7-8] CONSIDERATIONS OF IF PREAMPLIFIER DESIGN 371 



large input conductance of the grounded-grid stage. Since such a feedback 

 conductance is not a noise source, less noise exists than in the case when 

 such network damping is obtained by a physical resistor. 



Wide-band neutralization of the input tube is employed to stabilize the 

 admittance appearing at the input grid. Such stabilization allows the widest 

 bandwidth for which a low spot noise figure is obtained by minimizing the 

 variation of the last term of Equation 7-30 with frequency. 



To minimize the variation of the grid admittance with frequency, a 

 double-tuned (two-pole) mixer-IF coupling network is employed with the 

 cascode input circuit. When this circuit is designed for a flat (Butterworth) 

 response, the bandwidth is given by 



B = -4^ (7-35) 



7rV2C 



where gs is the value of source conductance required for minimum F 



B is the 3 db bandwidth 



C is the total capacitance appearing at the input of the first tube. 



The signal transmission bandwidth is slightly wider than this value because 

 of the loading of the source caused by coil losses and the input conductance 

 of the tube. 



In high-PRF radars (such as the pulsed-doppler systems described in 

 Chapter 6) and where very short range with high accuracy is required, the 

 double-tuned mixer-IF coupling network is found to introduce objectionable 

 transients following the transmitter signal. These transients result from the 

 nonlinear loading on the network by the mixer crystals. In such cases a 

 grounded-grid input stage is employed. The transmission bandwidth of 

 the mixer-IF coupling is very wide because of the heavy damping caused 

 by the input conductance of the grounded-grid stage. The heavy damping 

 by the tube minimizes transients resulting from the crystal mixer IF 

 admittance variation when the transmitter signal is present at the mixer. 



A typical example of IF preamplifier performance is given by the 

 following: 



CIRCUIT AND TUBE PARAMETERS 



g,n = 20 X 10-^ mho 



M = 44 

 figt = 5.0 X 10-^ mho 

 ^1 = 10~^ mho 

 Rn = 150 ohms 



