220 



BELL SYSTEM TECHNICAL JOURNAL 



microwave amplifiers were known which gave promise of yielding an ade- 

 quate ratio of output power capacity to noise figure, and there was con- 

 siderable doubt of our ability to reduce, sufficiently, the feedback from the 

 transmitting antenna to the receiving antenna. 



These difficulties with straight-through amplification can be avoided by 

 a repeater amplifier such as is shown schematically in Fig. V-1. The in- 

 coming signal is converted to an intermediate frequency, IF, where better 

 amplifiers are available and where the major part of the required gain is 

 supplied. The amplified IF signal is then converted back to the microwave 

 frequency/ + A/, where A/ is relatively small. The difference A/ between the 

 incoming and outgoing frequencies permits the use of circuit selectivity to 

 counteract feedback troubles, and the radio frequency amplifier following the 

 transmitting converter can have a relatively large noise figure. 



INPUT 



SIGNAL 



(f) 



RECEIVER 

 CONVERTER 



MODULATOR 

 (- 8 DB) 



BEATING 



OSCILLATOR 



(f-IF) 



I-F 



AMPLIFIER 



(+76 DB) 



35 MW 



AUTOMATIC 

 VOLUME 

 CONTROL 



TRANSMITTER 

 CONVERTER 



MODULATOR 

 (-11 DB) 



BEATING 

 OSCILLATOR 

 (f+Af-lF) 



R-F 



AMPLIFIER 

 (+20DB) 



2.8 MW 



OUTPUT 

 SIGNAL 



280 MW (f'+Af) 



Fig. V-1. — Schematic of a repeater amplifier. 



Our initial research on microwave repeaters was directed toward solving 

 the problems associated with an amplifier of the type shown in Fig. V-1. 

 The gain and level figures shown in this figure apply to the example of an 

 eight-link relay system given in section II. They indicate approximate 

 minimum objectives for the various components of the repeater amplifier 

 to be discussed later. 



Choice of I.F. Frequency — When selecting the intermediate frequency for 

 a multichannel repeater circuit utilizing the interleaved radio frequency 

 plan of Fig. IV- 1 and the intermediate frequency type repeater amplifiers 

 of Fig. V-1, the relative position of the various discrete frequencies and 

 frequency bands shown in Fig. V-2 must be considered. In order to mini- 

 mize the possibility of crosstalk from the image bands and interference from 

 the beating oscillators caused by insuflicient shielding, it is desirable to 

 choose the intermediate frequency in such a way that the image bands fall 

 midway between the active bands, and the oscillator frequencies fall midway 

 between the image and active bands. These conditions are realized, as 

 shown in Fig. V-2, if the intermediate frequency satisfies the relation 



