A\ndy, Those are very good answers. Keep up the good work.
Lee Ingraham, Andrew wrote: > > As the speed of digital signals gets faster and faster, people begin being > > concerned with the distance for electric charge to move on power and > > ground planes of multilayer PCB during the signal rise time from a > > decoupling capacitor (cap) to a chip it serves. I would like to raise two > > questions. > > > > (1) The charge is moving in a metalic plane, not inside the dielectric > > between pwr and gnd planes. Please let me know why you have to use the > > propagation velocity in the dielectric, instead of that in the metal. > > The charge may be moving in the metal, but the energy (which makes the > charge keep moving) is primarily in the electro-magnetic field between the > planes, in the dielectric. The charge won't move unless there is an E-M > field to push it. > > It is just like an ordinary transmission line such as stripline. The > propagation velocity of a trace is that of the dielectric, even though the > charge moves only in the metal trace and planes. > > > (2) The second question is regarding distance between the cap and the > > chip. Do we really have to limit the distance letting the charge have > > enough time to move from the cap to the chip during the rise time > > interval? I doubt it. > > That depends whether you need the capacitor to help during the rise time > interval itself. > > If you had a single 1.0 Farad cap and attached it with 20 foot long jumper > cables to your chip, it would do nothing to help the chip during the rise > time interval. The jumper cable is a transmission line. If the voltage > sags at your chip, it takes many nanoseconds for the sag to reach the > capacitor. Until the sag reaches it, charge doesn't even start moving out > of it, i.e., the cap might as well not be there. > > Now flatten the jumper cable into two planes. The planes are a fat > transmission line (really!). The voltage sag propagates outward from the > chip, consuming charge stored in the intrinsic capacitance of the planes bit > by bit (not all of it at once!), and eventually reaching external capacitors > which help hold up the voltage. > > > Take the running water system for example. When we open, then close the > > water faucet within one second, does the water we've got in basin come > > from water tower (or water station, or reservoir)? No, it is the water > > that resides in the pipe. As a matter of fact, we have a very large pipe - > > pwr/gnd planes. Well, of cause you know, I did not mean we don't need > > water tower - the cap. ...... > > Pwr/gnd planes are similar to a long pipe. At first they help hold up the > pressure, but without a tank, the pressure would disappear. > > Even with a tank, the pressure does drop a little when you open the faucet. > Open a very large faucet, and the pressure immediately drops a lot ... > especially if you are the house at the end of the water main. Open a very > large faucet right at the tank or pumping station, and the drop in pressure > is much less. (This analogy is not very good, however, because water pipes > also have resistance.) > > Regards, > Andy > > **** To unsubscribe from si-list or si-list-digest: send e-mail to > [email protected]. In the BODY of message put: UNSUBSCRIBE > si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. > si-list archives are accessible at http://www.qsl.net/wb6tpu > **** **** To unsubscribe from si-list or si-list-digest: send e-mail to [email protected]. In the BODY of message put: UNSUBSCRIBE si-list or UNSUBSCRIBE si-list-digest, for more help, put HELP. si-list archives are accessible at http://www.qsl.net/wb6tpu ****
