# [SI-LIST] Fwd: Re: impedance and Characteristic impedanece

• From: Bill Jones <novice_guru@xxxxxxxxx>
• To: si-list@xxxxxxxxxxxxx
• Date: Wed, 21 Mar 2007 15:53:19 -0700 (PDT)

```  Raj,

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.

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• » [SI-LIST] Fwd: Re: impedance and Characteristic impedanece