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BELL SYSTEM TECHNICAL JOURNAL 



operating at a frequency suitable for receiving the incoming signal. The 

 frequency of this oscillator is controlled by a high Q cavity and a servo 

 mechanism to 0.2 megacycles or better. ^^ One portion of the output of the 

 oscillator is used as the beat frequency in the receiving converter. A second 

 portion is combined with the output from a crystal oscillator operating at a 

 frequency equal to the difference, A/, between the incoming and outgoing 

 frequencies. In this way a beat frequency for the transmitting converter is 

 obtained which has the same variations as that for the receiving converter 

 except for negligibly small variations that may occur in the crystal oscillator. 

 As a result of this method of deriving the beat frequencies the outgoing 

 frequency always differs from the incoming frequency by an amount equal 

 to the crystal oscillator frequency and is not influenced by variations in 

 the high-frequency local oscillator. The result is that, except for the small 



MODULATOR 



MODULATOR 



f+Af 



I-F AMPLIFIER 



MODULATOR 



AMPLIFIER 

 (f-IF + Af) 



CRYSTAL 

 OSCILLATOR 

 {FREQ=Af) 



Fig. V-3. — Frequency control system for a microwave repeater. 



crystal oscillator variations, all radiated frequencies in a long circuit carry 

 only the variations in the transmitting oscillator of the originating terminal, 

 while the intermediate frequency of each repeater may vary by an amount 

 equal to the sum of the variation of its own local oscillator and that ofjthe 

 terminal transmitter frequency. 



Automatic Gain Regulation — The function of the automatic gain control 

 circuits is to hold the repeater output constant over the expected fading 

 range. As akeady stated an allowance for fades 20 db down and 10 db up 

 from free space have been made for 30-miIe paths at a wavelength of about 

 7 centimeters. In addition to knowledge of the fading range to be com- 

 pensated, it is necessary in the design of suitable circuits to know what the 

 maximum fading rate is likely to be. Analysis of the records of a number 

 of disturbed periods on the New York-Neshanic path indicate a maximum 

 rate of 5 db per second. 



»V. C. Rideout, "Automatic Frequency Control of Microwave Oscillators", Proc. 

 I. R. £., vol. 35, pp. 767-771, August 1947. 



