Microstrip Impedance Calculator Inaccurate?

I used this website to calculate microstrip impedance and found it to be inaccurate. The paramameters were: Trace Thickness = 2.08 mil, Substrate Height = 4.06 mil, Trace Width = 5.4 mils, Substrate Dielectric = 3.84 mils. The calculated impedance was 36.3 ohms but I believe it should be 50.6 ohms. When I google "impedance calculators pcb" this website is the first hit. The websites associated with hits 2-5 all calculate the microstrip impedance for this stackup to be 50.6 ohms. Can anyone explain this discrepancy?

Rick,

We are checking into this now. Thanks for bringing it to our attention.

I've questioned the accuracy too. See my comparison of three different online calculators.

chris

distances are in mils


[code lang="php" line="none"]source w s t h er Zd (ohms)
Mantaro 38 5 0.7 62 4.3 100.3
EEWeb 26 5 0.7 62 4.3 99.7
Saturn 37 5 .5oz 62 4.3 100.4

Mantaro 44 10 0.7 62 4.3 100.1
EEWeb 46 10 0.7 62 4.3 100
Saturn 43 10 .5oz 62 4.3 100.2

Mantaro 22 5 0.7 31 4.3 99.3
EEWeb 23 5 0.7 31 4.3 99.7
Saturn 21 5 .5oz 31 4.3 99.5

Mantaro 27 10 0.7 31 4.3 99.9
EEWeb 36 10 0.7 31 4.3 99.6
Saturn 26 10 .5oz 31 4.3 100.1[/code]

Some online calculators use a very simplified approximation to calculate impedance. These formulas work if your looking for a ball park impedance. Then a few years ago a committee put together the IPC-2141A Design Guide for High-Speed Controlled Impedance Circuit Boards. This effort was to come up with a set of approximation formulas in one standard. This is what EEWeb used for this impedance calculator.

To get back to the comparison that you made we have some data from Polar's impedance solver a high end simulation tool.

W=38, H=62, Er=4.3

Polar Zo=87.91
EEWeb Zo=90.2
Mantro Zo=90.23

In Regards to Ricks Questions I think it may be that you used the wrong calculator. Use this link for "Microstrip Impedance":http://www.eeweb.com/toolbox/microstrip-impedance/

I calculate 50.6 ohms

see attached screen shot

Thanks Cody,

I agree, it now seems to work. I can't explain the former discrepancy. Thinking I might have been using the wrong calculator, I went back and tried using the only other impedance calculator with 4 input parameters (Symmetrical Stripline) and got 33.1 ohms rather than 36.3 ohms. At this point I wish I took a screen shot of what I saw a few months back. The good news is that the calculator appears to be correct.

Have you verified your embedded impedance calculator?

Using http://www.eeweb.com/toolbox/embedded-microstrip-impedance/ with
T: 1.4, H1: 4, H2: 5.7, W: 5, Er: 4.2

I get 34.6 Ohms.

That seems like way too much drop from the basic surface calculator (http://www.eeweb.com/toolbox/microstrip-impedance/), which gives 54.7 Ohm for the same basic stackup.

In fact, it is very close to what the symmetrical calculator (http://www.eeweb.com/toolbox/symmetric-stripline-impedance/) gives for a 4 mil separation and other parameters the same.

It is also much lower than any other calculator I have found. Saturn and the old WindRiver spreadsheet give ~46 Ohms. Other web-based calculators give about the same. They all seem to be based on the same formula.

Assuming I am thinking about this correctly, this is looking really unlikely: Set embedded microstrip to match the embedded stripline using 1.4, 4, 9.4, 5, 4.2 and compare that to embedded stripline with 1.4, 4, 5, 4.2, physically exactly the same thing, but the second has a piece of metal on top, and the embedded microstrip gives a lower value.

Embedded microstrip: 32.9 Ohms
Embedded stripline: 34.5 Ohms

That is not physically possible. The copper layer cannot increase the impedance.

The copper layer absolutely changes the impedance. The additional copper layer increases the capacitance of the trace which decreases the impedance. A general formula for a transmission line is:

[tex]Z_{O}=sqrt{frac{L}{C}}[/tex]

Since C is in the denominator as C increase Zo decreases. Check out this "link":http://en.wikipedia.org/wiki/Telegrapher's_equations for more information.

I understand that the copper layer decreases the impedance. That was exactly my point.

The EEWeb embedded microstrip gives a lower, not higher, impedance than the stripline.

That claims the impedence is increased by the copper layer. I see no physical mechansim by which that could be correct.

More than that, I would expect the embedded microstrip impedance to be much higher than the stripline with comparable spacing to the plane layers as I believe I set up in my example parameters.

One case has a single plane at 4 mil and the other has two planes each at 4 mil. Other than that, I think I configured every other physical characteristic identically from the point of view of the equations.

these calculations are approximations at best even if everything is calculated correctly, but yeah i suspect there are errors in this calculator
http://www.eeweb.com/electronics-forum/stumed...dge-coupled-microstrip-impedance-calculation/

i posted about this, in the formulas presented in the web calc explenation i found several differences from the IPC-2141A

Attached screen shot shows a different result from it was 4 years ago. It is now 58.0 ohms. What I'd like to say is that this is not a reliable source, and the reason I found this posting is I had a question about the accuracy of its "Asymmetric Stripline Impedance Calculator". For a board we designed, the EEWeb calculates 47.9 ohms, while Mantaro gives 29.967 ohms (see other two screen shots). Quite a difference!

Unfortunately, we made the PCB based on the EEWeb calculation, and the impedance of the trace turns out to be close to the Mantaro's calculation.

I think the source of EEWeb comes from _he IPC-2141A (2004) “Design Guide for High-Speed Controlled Impedance Circuit Boards”_ . It is quire bothersome to see that there is quite a difference in the answers. Here is a "link ("")":http://www.ipc.org/4.0_Knowledge/4.1_Standards/2141A_Errata.pdf to the errata of the IPC2141A. Like what Cody send "IPC standard based their work off of the Transmission Line Design Handbook and that the equations in the handbook is what they intended."
Also referring to previous comments in this forum, https://www.eeweb.com/electronics-forum/stume...edge-coupled-microstrip-impedance-calculation

Hello
I am new member of this forum site. But, I have known that Microstrip Impedance Calculator Inaccurate

In about my numbers are:
Dielectric constant - 5.58
Dielectric height - 21mil
Signal line width - 23 mil
Copper thickness - 11z - 3.4mil