# 75 ohm digital cable

What does the 75 ohm rating mean? If I measure the resistance of this cable it measures .4 ohm which is the same as any other interconnect I measured.
rhljazz
3 responses
 05-28-2004 8:57pmneed to measure it (in parallel) when terminated @ one end, I believe. The rating is for it connected into a system, not by itself. -Ed 05-29-2004 12:10amIt is a 75 ohm IMPEDANCE, not resistance. Impedance has both a resistive and a reactive part, and is properly represented as a complex number of the form r+iw, where i is the square root of -1 (an imaginary number). The "r" part is pure resistance, and the "w" part is the reactive part. Direct Current (DC) only sees the "r", but Alternating Current (AC) sees the "r" and the "w". So measuring it with an ohmmeter only gives you the "r". The 75 ohms refers to the magnitude of the complex impedance (sqrt(r^2 + w^2) for the mathematically inclined). The "w" term is often a function of frequency.Thus you need a much more complex setup to measure true impedance. The 75 ohm value refers to how AC sees the cable.The above oversimplifie a lot of things. See a good textbook on cirsuits for the real lowdown ;). 05-29-2004 8:10am75 Ohms is called the "characteristic impedance" of the cable. www.bluejeanscable.com/articles/impedance.htm has a non-technical explanation of impedance and characteristic impedance. You can also find more information by searching for "characteristic impedance" on Google.