A SINGLE SIDEBAND MUSA RECEIVING SYSTEM 323 



lator is contained in a cast box mounted on rubber supports. The 

 inductance coils have an extremely low temperature coefficient and 

 are mounted on cast supports for rigidity. The condenser is also of 

 rigid construction. A variation in plate voltage of one volt gives a 

 frequency variation of about 6 cycles at 18,000 kc. 



A buffer amplifier connects between this oscillator and a push-pull 

 power amplifier which delivers ten watts of output. This output is 

 delivered to a transformer located on the center bay of the row of 

 first demodulator bays. Coaxial cables of equal length distribute it 

 to the first demodulators on the adjacent bays. A vacuum tube 

 voltmeter connected across this transformer gives an alarm if the 

 voltage fails. 



In the automatic tuning system the incoming carrier at approxi- 

 mately 100 kc. is beaten with the local 100 kc. oscillator. The phase 

 of the beat frequency is then split and the resultant two-phase output 

 applied to a motor which drives a condenser in the first beating oscil- 

 lator circuit until the beat frequency is reduced to zero. In order to 

 avoid interruption of control due to fading, all three diversity branch 

 carriers are simultaneously connected to the circuit. 



The second beating oscillator operates at 3,000 kc. and is a standard 

 broadcast oscillator which has been slightly modified. 



The 100 kc. oscillator is required to have the same frequency as 

 the center of the pass band of the carrier filters. Since these are only 

 40 cycles wide both filters and oscillators are made with low tempera- 

 ture coefficient crystals. The oscillator is of the bridge type described 

 by Meacham ^ but without temperature control. 



Automatic Volume Control and Final Demodulators 



Only the carrier is rectified for automatic volume control purposes. 

 In the 100 kc. amplifiers where the carrier and sideband are amplified 

 separately, separate automatic volume control circuits are used. 

 The time-constant of the carrier amplifier control is made 0.1 second, 

 which is as fast as is practical with the narrow carrier filter used, and 

 the time-constant of the sideband volume control is made variable but 

 is generally set at a value of 1 second. 



With the common automatic volume control used with diversity 



receivers the rectifiers are so connected as to give outputs which vary 



according to the square or first power of the branch input. The sum 



of the rectifier outputs is held substantially constant. If the combined 



signal output is also to be held constant, the final demodulators of the 



«L. A. Meacham, "The Bridge-Stabilized Oscillator," Proc. I.R.E., Vol. 26, No. 

 10, p. 1278, October 1938; B.S.T.J., Vol. XVII, No. 4, p. 574, October 1938. 



