THE RADAR RECEIVER 743 



to obtain a somewhat wider band under average tube conditions so that 

 under subnormal but acceptable tube conditions the over-all performance of 

 the amplifier is still within requirements. The use of stagger-tuned inter- 

 stage designs will also result in increased performance over the basic syn- 

 chronous single-tuned variety, but the maintenance of the over-all perform- 

 ance of a radar IF amplifier of this type involves relatively complex measure- 

 ments not always possible under military conditions. 



2.24 Second Detector Design 



The final conversion of the IF signal to the video form, as required by the 

 following radar display device, is accomplished by simple detection or recti- 

 fication. For this purpose either a diode rectifier or a triode operating as 

 plate circuit detector is usually employed. The second detector design 

 follows the practices generally developed for television receivers prior to the 

 war. The diode second detector method has the advantage of simplicity 

 with no plate supply voltage being required, but the performance of such a 

 detector is somewhat limited for the frequencies employed in radar systems. 

 The linearity of rectification of a diode depends on maintaining a high load 

 impedance relative to the internal impedance of the tube. The external 

 load impedance is limited, by the presence of tube and parasitic circuit 

 capacitance and the video band width required, to somewhat less than 1000 

 ohms for the typical radar case. The internal impedance of the usual avail- 

 able diode is of the order of several hundred ohms so that the linearity of 

 detection suffers. The low value of the diode load resistance is also reflected 

 in the termination of the last IF amplifier stage and affects the gain of 

 this stage. 



The plate circuit detector often employed consists of a triode operated 

 near plate current cutoff. Here the detector load impedance is effectively 

 isolated from the plate circuit of the last IF amplifier stage. The linearity 

 that can be obtained from this type of second detector is essentially the same 

 as with the usual diode detector. 



The polarity of the detected video output signal may be chosen of either 

 sign by proper circuit arrangement for convenience in the video amplifying 

 and limiting circuits which follow. It is desirable to reduce the ampHtude 

 of the IF signal which appears at the output of the second detector to prevent 

 overload and interference in the video amplifier following. This is accom- 

 plished commonly by the inclusion of a low-pass filter of simple form in the 

 output circuit of the second detector. 



2.25 Typical Component Designs 



In the military radar system design it has been observed that a maximum 

 video output signal of the order of one volt of noise is desirable as a design 



