Raj, To add comment to others, the 50 ohm controlled Zo refers to maintaining the same cross-section versus length (same line width and distance from return path). The original impedance calculation is usually made using closed-form formulas or lossless solvers, or assuming the high freq. limit. In reality the lossy Zo is complex where the real part gets bigger and and the mag. of the imaginary part gets bigger at lower freqs. So for an example lossy high freq-limit 50 ohm controlled package line, the complex Zo=79-30i at 10MHz, 54-6i at 100MHz, and 52-1.5i at 1GHz, and only approaches a mostly real 50.0 above 5GHz. Only if this freq-dependent cmplx Zo is considered will you see the proper slope on the, eg, "flat parts" of a lossy line TDR simulation. However, for a great many board problems, assuming a low-loss or lossless line with real Zo=the high freq. limit (or as an alternative, lossless) value is practical, so that is what you see done. N_G. -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On Behalf Of nagaraj Sent: Tuesday, March 20, 2007 9:04 PM To: si-list Subject: [SI-LIST] impedance and Characteristic impedance When I say impedance, it depends on the capacitance and inductance. So, these two components depend on frequency = (wL and 1/wC, where w=3D 2xpixF). When I say characteristic impedance, it too has capacitance and inductance involved. So when I say I maintain 50 ohm characteristic impedance on my board... there may be lot of frequencies involved in the design resulting in different characteristic impedance. then how the board is said as a 50 ohm impedance controlled..? Thanks and regards, Raj. --------------------------------- --------------------------------- We won't tell. Get more on shows you hate to love (and love to hate): Yahoo! TV's Guilty Pleasures list. ------------------------------------------------------------------ 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 technical documents are available at: http://www.si-list.net 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