MICROWAVE REPEATER RESEARCH 223 



A conventional delayed automatic gain control circuit has been used in 

 which a d.c. voltage, supplied by a peak rectifier in the output of the last 

 IF stage, is amplified and fed back to bias several of the IF stages. Analy- 

 sis of the transient response of the last repeater in response to a step function 

 disturbance in the input to the first repeater, where the number of repeaters 

 is of the order of 5 or more, shows that great care must be taken in shaping 

 the frequency characteristic of the feedback circuit. 



General Requirements of Components — The important factors bearing on 

 the basic layout of the amplifier components have been discussed. Others, 

 affecting the design of these various components will now be considered, 

 after which a brief description of the research work on each component will 

 be given. 



Different repeater circuits will, in general, have different numbers of 

 repeaters; also it may be necessary to feed signals into and extract signals 

 from them at any point. Under these conditions it is impractical to specify 

 only the over-all characteristics for a given number of repeaters. Each 

 repeater itself should be individually good and should not depend upon any 

 systematic compensation or equalization at any other point in the system. 

 The repeater was designed in accordance with this philosophy and, in the 

 interest of flexibihty and ease of testing, the same line of reasoning was 

 extended to cover the design of the components of the repeater as well. 



In accordance with the above considerations major emphasis was placed 

 on obtaining a minimum of amplitude variation and phase distortion over 

 a 10-megacycle band for each repeater component. However, it was appre- 

 ciated that even with the simplest circuits the inherent phase distortion 

 would be excessive in long relay systems. Phase equalizers can be designed 

 to equalize this distortion, but the difficulties of design and ahgnment in- 

 crease with the magnitude of the distortion to be equalized. Accordingly, 

 simple circuits were used wherever gain requirements would permit and at 

 the same time parallel research was carried out on the problems of designing 

 and testing appropriate phase equalizers. 



While our aim was to provide a repeater 10 megacycles wide suitable for 

 any type of modulation, it soon became apparent that it would be un- 

 economical to attempt to provide the extreme degree of linearity that would 

 be required; for example, for a long relay circuit carrying an amplitude- 

 modulated television signal. However, early tests indicated that very satis- 

 factory transmission could be obtained with low-index frequency-modulated 

 signals, for which reason the later stages of the work were aimed at providing 

 an amplifier to be used for such signals. Nevertheless an attempt was 

 made to limit the compression in each unit except the R.F. amplifier to 

 0.1 db at maximum rated load. 



