SILICON CRYSTAL RECTIFIERS 21 



Specification proof-test levels are, of course, not design criteria. Since 

 the units are generally used in combination with protective devices, such 

 as the transmit-receive switch, it is necessary to conduct tests in the circuits 

 I of interest to establish satisfactory operating levels. 



I In general, however, the units may be expected to carry, without deteriora- 

 tion, energy of the order of a third of that used in the single d-c spike proof- 

 ; test or peak powers of a magnitude comparable with that used in the multiple 

 I flat-top d-c pulse proof-test. The upper Hmit for applied continuous wave 

 i signals has not been determined accurately, but, in general, rectified currents 

 i below 10 milliamperes are not harmful when the self bias is less than a few 

 tenths of a volt. 



' The service life of a crystal rectifier will depend completely upon the 

 ; conditions under which it is operated and should be quite long when its 

 ! ratings are not exceeded. During the war, careful engineering tests con- 

 ! ducted on units operating as first detectors in certain radar systems revealed 

 j no impairment in the signal-to-noise performance after operation for several 

 [ hundred hours. A small group of 1N21B units showed only minor impair- 

 I ments when operated in laboratory tests for 100 hours with pulse powers 

 I (3000 megacycles) up to 4 watts peak available to the unit under test. 



Another important military application of silicon crystal rectifiers was 

 as low-power radio frequency rectifiers for use in wave meters or other 

 items of radar test equipment. Here the rectification properties of the 

 unit at the operating frequency are of primary interest. Since units which 

 are satisfactory as converters also function satisfactorily as high-frequency 

 rectifiers special types were not required for this application. 



Units were also used in military equipment as detectors to derive directly 

 the envelope of a radio frequency signal received at low power levels. 

 These signals were modulated usually in the video range. The low-level 

 performance is a function of the resistance at low voltages and the direct- 

 current output for a given low-power radio frequency input. These may 

 be combined to derive a figure of merit which is a measure of receiver 

 performance.^ 



Typical direct-current characteristics of the silicon rectifiers at tempera- 

 tures of —40°, 25° and 70°C are given in Fig. 10. It will be noted in these 

 curves that both the forward and reverse currents are decreased by reducing 

 the temperature and increased by raising the temperature. The reverse 

 current changes more rapidly with temperature than the forward current, 

 however, so that the rectification ratio is improved by reducing the tempera- 

 ture, and impaired by raising the temperature. The data shown are for 

 typical units of the converter type. It should be emphasized, however, 



'' R. Beringer, Radiation Laboratory Report No. 61-15, March 16, 1943. 



