What is the difference between impedance and the characteristic impedance of a line?

Thread Starter

hoyyoth

Joined Mar 21, 2020
196
Dear Team,

May I know the difference between Characteristic impedance and impedance?

Why we are always concerned about the characteristic impedance of a TX line.

In the TX Line impedance is not equal to the characteristic impedance,then the circuit not work?

Arai
 

Papabravo

Joined Feb 24, 2006
18,070
The characteristic impedance of the line depends only on the geometry of the transmission line. It is generally understood to be a constant. If you take a particular length of coaxial cable with distributed inductance and distributed capacitance and sweep it on a VNA (Vector Network Analyzer) you will see the actual impedance as a function of frequency.

Then if you look at the partial differential equations for a transmission line you will see both the characteristic impedance and the actual impedance, usually as a ratio or a rational function. (I may be a bit fuzzy on this, since I haven't looked at it in a long time).
 

LowQCab

Joined Nov 6, 2012
1,913
Its basically the difference between how it's "supposed" to work,
and how it "actually" works in a Circuit with other Components.
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Thread Starter

hoyyoth

Joined Mar 21, 2020
196
The characteristic impedance of the line depends only on the geometry of the transmission line. It is generally understood to be a constant. If you take a particular length of coaxial cable with distributed inductance and distributed capacitance and sweep it on a VNA (Vector Network Analyzer) you will see the actual impedance as a function of frequency.

Then if you look at the partial differential equations for a transmission line you will see both the characteristic impedance and the actual impedance, usually as a ratio or a rational function. (I may be a bit fuzzy on this, since I haven't looked at it in a long time).
Hi Papa,

Thank you very much.

What I understood from your answer is characteristic impedance is a constant and impedance is a variable.Please correct me if I am wrong.

When we are designing a PCB for a high-speed signal do we need to take care of both characteristic impedance and line impedance or only characteristic impedance.

Regards
HARI
 

Thread Starter

hoyyoth

Joined Mar 21, 2020
196
Its basically the difference between how it's "supposed" to work,
and how it "actually" works in a Circuit with other Components.
.
.
.
Hi LowQCab,

Thank you very much.

If you don't mind could you please elaborate on your answer.

Regards
HAri
 

Papabravo

Joined Feb 24, 2006
18,070
Hi Papa,

Thank you very much.

What I understood from your answer is characteristic impedance is a constant and impedance is a variable.Please correct me if I am wrong.

When we are designing a PCB for a high-speed signal do we need to take care of both characteristic impedance and line impedance or only characteristic impedance.

Regards
HARI
I don't have much experience in PCB design. What I can tell you is that characteristic impedance, a constant real number, is used to match the source impedance to the transmission line and to match the transmission line to the load impedance. In instances where there is no transmission line, or it is very short, there is a conjugate match. Using a conjugate match is using the actual impedance rather than the characteristic impedance.
 

LowQCab

Joined Nov 6, 2012
1,913
The Higher the Frequency goes, the more various unexpected deviations from ideal start to occur.
Are You working with a Coax-Cable, or a Twisted-Pair,
or are You working on a Printed-Circuit-Board ?
What Frequency-range are You working with ?
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Irving

Joined Jan 30, 2016
2,524
My take on this is that a transmission line aims to connect two entities. If they have the same impedance eg 50ohm then the line must also be 50ohm in order to facilitate maximum transfer of energy - that is, the line must have a characteristic impedance matching the impedance of the end points.

If the end points differ in impedance, eg a 50ohm driver matching to an odd length dipole aerial then a matching network is needed. This often takes the form of a transmission line which, while having a known characteristic impedance, exhibits a specific impedance by virtue of its length being specific such that it bridges the gap between the end impedances, to facilitate maximum energy transfer and minimum reflectivity.
 

Papabravo

Joined Feb 24, 2006
18,070
My take on this is that a transmission line aims to connect two entities. If they have the same impedance eg 50ohm then the line must also be 50ohm in order to facilitate maximum transfer of energy - that is, the line must have a characteristic impedance matching the impedance of the end points.

If the end points differ in impedance, eg a 50ohm driver matching to an odd length dipole aerial then a matching network is needed. This often takes the form of a transmission line which, while having a known characteristic impedance, exhibits a specific impedance by virtue of its length being specific such that it bridges the gap between the end impedances, to facilitate maximum energy transfer and minimum reflectivity.
You also want to avoid an impedance discontinuity which would give rise to reflections, which of course affects the transmitted power.
 

crutschow

Joined Mar 14, 2008
29,466
When we are designing a PCB for a high-speed signal do we need to take care of both characteristic impedance and line impedance or only characteristic impedance.
It depends upon the frequency of the AC signal or the rise/fall times of a digital signal compared to the trace length as to whether you need to be concerned about the trace impedance (and use strip line or microstrip PCB design).
Here's a good reference on that.
 
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