TRANSMISSION LINES 293 



this means. No difficulty was experienced in maintaining a ratio of 

 at least 60 db between the carrier voltage applied to the unit and the 

 residual carrier current not completely balanced out. Over short 

 periods an even higher degree of balance can be readily obtained. 



There is a certain amount of electrical noise generated in the recti- 

 fying disks over and above that caused by thermal agitation ^' ^ 

 effects. The amount of this noise compared with the maximum per- 

 missible modulation output determines the volume range possibilities 

 of a modulator of this type. Measurements indicated that this range 

 was approximately 90 db, which obviously was more than sufficient to 

 meet the requirements desired. 



The circuit includes a relay and a meter through both of which 

 flows the d.-c. component produced by the rectification of the carrier 

 frequency. These supplementary units give a check on the magnitude 

 of the carrier supply and afford an alarm in case of failure. From the 

 modulator unit the circuit is connected to the band filter which trans- 

 mits only the lower sideband lying between approximately 25,000 and 

 40,000 c.p.s. and the vestige of the upper sideband. From the band 

 filter the currents are led to an amplifier and thence to the line circuit 

 leading to the farther terminal. 



It may be noted that at the transmitting terminal the 40,000-cycle 

 carrier current is derived from a 20,000-cycle oscillator by passing its 

 output through a series of copper oxide rectifiers connected to form a 

 frequency doubler. Part of the originally generated 20,000 cycles also 

 is connected to the input of the transmitting amplifier and sent over 

 the line to be used in producing the 40,000-cycle carrier supply for 

 demodulation. 



At the receiving terminal a similar modulation or demodulation 

 process occurs through the use of copper oxide disk circuits. A relay 

 and meter also are included in the circuit to check the carrier supply, 

 in this case providing also a check or pilot of the transmission over the 

 long line circuit. The 20,000-cycle synchronizing current is selected 

 at the receiving terminal, amplified and applied to a frequency doubler, 

 and thence applied to the demodulator circuit. The input of this 

 carrier supply circuit includes also a phase adjusting variable con- 

 denser arrangement so that the phase of the carrier supplied to the 

 demodulator may be adjusted properly in relation to that of the 

 carrier supplied to the modulator at the sending end. An interesting 

 feature of the receiving terminal carrier supply is the quartz crystal 

 filter employed to select the 40,000-cycle carrier after frequency 

 doubling. The transmission characteristic of this extremely selective 

 filter is shown in Fig. 5. 



