582 BELL SYSTEM TECHNICAL JOURNAL 



Our field strength distribution surveys and studies have shown that, 

 for 5-kw stations separated by two or three hundred miles, a field 

 strength ratio of 20 db is obtained only at points well within the nor- 

 mal service area of the station. On the other hand the limits of the 

 10-db ratio lie for the most part outside the normal service range of the 

 station. Thus if such a station is to be operated on a common fre- 

 quency chain, the carriers must be maintained approximately in iso- 

 chronism if a large portion of the listeners within the normal service 

 range of the station are not to receive a seriously impaired program. If 

 approximate isochronism is maintained, the service area of each of these 

 stations should not differ materially from that which selective fading 

 and interference would establish for that station transmitting alone. 



In order to maintain unimpaired reception in the region where the 

 field strength ratio is between 10 and 20 db, it is necessary that the 

 stations be operated so that their carriers are not permitted to differ in 

 frequency by more than one cycle in 10 seconds and this demands a 

 frequency stability of an entirely new order of magnitude for commer- 

 cially available independent oscillators. However, at the time that 

 this development was undertaken for the Central Broadcasting Com- 

 pany, previous tests had shown that a newly developed crystal con- 

 trolled oscillator unit designated as the No. D-90684: oscillator-ampli- 

 fier possessed an exceptional frequency stability for commercial equip- 

 ment and that minor modifications would give it the stability required 

 for the simultaneous operation of a small group of stations on a com- 

 mon frequency. 



It was therefore planned to replace the existing crystal control 

 equipment by one of these new units located at each station and sup- 

 plemented by a monitoring receiver located midway between the trans- 

 mitters. 



The No. D-90684 oscillator-amplifier is a relay rack mounted assem- 

 bly consisting of a shielded unit containing a constant temperature oven 

 and a crystal oscillator, an amplifier having a maximum power output 

 of thirty watts, and the necessary power control equipment. The 

 amplifier tubes, instruments, and controls are mounted on the front of 

 the panels as is shown in Fig. 3 and all other apparatus is mounted in 

 the back and enclosed by a metal locker. The assembly of the various 

 components inside the locker is shown in Fig. 4. The power equipment 

 is placed in the lower part, the constant temperature oven and crystal 

 oscillator unit is mounted on slides in the middle compartment, while 

 the upper shielded section isolates the buffer and output stages from 

 the rest of the transmitter. The door of the locker is fitted with safety 

 switches which automatically disconnect all high voltages from the 



