I want to model a simple signal transmission system by considering it as travelling through a transmission line.
Here is the system:
The original signal generated by the transducer is 0 to 10V step function like signal with very low 3Hz fundamental frequency.
So the signal has high harmonics which matters since it is squarewave-like step function signal.
This is a general question which is related to my previous question: Passive low-pass filtering question for a transducer output
Basically I have confusion when to use which model. Transmission line theory or lumped model…
So I want to model this in LTspice as a Lumped LC Model or something in that sort to see phenomena like ringing.
To achieve an approximate model I will write about each sections from left to the right of the circuit:
Questions when trying to use lumped model:
1-) Rload in the above illustration represents a scope or very high input impedance of a measurng instrument which is known.
Rout in the above figure is the output of a transducer which is unknown. How to measure that if it is not a function generator with a known output impedance?
I will use the following method to find the output resistance:
Do I really need the output impedance or can I take it roughly some hundreds of ohms?
2-) Coaxial-cable in the above illustration is a standard 50 Ohm intrinsic impedance BNC cable. I know its length and lets say it is 20 meters long.
Since we use lumped element model we will not use 50 Ohm right? And capacitance and the inductance will vary with length?
In other words, how can I model this cable in LTspice?
A question when trying to use impedance matching in transmission line theory:
So now we have a transducer output impedance which is Rout=Zout, coaxial cable impedance which is 50 Ohm, and we have a load Rload=Zload which is lets say 100M.
So in this case to achieve impedance matching throughout the line I need a 50 Ohm resistor in parallel with Rload just before Rload to make the Rload 50 Ohm.
And I also need to know the Rout and I need to add a series or parallel resistor to it such that its equivalent or Thevenin would be 50 Ohm.
Is this method/model right? If it is than I will have the following issue:
The 50 Ohm parallel resistor will load the transducer and I will have more error right?
It seems to me for sending data this might not be problem but in this case the signal's voltage level is important.
What would you suggest in this case?