Focus Electrode 

 Modulating Anode 

 Tube Body 



11-5] RADAR SYSTEM EMPLOYING A HIGH-GAIN AMPLIFIER 601 



insulated from the main body of the 

 tube which contains the RF cir- 

 cuitry. If this modulating anode is 

 run at the potential . of the main 

 body of the tube, the electron beam 

 will pass through the hole in the 

 modulating anode and continue 

 through the interaction circuitry. 

 When the modulating anode is at 

 cathode potential, it completely 

 shields the cathode from the positive 

 potential of the tube body and no 

 current will be drawn from the 

 cathode. It is not meaningful so use 

 the technically correct definition of 

 mu in this structure. Because the 

 tube body is completely shielded 

 from the cathode, the control mu is 

 technically infinite. .It is of more in- 

 terest to the system designer to note that the control voltage must be 

 relatively large. The pulse voltage will be equal to the voltage required for 

 cathode modulation if the modulating anode is' operated at tube body 

 potential during the on period. 



The modulating anode can be so designed that it need not be operated at 

 the tube body potential during the on period, and the modulating voltage 

 required can thereby be reduced by perhaps a factor of 2. More important 

 is the fact that the modulating anode does not draw much current. Nearly 

 all of the beam current will pass through the aperture in the modulating 

 anode. This permits greater flexibility in modulator design. 



Fig. 1 1-27 Modulating Anode for Con- 

 trol of an Electron Beam. The Modula- 

 ting Anode Is at Cathode Potential or 

 Lower When the Beam Is Off, and Is 

 Pulsed Toward the Potential of the 

 Tube Body to Turn the Beam On. 



11-5 



A TYPICAL RADAR SYSTEM EMPLOYING A HIGH- 

 GAIN AMPLIFIER 



A typical radar system employing a high-gain amplifier is shown schemat- 

 ically in Fig. 11-28. A low-power stable oscillator drives an intermediate 

 amplifier which is synchrodyned to shift the output frequency from the 

 input frequency by an amount equal to the intermediate frequency of the 

 receiver. The intermediate amplifier then drives the high-power, high-gain 

 final amplifier, and the stable oscillator provides the local oscillator signal 

 for the receiver. With this system, no AFC circuit is required to keep the 

 receiver in tune with the transmitted signal. In addition, phase coherence 

 is maintained continuously through a chain of transmitted pulses. A system 

 of this type gives excellent MTI performance when properly designed. 



