SILICON CRYSTAL RECTIFIERS 15 



structure were held in abeyance during 1942 and 1943. However, an 

 opportunity for realizing the advantages inherent in the shielded design 

 was afforded later in the war and a sufficient quantity of the units was pro- 

 duced to demonstrate its soundness. As anticipated from the construc- 

 tional features, marked uniformity of electrical properties was obtained. 



Types, Applicatioks, akd Operating Characteristics 



Various rectifier codes, engineered for specific military uses, were manu- 

 factured by Western Electric Company during the war. These are listed 

 in Table II. The units are designated by RMA type numbers, as 1N21, 

 1N23, etc., depending upon their properties and the intended use. Letter 

 suflixes, as 1N23A, 1N23B, indicate successively more stringent perform- 

 ance requirements as reflected in lower allowable maxima in loss and noise 

 ratio, and, usually, more stringent power proof-tests. In general, different 

 codes are provided for operation in the various operating frequency ranges. 

 For example, the 1N23 series is tested at 10,000 megacycles while the 1N21 

 series is tested at 3,000 megacycles and the 1N25 at 1000 megacycles, 

 approximately. Since higher transmitter powers are frequently employed 

 at the lower frequencies, somewhat greater power handling ability is provided 

 in units for operation in this range. 



One of the more important uses of sihcon crystal rectifiers in military 

 equipment was in the frequency converter or first detector in superheter- 

 odyne radar receivers. This utilization was universal in microwave re- 

 ceivers. In this application the crystal rectifier serves as the non-linear 

 circuit element required to generate the difference (intermediate) frequency 

 between the radio frequency signal and the local oscillator. The inter- 

 mediate frequency thus obtained is then amplified and detected in conven- 

 tional circuits. As the crystal rectifier is normally used at that point in 

 the receiving circuit where the signal level is at its lowest value, its perform- 

 ance in the converter has a direct bearing on the overall system performance. 

 It was for this reason that continued improvements in the performance of 

 crystal rectifiers were of such importance to the war effort. 



For the converter application, the signal-to-noise properties of the unit 

 at the operating frequency, the power handling ability, and the uniformity 

 of impedance are important factors. Tlie signal-to-noise properties are 

 measured as conversion loss and noise ratio. The loss, L, is the ratio of 

 the available radio frequency signal input power to the available inter- 

 mediate frequency output power, usually expressed in decibels. The 

 noise ratio, Nr, is the ratio of crystal output noise power to thermal (KTB) 

 noise power. The loss and noise ratio are fundamental properties of the 



