Tom, If I'm not mistaken, the formula you gave is off a little and should be: Zcable = Zsource * Vcable / (Vsource - Vcable) ----- Original Message ----- From: Tom Dagostino To: James_R_Jones@xxxxxxxx ; peter.baxter@xxxxxxxxxxxxx ; si-list@xxxxxxxxxxxxx Sent: Friday, March 07, 2003 8:58 AM Subject: [SI-LIST] Re: Instruments / Methods for measuring a cable's chara cteristic impedance An alternative is to take a good pulse generator with a known output impedance of 50 Ohms. Attach it to the cable of interest. At the attachment point probe the junction with a good FET probe (to minimize C loading). You can then compute the voltage divider of the open circuit pulse output to the measured initial level at the cable. For example, assume you have a 10 volt pulse open circuit and the output impedance is 50 Ohms. If you observe a 6 volt level at the beginning of the edge when it is loaded by the test cable then you would calculate the effective Zo of the cable is 75 Ohms. Zo = V*Zo/(V-X) Where X is the voltage at the cable V is the input open circuit voltage Zo is the impedance of the source Zo is the impedance of the cable under test in this case Zo = 6*50/(10-6) = 300/4 = 75. Tom Dagostino Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC 2926 SE Yamhill St. Device Modeling Division Portland, OR 97214 13610 SW Harness Lane Beaverton, OR 97008 http://www.teraspeed.com 503-430-1065 tom@xxxxxxxxxxxxx -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of James_R_Jones@xxxxxxxx Sent: Friday, March 07, 2003 7:58 AM To: peter.baxter@xxxxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Instruments / Methods for measuring a cable's chara cteristic impedance Peter, A simple, cheap trick that I have used before to measure Z0: Hook up a pulse generator with known Rout on one end of unterminated cable. Place a potentiometer in series with the source driving the cable. Also attach a scope probe to the cable. Turn on the pulse generator. Now vary the potentiometer and watch how the reflections change. You will get a 100% reflection at the unterminated end of the cable, and it will bounce off the source as well if the source is not matched correctly. This will be visible on the scope. So tune the potentiometer until the source reflection goes away, then measure the pot. Sum the pot resistance with the source resistance, and you have Z0. (Beware of parasitic C) If you are looking for something quick and dirty that you can do with some standard lab equipment, this works. Check it out. Of course if you want to take more accurate Z0 measurements, then I recommend the Tek TDS8000 with 80E04 sampling head. Hope this helps, James R. Jones Dell -----Original Message----- From: Peter Baxter [mailto:peter.baxter@xxxxxxxxxxxxx] Sent: Friday, March 07, 2003 4:19 AM To: si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Instruments / Methods for measuring a cable's characteristic impedance Hi, I've been using the LC method to determine the characteristic impedance of a 1 metre long transmission line (flat) cable (around 60 ohms at 100kHz). This method works fine for me. However getting access to the "beautiful / expensive" HP LCR meter (on another companies premises) is getting harder and harder. I'm now considering what alternatives I might consider. For the work I do, purchasing gear costing US$10,000 up, isn't practical. Might readers be able to suggest alternate methods, or products that I might consider? (Recommendations too, for second hand test equipment outlets - in USA or the rest of the world - others may also be interested). Could a network cable tester be adapted to the application? Or is my cable length too short? Thanks Peter Baxter ------------------------------------------------------------------ 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 ------------------------------------------------------------------ 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