Impedance matching

[Kevin Weddle]

I want an easy way to determine the matching characteristics of a line. I want the output device to be any resistance, the input device to be any resistance, and the cable a 50 ohm. What is the length? How can we determine this without going to any trouble?

 

[MP1]

You will also have to provide the gauge of the wire to determine the resistance. Resistance can be determined with these two known values (thickness or gauge and length).

MP

 

[Kevin Weddle]

I want to use a 50ohm cable. This gives you capacitance as well a inductance. Somehow the load impedance will affect the line impedance. If you say the load impedance is closer to a short, then the length of the cable should be chosen to be a series inductor capacitor at the resonant frequency. This is the 50ohm seen by the source. I want to make sure of this with the oscilloscope. A standing wave will appear as an offset of the signal. Am I right?

 

[Kevin Weddle]

This will require the formula

wavelength = (.67)100x10E6 / frequency

What do you do with the wavelength? You also have to incorporate the load resistance. I would make the length some fraction of the wavelength. I would assume the load to be a short because it will be closer to a short. If you make the length the correct fraction of the wavelengh, you will get the line at it's resonant frequency. You don't want it to appear inductive or capacitive, but do you want it to be parallel resonant frequency or series resonant frequency? I think it should be series resonant frequency because a 50 ohm cable is closer to a short. Any suggestions about measuring the end result.

 

[MP1]

kevin, perhaps I am not understanding, but I thought you wanted to find the resistance of wire when knowing the thickness and length. If this is the case, there is a nice little calculator here at the link:
http://www.mogami-wire.co.jp/e/cad/wire-gauge.html

MP