03-10-2016, 07:42 AM
(This post was last modified: 03-10-2016, 07:43 AM by ppppenguin.)
Whether you choose 1/10 of a wavelength or any other number is a bit arbitrary. Ultimately it depends on how much error you're prepared to ignore by treating a transmission line as lumped C and L. At 1/4 wavelength the errors are likely to be severe. At 1/10 it may or may not matter depending on your requirements.
For analogue video, connections outside a piece of kit are always properly terminated at both ends. This keeps the amplitude correct as well as eliminating visible ghosting and frequency response errors. At least with terminations both ends if you manage to leave out the term at the receiving end the transmitting end will absorb the reflection. Us video types hate having any 50R BNC terminations around. Just the thing to cause a "hard to find" amplitude error in a complex system. 50R cables give more subtle problems so we don't like those either. The serial digital video (SDI) standards were designed to use existing analogue video cables and connectors to ease the transition from A to D. In practice it often works though all the minor transmission line discontinuities caused by older patch panels etc can give trouble at 270MHz. Especially as you could be right on the edge of a digital cliff and not know it. This got worse with HD-SDI (1.5GHz) and its extension to 1080/50p etc which runs at 3GHz.
For PCB design with fast logic a very rough rule of thumb is that the electrical length of a track must be less than the rise time of the logic signals. If it isn't then you need to think about some kind of termination and looking after the characteristic impedance of the track.
For analogue video, connections outside a piece of kit are always properly terminated at both ends. This keeps the amplitude correct as well as eliminating visible ghosting and frequency response errors. At least with terminations both ends if you manage to leave out the term at the receiving end the transmitting end will absorb the reflection. Us video types hate having any 50R BNC terminations around. Just the thing to cause a "hard to find" amplitude error in a complex system. 50R cables give more subtle problems so we don't like those either. The serial digital video (SDI) standards were designed to use existing analogue video cables and connectors to ease the transition from A to D. In practice it often works though all the minor transmission line discontinuities caused by older patch panels etc can give trouble at 270MHz. Especially as you could be right on the edge of a digital cliff and not know it. This got worse with HD-SDI (1.5GHz) and its extension to 1080/50p etc which runs at 3GHz.
For PCB design with fast logic a very rough rule of thumb is that the electrical length of a track must be less than the rise time of the logic signals. If it isn't then you need to think about some kind of termination and looking after the characteristic impedance of the track.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
