# How to measure the response of resonating line?

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#### sviodo

##### Member level 4
substrate parameters

Recently i saw paper that describes method of er measurement by the response of resonating lines (or sth similar, don't remember exactly).
Do you, any of you know this paper/method and could attach appropriate pdf?

Regards,

sweep frequency

You measure the input impedance of an open line. The lowest frequency that gets a near zero impedance is the one the line is a quarter wave long electrically. Compare the physical length and the calculated free space quarter wave length and take the square of the ratio. This will get you close. If you want even closer, use the attached program which will give you the line wavelength for your particular trace. You put the first estimate of esubr in and see how close the calculated wavelength is and then cut and try small variations in esubr.

Measurement of Er

You can also do this most effectively with a ring (for microstrip measurements). The mean diameter of the ring is equal to the guide wavelength. I posted an internet link to NIST recently which contains lots of info on Er measurement. Let me know if you are interested and I'll re-post

Timelord

Re

hi timelord

If the substrate is thin, you can make a capacitor, this is the simpliest way.

Rogers

Rogers corp uses the new board with copper on both sides and measures the capacitance. This has some drawbacks becasuse most people have capacitance meters that operate in the lower frequency range under 1 MHz. You also have to be able to measure the board thickness. You can probably trust the metal thickness specifications and subtract that from the total thickness measurement.

There are several good methods.

1.) There is an easy method:
You just build two microstrip THRU-lines of different lengths.
Then you subtract the phase of the short line from the long line.
From this and the knowledge of the length difference of the lines you can derive the EFFECTIVE dielectric constant. You can use then the analytical formula to calculate the dielectric constant from the effective dielectric constant.

2.) You can build a notch filter and compare the resonant frequency with a 2.5D or 3D EM-simulator.

3.) You can build a microstrip line and measure it. Then you can plop a substrate ontop of it and see how its characterics change and derive epsilon_r.

There is even a paper discussing the errors involed in method 1 and 2, but I would have to dig it out.

have to say that few ideas seem to to practical :>

Rgz,

I atually used method 1) and 2) to derive the characteristic impedance of G/E/T/E/K and FR4 material. No1 seems to be best.
The board mfg. often only specifiy the epsilon at 1MHz (except for R/O/GERS. If you see a plot with epsilon over frequency that shows a LINEAR curve down to 1 MHz, be very careful.
The epsilon actually slopes up at the low frequ. end.

harkonnen said:
I atually used method 1) and 2) to derive the characteristic impedance of G/E/T/E/K and FR4 material. No1 seems to be best.
The board mfg. often only specifiy the epsilon at 1MHz (except for R/O/GERS. If you see a plot with epsilon over frequency that shows a LINEAR curve down to 1 MHz, be very careful.
The epsilon actually slopes up at the low frequ. end.

have same material to check thx budy

There is a simple and accurate method for measuring Er.
Make a very looseley (wide gap) coupled half wavelength filter (a microstrip line with two wide gaps). Measure the resonant freq. Calculate the Er from analitical formulas.

If you want to know it for high frequencies and you have on-board mesurements setup, is better to use CPW's instead of ustrip's.

You skip the via to ground

Measurement of ER

Sviodo

Here is the paper I promised you on how to calculate ER using ring resonators

Timelord

I heard there are materials whose dielectric constant is more than 1000....I 'ld like to know what your idea about it, if you confirm it and give me some information.

Lupin

i guess you don't mean low loss microwave substrates, because if you mean there are some about er=80 and that's the high...

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