On 1 December 2011 15:30, peter green wrote: > Justin Richards wrote: >> Is there an implied frequency when discussing characteristic impedance. >> When we need to analyze cables with respect to their RF >> characteristics we end up with a large table S parameters that have >> been recorded over a range of frequencies. >> >> So when a cable is quoted at 75 ohms, that must be for a given >> frequency. =A0If so what is it. >> > An ideal coaxial cable (made up of perfect conductors and perfect > dielectrics in a perfect geometery) will have a constant characteristic > impedance across all frequencies. I'm not sure on the details but my > understanding is that if the characterstic impedance is changing > significantly with frequency you are probablly above the cable's usable > frequency range. > Justin, Cable characteristics are normally referred to at a standard frequency, depending on cable likely application. 200MHz, 400MHz and 1GHz being typical but impedance should be pretty stable above a few MHz. Mathematically the impedance is give by Z=3Dsqrt((R+jWL)/(G+jWC)) Since G is normally close to zero in most cases this reduces to sqrt(L/C) at high frequencies - where WL is >> R. (W =3D 2 * PI * f) .. The inductance is not constant although it only changes slightly as skin effect moves the current to the outside of the centre conductor. The R term is also not constant with frequency, it increases due to skin effect (with sqrt(frequency)) and linearly with frequency as dielectric loss starts to dominate. There is also a loss component proportional to frequency involving the screen, but this is normally not significant. Insulators like PVC have a very frequency dependent dielectric constant also (This adds to the G term IIRC), which is why PVC is not used for RF cable insulation - although the effect is present with other materials as well. Richard P --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .