272 BELL SYSTEM TECHNICAL JOURNAL 



Standard repeaters, the voice currents are further amplified and em- 

 ployed to modulate the plate voltage of an oscillator consisting of two 

 250-watt tubes connected in a push-pull circuit and oscillating at the 

 frequency of the carrier which is to be transmitted. The frequency 

 of such an oscillator, if not carefully controlled, will wander outside of 

 the assigned frequency band, thus causing interference with other 

 services and it will also suffer variations during the modulation cycle 

 which contribute to fading phenomena encountered at the distant 

 receiving station. In order to reduce these effects the oscillator is 

 held in step at the desired carrier frequency by means of a second 

 oscillator which is electrically removed from the reactions normally 

 influencing and tending to vary the frequency of the controlled oscil- 

 lator. Every precaution is taken to maintain accurately the frequency 

 of the second oscillator and among other things it is governed by a 

 piezo-electric quartz crystal whose temperature is regulated closely. 



Since it is impractical to use crystals cut sufficiently thin to oscillate 

 directly at frequencies in the range 10,000 to 20,000 kilocycles, thicker 

 crystals of lower frequency are used in combination with harmonic 

 generators which multiply the crystal frequency first by two or three 

 and then by one or two as the case requires. By virtue of the wide 

 differences between the input and output frequencies of the harmonic 

 generators these intermediate steps tend to isolate the crystal oscillator 

 from the other radio circuits and thus aid in stabilizing the frequency. 



The modulated radio frequency output of the controlled oscillator is 

 applied to the grids of a two-stage power amplifier employing water- 

 cooled tubes designed for operation at these frequencies. The first 

 stage contains two tubes and the second stage contains six. The 

 tubes are arranged in push-pull circuits, the entire system being care- 

 fully balanced to ground. The carrier output power from the last 

 stage is 15 kw. With 100 per cent modulation this corresponds to 60 

 kw. at the peaks of the modulation cycle. In other words, a radio tele- 

 phone amplifier of this type, rated at 15 kw. when provided with a 

 sufficiently large d-c. power source, could be used as a 10,000-kilocycle 

 continuous wave generator of 60 kw. capacity. 



The radio signal delivered by the amplifier is conveyed to the antenna 

 by means of a 600-ohm open wire transmission line. The antenna 

 itself is both a very efficient radiator and a highly directive one. 



Transmitting Equipment 



At the transmitting station the apparatus for each channel comprises, 

 (1) wire terminal equipment and repeaters, (2) a voice frequency control 



