Some distributed systems questions ... - RF Cafe Forums

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taz

Post subject: Some distributed systems questions ...

Unread postPosted: Thu Oct 07, 2004 12:12 pm

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Lieutenant

Joined: Thu Oct 07, 2004 11:52 am

Posts: 2

hi,

I have two basic but important questions, I hope you can help me:

1. Why do we use distributed model (and not lumped) in frequency applications\problems ??? what are the advantages\disadvantages ?

2. Why there are incident waves and reflected waves in a transition line ? this happen for both sinusoidal and step function….why ???

Thanks,

taz

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Itay

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Unread postPosted: Thu Oct 07, 2004 12:36 pm

Greetings,

No clue for your first question. If you will be more specific I might give you an answer. Basically, distributed elements can be modeled as lumped elements for the the purpose of analysis; for example, this is the way to derive the telegraphic equations of a transmission line.

For your second question: Incident waves are voltage waves that flows from a source eg: Generator to a load. In an ideal circuit all the energy of these waves is being absorbed in the load. In the practical world, a portion of this energy is reflected back due to impedance mismatch. The impedance of the load should be equal to the characteristic impedance of the system eg Zo, which is usually 50 ohm in RF systems, 75 ohm in Video applications and 600 ohm in Audio applications. The amout of the mismatch determines the size of the reflected waves. The reflection coefficient determines the size of the reflections. It is defined as: rho=(ZL-Z0)/(ZL+Z0). From here you can also derive the return loss, which is the amount of the reflected wave in relation to the incident wave: RL=-20 log |rho|. That means how lower is the reflected wave comparing to the incident one (It should be as low as possible of course)

Hope this helps,

Itay

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taz

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Unread postPosted: Thu Oct 07, 2004 1:02 pm

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Lieutenant

Joined: Thu Oct 07, 2004 11:52 am

Posts: 2

I will start with your second answer, I agree with what you said 100%, and for the present time I accept it as an axiom. This is why I'm asking these questions …why mismatch is causing reflected waves ? why In the practical world, a portion of this energy is reflected back ??? why match circuits don't have reflected waves ???

I understand the math, but I don't understand the physics behind it ….

And for my first question, for my best understanding …these are two different theories , but according to what you said – they are derived from another . is that a correct interpretation ???

by the way, why in speakers wiring there's no indication for impedance ??

again…thanks,

taz

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Itay

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Unread postPosted: Thu Oct 07, 2004 1:42 pm

Greetings Taz,

You are welcome.

Yes the theory of transmission lines is derived from an analysis of lumped model which defines the transmission line as LPF circuit that includes L,G,R,C elements and also their values per unit length. From this theory you also derive the Zo and Gamma coeffiencient: Zo= sqrt (L/C) - In loseless tranmission lines and Zo= sqrt ((R+jwL)/(G+JwC)) in lossy transmission lines and many more definitions...

I advise you to read more in text books regarding your questions and find better answers. Posting questions like these in a forum won't bring the answers you would expect, as there are many lines to write about it and people aren't that patient to write them...