How to measure the Er and the electric loss factor of a substrate/material ? - Page 1
Forum section: electronics / rf-microwave

author: jujun
author_rank: Regular Contributor
timestamp: September 29, 2018, 08:17:07 am
content: Hello,I am looking if it's possible with usual lab material (VNA, signal generator ... ) to measure the Er and the electric loss factor of a substrate.I don't really want a very accurate measurement, it's mainly to have a rough idea of what I have in the hand.Maybe by making a 50 ohm trace and see how it perform, it's maybe possible to do the calculation in reverse. (with losses and impedance to know the Er and TanD) But I don't know at all if it's possible. Maybe there are others ways ?Thank youJ

author: Kleinstein
author_rank: Super Contributor
timestamp: September 29, 2018, 12:09:27 pm
content: It depends on the frequency range. A reasonable easy way would be to have an geometrically well defined test structure and measure with an VNA or similar. The test structure response depends on the Er and loss. The test structure could be just a (e.g. 50/75 Ohms) transmission line (Speed and loss depend on Er and loss), but also a kind of strip line resonator (frequency depends on Er, Q on loss).It might be a little difficult to get tan D from the transmission line, I would expect the resonator to be more sensitive, but there is also conductor loss and not all resonators are the same.For simple structures there should be reasonable accurate formulas or computational methods.Another method would be to build a kind of plate capacitor with defined geometry and measure the capacitance difference from just air to having the substrate material inside. The loss measurement would be rather difficult unless one has a very good cap meter that can do good loss readings.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 29, 2018, 01:17:08 pm
content: Yes, that is how they do it. The formulas are not simple, depending on frequency (a resonator or TL depends on frequency anyway). Once that's known, you get the plots shown by laminate manufacturers, well, the good ones on the good materials anyway.Searching on those terms should produce results for, say, a Keysight measurement fixture and how-to-use manual.Tim

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 29, 2018, 04:35:25 pm
content: One way is to make a resonator on a known material (microstrip resonator on a good dielectric) and then measure the resonance in air/vacume. Then placing the dielectric you want to measure on top of the resonator, and observing how this changes the frequency and Q of the system.I've seen this used for measuring dielectrics at tens/hundreds of GHz.Keysight have anappnote about measuring dielectrics

author: Zeren
author_rank: Newbie
timestamp: September 29, 2018, 05:32:05 pm
content: You can use two lines with different lengths and measure difference in phase and attenuation. From this you compute complex permittivity. I used this method from few hundreds MHz to 67 GHz.You can verificate extracted permittivity with measurement of impedance step from 50 ohm to 100 ohm and back to 50 ohm.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: September 29, 2018, 07:33:43 pm
content: Quote from: Zeren on September 29, 2018, 05:32:05 pmThe problem here is that you are relying on the repeatability of your manufacturing process. Other measurements (free space, coaxial probe, cavity, parallel plate) are less dependent on this because of the fixture not changing, you just add or remove the dielectric sample. I think especially for loss this can be an issue, if you are measuring very low loss substrate at high frequencies, it can be hard to isolate manufacturing issues from true dielectric loss. I believe DuPont actually have a set of slides on the issues they ran into trying to measure ultra-low-loss PTFE substrates.link

author: Zeren
author_rank: Newbie
timestamp: September 29, 2018, 08:01:15 pm
content: I don't repeatability is issue in this measurement but issue is separation dielectric loss from loss in conductor.

author: Wolfgang
author_rank: Super Contributor
timestamp: October 01, 2018, 07:41:54 pm
content: Quote from: TheUnnamedNewbie on September 29, 2018, 04:35:25 pmI think the E5061B-3L5 is the machine of choice that Keysight recommends for material property tests. This works from 5Hz to 3GHz, so should cover all kinds of base materials except microwave.The low limit looks awkward, but can help to identify losses due to dielectric soakage.