374 THE RADAR RECEIVER 



alent noise voltage that must be amplified may be determined by computing 

 the total noise voltage at the input of each stage caused solely by the total 

 grid conductance, the shot noise in the tube, and total grid admittance. 

 This voltage is then referred to the input by dividing by the total gain from 

 the input to the noise considered. Even though consideration is given to 

 the design of a low-noise IF preamplifier, the process of referring all of the 

 noise sources in the receiver to the input is extremely important, especially in 

 multiple-conversion receivers such as are employed in some forms of doppler 

 radar receivers. This method sometimes reveals factors such as noise on 

 beating oscillator signals or noise caused by a method of selectivity dis- 

 tribution that would degrade the IF signal-to-noise ratio and result in a 

 sensitivity poorer than would be estimated from consideration of the IF 

 preamplifier noise figure and overall receiver selectivity only. 



A typical equivalent input noise for an ordinary radar receiver having an 

 overall bandwidth of 5.0 Mc is about 3.0 Mvolts rms. For signal detection 

 alone, a voltage between 1 and 2 volts rms at the input to the IF envelope 

 detector is satisfactory. Thus the required overall amplifier gain is on the 

 order of 105 to 115 db. 



To obtain the voltages for the cathode ray tube an additional gain on the 

 order of 40 db is then required. (Included in this figure is a loss of 6 to 10 db 

 that is usually produced in wide bandwidth second detectors.) 



Where the envelope of the signal must be accurately demodulated, higher 



voltages may be applied to the envelope detector to recover larger negative 



peak modulation with less distortion. However, dynamic range of the 



amplifier must be exchanged for the higher operating level. In tracking 



receivers, use of a range-gated amplifier ahead of the envelope detector 



allows such an exchange to be made. The detector average output is usually 



regulated to a relatively fixed level, and noise modulation positive peaks 



have very small probability of exceeding a level more than 12 db above the 



regulated level. In a typical case of a receiver having an IF bandwidth of 



5 Mc, incremental gain can be maintained for a range of IF signal from zero 



to about 12 volts rms at the input to the IF envelope detector. Thermal 



noise can therefore be amplified to a level of about 3 volts rms at the 



detector input. It is obvious that this does not appreciably alter the IF gain 



requirements. In the case of very narrow band receivers such as are 



employed for detection and tracking of targets by means of their difference 



in doppler frequency, receiver bandwidths are on the order of several 



hundred cycles per second. Considerably more gain is therefore required 



over that encountered in conventional radar sets. For example, with a 



bandwidth of 500 cps, the required gain to the envelope detector would be 



... , 5,000,000 ,^ ^, , . , r . • , J 



^°gio — 77^7^ — = 40 db more than m the case or the conventional radar 



receiver of 5 Mc IF bandwidth. 



