance of 

 bypass capacitors and their connecting leads 



5. Coupling between heaters 



6. Coupling between input and output caused by ground currents. 

 (The impedance of the chassis is not negligible. It is necessary that 

 the output and input currents not flow through the same part of 

 the chassis to ensure stable operation.) 



The grid-to-plate feedback can be partly compensated by proper circuit 

 design. It is advisable that a common bypass capacitor be employed for 

 the screen grid and plate return of the amplifier. Appropriate choice of 

 this component then enhances the amplifier stability. In high-frequency 

 stages which are gain-controlled it is also desirable that feedback be intro- 

 duced in the cathode lead to stabilize the input susceptance of the tube. 

 These circuits are shown in Fig. 7-6. It is further desirable that such stages 

 employ vacuum tubes with separate suppressor grid terminals to minimize 

 the feedback from plate to cathode. This feedback path leads to instability 

 when the input susceptance of tubes having internal suppressor grid-to- 

 cathode connections is to be stabilized. 



The most economical distribution of gain and selectivity in the IF 

 amplifier occurs when the stages are made identical. However, this con- 

 dition does not al<Vays provide the most stable operation. The latter IF 



