224 BELL SYSTEM TECHNICAL JOURNAL 



A further important requirement placed on all components was that their 

 input and output impedances should match the corresponding impedances 

 of the components to which they were to be connected with a minimum of 

 reflected power over the 10-megacycle band. This requirement was imposed 

 on the separate units to provide for flexibility in testing and to permit easy 

 patching in of spare units in case of failure. 



Receiving Converter* 



The receiving converter, together with the input to the first stage of the 

 intermediate-frequency amplifier, occupies a unique position in the repeater 

 amplifier in that it is located at that point in the circuit where the signal 

 level is lowest. As a consequence, research on receiving converters has been 

 directed toward insuring that the least possible noise be added to the signal 

 to be amplified. An extensive background of microwave converter design 

 information was available from the work done on converters during the 

 war2®>27 which led to the selection of a balanced converter using a waveguide 

 hybrid junction and 1N23-B silicon point contact rectifiers. The informa- 

 tion already available would probably have been adequate except for the 

 two additional requirements imposed by the repeater amplifier: first, that 

 the standing wave ratio at the input have a low value and, second, that 

 uniform conversion efficiency be maintained over a band of at least 10 mega- 

 cycles. 



A receiving converter with its associated input and output circuits is 

 shown in Fig. V-4. Frequency conversion is accomplished in a device of 

 this kind by virtue of the fact that when two sinusoidal voltages (in this 

 case the signal and the beating oscillator) are applied to a non-linear imped- 

 ance such as a rectifier, new frequencies given by the sums and differences 

 of the applied frequencies and their harmonics are generated. The dif- 

 ference frequency may thus be selected as the desired output frequency and 

 passed through the IF transformers to the IF amplifier. The performance 

 of a converter is, however, influenced by the impedances encountered by 

 some of the other frequencies generated. For this reason, the separation 

 S between the input filter, more properly a component of the channel se- 

 lecting network, but here shown as part of the converter, and the converter 

 must be given consideration, since it determines the phase of the reflection 

 from the filter at the image frequency (the difference between the beating 



* This section prepared by C. F. Edwards who was responsible for the research on the 

 receiving converters. 



* "Developments of Silicon Crystal Rectifiers for Microwave Radar Receivers", J. H. 

 Scaff and R. S. Ohl, B. S. T. /., vol. 26, pp. 1-30, January 1947. 



*^ Descriptions of several converters developed prior to and during the war as well as 

 a more complete description of the present converter are given in C. F. Edwards* paper, 

 "Microwave Converters", Proc. I. R. E., vol. 35, No. 11, pp. 1181-1191, November 1947. 



