[SI-LIST] Re: Impedance without a reference plane
- From: chand basha <chand_374@xxxxxxxxx>
- To: erdinih@xxxxxxxxx, nagaup1@xxxxxxxxx
- Date: Mon, 4 Sep 2006 23:18:04 -0700 (PDT)
If the Board is there physically in hand then you can test its characteristic
impedance by passing the current through the trace.
Ihsan Erdin <erdinih@xxxxxxxxx> wrote: Z0=sqrt(L/C) is the characteristic
(wave) impedance of a line that
predominantly supports TEM mode of propagation, which strictly depends on
the line width and the electrical distance to the reference conductor beside
other factors in the line cross-section. If the separation between the
signal and the reference conductor is not within a fraction of the
wavelength of the highest frequency content of the signal spectrum TEM mode
stops being the dominant mode of propagation and higher order (waveguide)
modes begin to propagate, which renders the Z0=sqrt(L/C) of little use.
In short, transmission line wave (characteristic) impedance is not a simple
one-cut-for-all-structures formula that could be used disregarding the
signal spectrum.
Regards
Ihsan
On 9/4/06, nagaraj wrote:
>
> Bharath,
> The characteristic impedance is given by,
>
> Z0 = sqrt (L/C) , where L inductance/sq-inch and C Capacitance/sq-in
>
> If we look at the formula for C = EA/D E- dielectric constant, A Area of
> the plate and D spacing between the plates.
>
> In your case the D means is infinity hence your impedance is also
> infinity ( directly proportional). In practical, the impedance is of large
> value.
>
> If no reference place then signal might have used a reference trace for
> the signal to complete. So it becomes a two wire system as such.
>
> Hope this is of some help.
> Thanks,
> Nagaraj.
>
> Bharathkumar Raju wrote: Hi,
> Have any one come across the situation where you need to
> simulate, calculate, measure, the characetristic impedance without a
> reference plane.
> I have a design where I need to decide upon the impedance of
> an isoalted trace (no other traces in the neighbourhood) of about 25 mil
> width withour any reference plane underneath.
> You inputs, sugestions will be of great value.
> Bharath
>
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