Some Distributed Systems Questions ... - RF Cafe Forums
Post subject: Some distributed systems questions ...
Unread postPosted: Thu Oct 07, 2004 12:12 pm
Joined: Thu Oct 07, 2004 11:52 am
I have two basic but important questions, I hope you can help
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 ???
Unread postPosted: Thu Oct 07, 2004 12:36 pm
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
Hope this helps,
Unread postPosted: Thu Oct 07,
2004 1:02 pm
Joined: Thu Oct 07,
2004 11:52 am
Hi Itay, thanks for your answer !
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 ??
Thu Oct 07, 2004 1:42 pm
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...
Regarding your question about speakers, please read the following:
have a varying load impedance and there is no meaning for the impedance
of the wiring.
I hope this helps,