Means of determining microstrip impedance

Typo. This should be "Z0 is 40 ohms."

Hi Joerg,

Well, that's how I designed them -- assuming a geometry, dielectric constant, etc. However, the material I'm doing this on is "generic FR-4" so the dielectric constant is a little iffy (I assumed 4.6, but I believe people tend to use anywhere from 4.4-4.8); hence the desire to see what actually happened and then back-calculate a few parameters.

I don't necessarily expect such a generic (read: cheap) dielectric to be good past a couple of GHz, but it would be nice.

Hmm... just get myself a bunch of 1% resistors and play, eh? Not a bad idea; thanks!

---Joel

I use TDR, a Tek 11801 scope with an SD-24 sampling head. That time-resolves the whole path, so you can separate out things like connector bumps and termination mismatches, and see the impedance at any location along the trace. To see where things are, you just run your finger along the trace and follow the bump that makes; great fun.

I think one can FFT the S11 data and theoretically reconstruct a TDR-like plot of impedance vs distance.

Send me a board with a test trace and I'll TDR it for you. It would be interesting to compare to a frequency-domain measurement.

John

Hello Joel,

Strange. If it was me I'd find out the material, measure the thickness, the trace width and calculate the impedance. The math is in the old Moto MECL databooks or the Fairchild version.

Else I'd probably "cheat" and change the termination resistor until no resonance is seen anymore. That would be the line's Z.

Regards, Joerg

Hello Joel,

I have seen a 5GHz antenna done on FR4. It may not be perfect but cheap. Maybe you could find the dielectric constant by taking a really large sheet and measuring the capacitance. On a multi-layer there can be a substantial difference between pre-preg and the other layers, likely due to the different number of fibers per unit.

No need for 1%. Just take SMT resistors where you are sure they are low inductance. Probably it's safe to start high and stack up to three. When things jibe and there is no resonance just ohm it out.

As a consultant I am trying to stay low on the food chain when asking for equipment access at a client's site. The big network analyzer would be nice, but.... So, another trick I have used was to place a lower value resistor and then gently taking the dremel to it until "Z=Ideal" happened, then measure. But be careful with respect to the dust. Some might even be a bit unhealthy.

Regards, Joerg

If the trace is long enough, and you have a scope fast enough, and if you can put a fast edge on it, you can measure Zo using the voltage divider approach.

The idea is that you would put a fast-edge in series with, say, a 47 Ohm resistor, then measure the ground referenced voltages (using an oscilloscope with active probes) on both sides of the resistor.

You want to use the first plateau voltage, and ignore the step caused by the reflection from the end of the line.

For some period of time after the edge starts to rise, what you have is a Voltage divider formed by Rseries and Zo. So the rise time of the edge must be significantly shorter than the two-way flight time down the trace and back for this to work.

Note, I've never actually done this with a circuit board, although I have done it with coax, just for fun.

--Mac

Theory is great. But as a practical matter, if you want to know the Z0, it's best to just measure the Z0. I always used a 100 ohm 1/4" pot at the end of the trace and a TDR at the beginning. Tweek the pot till the flat part after the glitch caused by the pot is equal to the flat part before the part, the transmission line. Measure the resistor when you're done.

In a perfect world, you could do the same with a network analzyer, but I've never been able to get frequency domain measurements to yield results in a less than perfect world.

As another practical matter, if you need accurate Z0, you've got a severe production problem using FR-4. Your vendor probably doesn't have any idea what the dielectric constant is for each batch. You MUST specify the exact layup you want. That won't keep 'em from changing it on you, but you can get your boards rebuilt when they do. I've seen problems caused by too much or too little pressure when they mashed the layers together. When there's only a few mils of dielectric, it don't take much squishing to change it radically. I found it better to make sure that critical traces and the associated ground plane were on opposite sides of the same piece of dielectric with thickness specified very tightly. Sometimes it takes a non-sandard layup to make that happen.

Don't get me started on what happens two years down the road when some bean counter decides he can save a buck by changing vendors. Make sure EVERYTHING is written into the board spec. and that incoming inspection checks for it. mike

...and have care with the termination resistor(s). For 50 ohms, use two 100 ohm 1/8 watt carbon composition, minimum lead length, spread in a "Y" from the microstrip to the ground plane (i think30 degrees may be optimim). With chip resistors, one might find that two 100 ohm SMD placed in a similar "Y" configuration may be better than one 49.9 ohm SMD. ..and beware of ground plane under the resistors... For the carbon comps, you want that; for the SMDs, you may want the ground plane to go no further than half the length (at most).

Perhaps a PCB material that is more uniform would be better. Megtron 5 is price competitive with FR-4:

That's really what I'm doing now (just that I'm using a method that doesn't require knowing the actual length of the tracee). I was hoping there was some way that you could visually get an indication of relative impedance (relative to the termination, which is the same impedance as the reference for the Smith chart) as a function of frequency... similar to how you can estimate Q of a tuned circuit from the size of the 'loop' it makes on a Smith chart.

---Joel

Hello Robert,

At those frequencies you really have to use SMT resistors.

Regards, Joerg