Published: TV Technology Magazine

Issue:  27 July 1998



Distribution of Serial Digital Over Coax



By Joe Fedele





BURLINGTON, Ontario

  The ability to effectively distribute serial digital signals over coaxial cable at high data rates is 

directly related to the cable grade and length.

  Coax that is made for high-speed serial digital applications is specifically designed by cable 

manufacturers to match the varied characteristics of signals at different frequencies. In such 

applications, the high-frequency components of a signal tend to ride on the outer skin of the coax's 

center conductor. 

  This "skin effect," as it is called, is identical to that of microwave transmissions through 

waveguide where virtually all the energy travels on the conductor's outer surface - as a result, the 

center isn't used and the cable remains hollow. But unlike microwave transmissions, SDI signals 

contain a low-frequency component that travels through the middle of the center conductor. 



IMPEDANCE PROBLEMS

  The net result is that if the center conductor is not made of solid copper, as in the case of some 

lesser grades of analog coax, there would be impedance instabilities and signal reflections that 

would increase attenuation in the cable to unacceptable limits.

  So-called precision-grade coax has been designed for both analog and digital applications. Analog 

cable will work at the higher-digital frequencies, but newer cables, which were designed 

specifically for digital transmissions, are needed for optimum performance. For example, the 

maximum transmission distance recommended by Belden Wire & Cable Co. for digitized 

composite NTSC at 143 Mbps is 1,286 feet for the company's 8281 coax product. 

  By contrast, the same cable operating at 1.485 Gbps would yield only 238 feet. And remote trucks 

or patchbays wired with Mini RG-59U cables, such as Belden 1865A, could expect only 150 feet 

worth of usable transmission capabilities at 1.485 Gbps (see chart for more details). 



Editor's Note:  This reprint updates several typographical errors from the original printed version.