Plastic Dielectric Constant Measurement using TDR and Copper Foil Tape - Page 1
Forum section: electronics / projects

author: neilhao
author_rank: Regular Contributor
timestamp: December 04, 2019, 07:44:53 am
content: Let us try to find out the easiest method for plastic dielectric constant measurement~For RF design, the plastic dielectric constant control become crucial when the RF circuit does not have sufficient clearance from plastic structure. Plastic structure near the antenna causes that the resonant frequency shifts to a lower frequency about 100Mhz to 200Mhz in general . This kind of situation become common due to small product dimension and flexible adhesive antenna using wildly. One way to deal with the resonant frequency shifting is wideband antenna. However, the batter solution is measuring plastic dielectric constant and then modeling the antenna with the necessary plastic structure, casing, etc. More accurate resonant frequency always can be calculated using more accurate model and the accurate dielectric constant is always required by accurate model. Therefore, it is meaningful to discuss the plastic dielectric constant measurement using TDR.More information could be found at:http://uniteng.com/index.php/2019/12/04/plastic-dielectric-constant-measurement-using-tdr-and-copper-foil-tape/Special thanks to w2aew, I was inspired by his pulse generator design and several YouTube videos related to TDR.

author: David Hess
author_rank: Super Contributor
timestamp: December 04, 2019, 06:41:52 pm
content: A TDR certainly works but the papers I have read on the subject usually rely on a microstrip resonator as part of an oscillator.

author: neilhao
author_rank: Regular Contributor
timestamp: December 05, 2019, 02:31:24 am
content: Quote from: David Hess on December 04, 2019, 06:41:52 pmDid you mean this method:http://www.keycom.co.jp/eproducts/dps/dps50/page.htmlI think this method is a little bit hard to be built from scratch by using plastic sample card, Copper Foil Tape, Craft Knife and Stainless Steel RulerI knew the Dk can be measured using differential length method, IPC clamped stripline resonator, full sheet resonance, split post dielectric resonator. However, for plastic, I think non of them are easy to be build from scratch.Generally we have two kind of methods, resonator method and transmission reflection method(non resonant). Resonant methods are used to test the dielectrics under a single frequency or several frequencies. Another methods are used to test the dielectrics under a frequency range. Different method may give different resultFor the microwave application, the proper dielectric constant test method should to be selected.

author: TheUnnamedNewbie
author_rank: Super Contributor
timestamp: December 05, 2019, 07:36:51 am
content: Quote from: neilhao on December 05, 2019, 02:31:24 amColleagues of mine have done measurements with high-Q resonant structures and then use a VNA. The main downside is that you need a different structure for every frequency, and that the frequency will shift (slightly, if you know a rough estimate of the permittivity already) from design to practice.If all you care about is the real value of the permittivity, measurements are 'easy'. Once you care about losses aswell, things become trickier. In resonant methods, you can use the Q to estimate the loss.For higher-loss dielectrics, a through measurement with VNA can yield good results. The hardest thing is doing extremely low-loss dielectrics (PTFE, PE, Alumina, etc.). In this case, the loss component gets buried in the measurement uncertainty. I believe one method that can be used is to measure the real part of the permittivity with extreme precision (which can be done using mirror-based open-resonant cavity techniques) over a wide frequency range, and then use the Kramers-Kronig relations to extract the imaginary part. Spectroscopy people do this a lot (or at least so I am told).On PCBs there is another interesting issue (rogers has a few good whitepapers on this) - the fact that most PCB materials, esp. those that have a glass weave, are actually anisotropic. As a result, depending on what type of transmission line/structure you are using, the permitivity is different. This is why in some Rogers datasheets the 'permittivity' and 'design permittivity' are different - one is measured by some standard IPC technique, the other is measured for a micro strip line.

author: David Hess
author_rank: Super Contributor
timestamp: December 05, 2019, 02:52:25 pm
content: HP/Agilent/Keysight makes or made dielectric analysers which only require a small sheet of material and work by placing it between two planar probes. But I think they only make their measurements at lower frequencies.

author: neilhao
author_rank: Regular Contributor
timestamp: December 05, 2019, 05:07:32 pm
content: Quote from: David Hess on December 05, 2019, 02:52:25 pmThe parallel plate method, also called the three terminal method in ASTM standard D15012, involves sandwiching a thin sheet of material or liquid between two electrodes to form a capacitor. The measured capacitance is then used to calculate permittivity.This method requires the surface of plastic sample to be very smooth

author: neilhao
author_rank: Regular Contributor
timestamp: December 05, 2019, 05:45:49 pm
content: Quote from: TheUnnamedNewbie on December 05, 2019, 07:36:51 amYou definitely spent a lot of time on the material research. The resonator method is too complex and time consuming for meI personally prefer the transmission reflection method using VNA, because it is relatively easy to be performed and I usually work with low loss materials. The accuracy was acceptable for sub 1Ghz to 6Ghz applications. However, the differential length method still requires two microstrips. For simplify, I tried to do the measurement by using one microstrip and TDR. I even want to find out another method which can measure the Dk without the microstrip.Yeah, the PCB is a troublemaker in both design and manufacturing. I was tripped by the manufacturing process control.http://uniteng.com/index.php/2019/11/20/the-considerations-of-antenna-design-for-iot-and-wearable-devices/Page3, PCB :The factors may affect PCB’s Dielectric Constant:a. The Dielectric Constant can be dependent on the substrate thickness, the testing frequency, the operation temperature, the type of copper used and the microwave structure.b. Two different microwave circuits using the exact same material will experience two different Dielectric Constant due to how the electromagnetic fields are using the different planes of the material.c. Dielectric Constant was determined by a very specific test method which may or may not correlate to the microwave application of interest. The Dielectric Constant which is reported for a material can be dramatically different by the test methods.d. The permittivity behavior is mainly attributed to dipolar moments and relaxation. Different dielectric materials will have different properties related to the dipole relaxation time.e. The low-end material such as FR-4 may have worse dielectric tolerance, moisture absorption, passive intermodulation (PIM), Dk/Df over Frequency and Temperature etc.f. In the manufacturing process control, the most of manufacturers in mainland China still used and only used legacy TDR equipment such as Polar CITS500s Controlled Impedance Test System. They may do not have capability to use the proper test method for the microwave application of interest.g. In the manufacturing process control, the manufacturers usually do not follow the material’s datasheet. The impedance is controlled by achieving the different pressout thickness. They use copper thickness, copper area, resin, estimated dielectric constant etc. to estimate the desired thickness. The Prepreg's thickness may very under the different cure temperature and pressure. The Core’s thickness will not be changed during the laminating process.