Hi Louis; Speed of light in a vacuum is 11.8 billion inches per secound- frequency is 90 MHz - without a calculator it is about 1200 inches. He mention only that his frequency is 90 MHz, of course if there is more to it then that then it changes the whole complextion of the problem but as the problem was stated it sounded pretty straight forward. By the way didn't we meet ? Justin -----Original Message----- From: Sanchez, Louis Sent: Friday, March 22, 2002 2:05 PM To: Tabatchnick, Justin Subject: RE: [SI-LIST] Re: Series termination value Hi Justin.......... I'm not sure how you arrived at the wave length. I may have missed part of the email thread on this subject, but I didn't see either data rates, edge rates, or line lengths mentioned in the original email message. Did I miss something? If I did, please let me know. I believe that these parameters are germain to the formation of a problem statement. I think that the problem can first be broken down according to whether it's a DC situation, or that the data and edge rates contain high frequency spectral components that cause PCB traces to look like transmission lines. If the edge rates are on the order of a nano-second or less, as is common in our SONET applications (I realize the application was not specified), then a reasonable estimate of the velocity of propogation in FR-4 material with a 71 ohm impedance is about 5.6 inches per nano-second. Let me present an example for our discussion: assume; edge rate=1Ns, trace length=12 inches, Vp=5.6 inches per nS. According to these conditions, the round trip delay of this line is about 4.3 nS. Since the edge rate of the pulse is 1 nS, the pulse doesn't know what is at the end of the line for 4.3 ns. Therefore, the driver will deliver a pulse of current into the line equal to the voltage amplitude divided by the sum of driver source impedance and transmission line impedance. This pulse of current will continue to be delivered until the first reflection comes back to the source 4.3 nS later. At that time the pulse of current will drop to zero (assuming the load impedance is very very high). When the trailing edge of the pulse occurs, the same thing will happen except that because this is occuring on the trailing edge of the pulse, the pulse of current delivered into the transmission line will be negative going. This behaviour under the stated conditions can be observed with SigExplorer (in Spectraquest) using the voltage and current probe features. Best Regards, Lou A. Sanchez LOC3-14 Optical Product Group Intel Sacramento -----Original Message----- From: Tabatchnick, Justin [mailto:justin.tabatchnick@xxxxxxxxx] Sent: Friday, March 22, 2002 1:04 PM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Series termination value I have to jump in here but from the looks of things this is just a standard transmission line problem. Impedance seen by the driver is a function of the load impedance, the line characteristic impedance , the distance between the impedance reference point ( in this case the 20 ohm resistor) and the load impedance, and the wave length. The wave length in this case is about 1000 inches ( in air ) and since your trace is probably less then a foot , I would say that you can treat this as a DC problem with the total load impedance seen by the driver as 91 ohms. I don't see a problem with attenuation due to loss or mismatch. Justin Tabatchnick SI Group Intel, Sacramento ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu ------------------------------------------------------------------ To unsubscribe from si-list: si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field or to administer your membership from a web page, go to: //www.freelists.org/webpage/si-list For help: si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field List archives are viewable at: //www.freelists.org/archives/si-list or at our remote archives: http://groups.yahoo.com/group/si-list/messages Old (prior to June 6, 2001) list archives are viewable at: http://www.qsl.net/wb6tpu