We know that the characteristic impedance of a microstrip line is given as follows:
In general the metal resistivity is so small that Z0 can be taken as sqrt(L/C). But if the metal resisitivity is not that small? Then according to the equation, this resistivity should affect the Z0. Is it true?
I did simple Agilent Genesys simulations of microstrip lines to see the effect of the metal resistivity on Z0 but surprisingly it didn't affect Z0. When I increase the metal resistivity, it only increases the attenuation. Any idea and help is appreciated.
Best Answer
If you are using a microstrip then it's likely that you are dealing with frequencies in the tens of MHz upwards. Let's use 100 MHz as an example and let's take the example of L being 250 nH per metre and C being 100 pF per metre. Together they produce a characteristic impedance of \$\sqrt{250/0.1}\$ = 50 ohms.
So how much is the impedance of 250 nH at 100 MHz - do the sums and you'll get 157 ohms.
So, if you increase your resistance per metre to something approaching or in excess of this then you are going to see changes to the impedance. It all boils down to how far you increased the resistance per metre and how you measured (or judged) the value of characteristic impedance.
I suspect that you went nowhere near 100 ohms per metre because the resistivity of a copper track 1 metre long would be about 1 ohm or less.