SYSTEM DESIGN 



NEWFOUNDLAND-NOVA SCOTIA LINK 



239 



the original frequency is selected. From this point it passes via the 

 high-pass directional filters through the amplifier and back to Sydney 

 Mines, where the level is measured on a transmission measuring set. 

 Information obtained in this way on each repeater can be compared at 

 any time with that obtained when the system was installed and any gain 

 variations localized. The test equipment provided has the additional 

 facility of an automatic sweep of the test frequency at 4 cycles and a 

 display of the returned-signal levels on a cathode-ray tube as in Fig. 

 12(5). 



Although the transmitted signals lie outside the band of the W-E 

 supergroup, the received signals, 520-528 kc, lie within the band of the 

 E-W supergroup, and two channels must be removed from traffic to 

 carry out the tests; these channels are in a "local" group. 



Piihe Method. 



As applied to the present system, the pulse method utilizes the over- 

 load characteristic of the amplifier to effect the frequency change in the 

 repeater. At Sydney Mines a continuous train of single-frequency 

 pulses is applied in the lower transmission band, such that either the 

 second or third harmonic is returned in the upper band, as in Fig. 13; 

 at Clarenville two-frequency pulses are applied in the upper band such 

 that either a second- or third-order difference product is returned in the 

 lower band. The pulse length is 0.15 millisecond, and the frequencies used 

 are given in Table I. At Sydney Mines the signals can be sent and re- 

 ceived either on the line itself or via the group equipment; in the latter 

 case only one group need be taken out of service. At Clarenville line 

 measurements only are provided for. 



The primary display is on a cathode-ray tube with a circular time- 

 base, and any one returned pulse can be accurately compared with the 

 reference pulse on a second tube with a linear time-base. The pulse 

 selected for such measurement is automatically blacked out on the pri- 

 mary display. 



Table I 



