Relation of track impedance to other characteristics

[binu G]

Relation of Track Z0 ?

hi,

what is the relation between the impedence of the track with respect to
length
width
thickness
dielectric height


I have noticed that there is no impedence change while increasing the track length.
can any one make the concept clear.

Binu G

 

[hcet]

Re: Relation of Track Z0 ?

look here:
h**p://www.polarinstruments.com/support/cits/cits_index.html#1. Introduction to Controlled Impedance.
i think polarinstruments explains it pretty good.

 

[dunn]

Re: Relation of Track Z0 ?

See document attached.

 

[Frikkie]

Re: Relation of Track Z0 ?

You will get but you will need a big track in H*W*L before
you notice.

 

[Johnson]

Re: Relation of Track Z0 ?

binu G, there is a exact relation between W, T, dieletric, ... and Z0, would you explaine your application in more detail?

 

[sinatra]

Re: Relation of Track Z0 ?

Hello all.
Unfortunately till now there is no closed formula for the calculation of the impedance of a microstrip line.
For years, many reserchers have worked to find approximate formulas that could for instance help circuit designers and also software (simulator) designers.
An example of approximate formula is the following:

Zo= (377/sqrt(er)) * (h/Weff) * 1/(1+1.735 * (er**(-0.836-0.0724*(W/h))))

Weff = W + (t/PI) * (1+Ln (2*h/t))

Where:
Zo= impedance of the microstrip line
er= relative epsylon of the dielectric (dielectric constant)
W=width of the microstrip line
h= thickness of the dielectric (substrate)
t= thickness of the microstrip track (thickness of the copper track for instance)
Weff= effective W

S.

 

[binu G]

Relation of Track Z0 ?

sinatra

you mean there will be no change in impedance with respect to the length?

Binu G

 

[smithchart]

Re: Relation of Track Z0 ?

Hi, BinuG:

What kind of configurations you are looking at? Microstrip/Embedded Microstrip/Stripline or....?

Generally speaking, you can use: Z0=sqrt(L/C) to calculate the impedance of a lossless trace. From the formula, it is easy to conclude that: Zo is independent of length as long as it is uniform. From the parallel plate capacior equation: C=Eps*A/d, where Eps is the dielectric constant of material, A is the area of the metal trace and d is the distance between the trace and the reference ground plane, we can conclude that increase the trace width, increase the Eps will increase the C value(change of C is dominant compared to the change of L), thus will decrease the Zo. on the other hand, increase distance will decrease the C and increase the L, resulting in a higher impedance value.

Attached is a very good tools that you can use to calculate the impedance of those kind of planar structures. In addtion, it can synthesize the geometry for a specified impedance, very useful!

Best regards,

 

[sinatra]

Re: Relation of Track Z0 ?

Hello binu G and all the others.
Binu G, yes, the impedance of a line doesn't depend on the lenght of the line.
You can see this for example in the case of the rg58 coaxial cables that you probably have already used. You probably noticed that you can by already made cables of many different lenghts and they are always 50 ohms.
Normally the impedance of a transmission line depends on many geometrical parameters and some other electrical parameters like epsylon, etc... but in general it never depends on the lenght.
S.

 

[amjad]

Re: Relation of Track Z0 ?

For stripline and microstripline there are 2 formulas they are attached[/img]