using microstrip line to measure the dielectric constant ?

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shahid78

Joined: 29 Jul 2006
Posts: 328
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07 Jul 2008 16:47 microstrip to measure dielectric constant

How do I measure the dielectric constant of a substrate using a microstrip line? Someone said that I could layout a microstrip line on a the substrate backed by a ground plane and use that fixture to do the measurement.
any help or comments are welcome

regards

RF-OM

Joined: 03 Aug 2007
Posts: 607
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Location: The Earth

07 Jul 2008 21:18 10ghz microstripline design

You may use TDR and measure the line impedance and then use any good microstrip line calculator to find the effective Er. After this you may calculate Er from effective Er. I think it is easier to have a square copper element on the board with known area and measure the capacitance. You also may to do a search books or Internet with Google and find how PCB vendors measure this parameter.

Phytech

Joined: 30 Apr 2008
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Location: USA

09 Jul 2008 14:43 capacitance, dielectric constant by using ic -555

It's much more accurate and comvenient to measure by the capacitance method rather than by the impedance method.

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RF-OM

Joined: 03 Aug 2007
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09 Jul 2008 17:21 ads 2008 tdr microstrip

Agree, but the question was about transmission line usage.

shahid78

Joined: 29 Jul 2006
Posts: 328
Helped: 28

10 Jul 2008 19:31 Re: using microstrip line to measure the dielectric constant

hi Phytech

i agree with RF-OM that i was asking about using microstrip line to retrieve dielectric properties

however i dont know about this capacitance method , so it would be good if you explain a bit about how to setup model to use this method

regards

Phytech

Joined: 30 Apr 2008
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Location: USA

11 Jul 2008 17:26 Re: using microstrip line to measure the dielectric constant

Hi shahid78

The capacitance method is just cut a piece of substrate with regular shape, such as square, rectangular, or round, where you can easily measure and calculate the area, as well as your substrate area. Then measure the capacitance value using a capacitance meter or LCR meter. Then use the parallel plate capacitor value formular to calculate the dielectric constant. (C=e0*er*area)/thickness

Actually, you can also use microstrip line to measure the dielectric constant. Here you use the microstrip line with length (L) amd width (W) as a half lamda resonator and you measure the resonant frequency (f) under weakly coulped conditions (e.g., with the measured insertion loss more than 35 dB). Then solve equation to find out er.

2*L*f=c/sqrt((1/2+er/2)+(er/2-1/2)/(1+12(thickness/W)^0.5)).

shahid78 wrote:

RF-OM

Joined: 03 Aug 2007
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11 Jul 2008 20:41 Re: using microstrip line to measure the dielectric constant

Yes, Phytech gave you the right method. I just want to add something that may help to improve an accuracy when it is important. When you measure the capacitance you need to know so called effective Er. This term counts how much electrical field lines actually closed through dielectric and how much closed through the air. In other words you are count the fringe effect. Practically it is better to have the copper square stopped about 3-to-5 substrate heights from the edge of the board. It will ensure that practically all electrical field lines are inside the dielectric. It also help to connect capacitor of approximately the same capacitance as was measured in parallel with your copper square. You also may measure capacitance again a few times, then calculate your copper square capacitance and take the geometric means from these calculations. In this case you will get the best accuracy.

madengr

Joined: 06 Apr 2005
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Location: Kansas City

11 Jul 2008 21:52 using microstrip line to measure the dielectric constant ?

Er for PCB is typically done with a resonator, such as a microstrip ring resonator. It's easy to measure resonant frequency.

FvM

Joined: 22 Jan 2008
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Location: Bochum, Germany

11 Jul 2008 23:28 Re: using microstrip line to measure the dielectric constant

Apart from the detail problems of determining Er accurately from microstrip or similar structures, a basic aspect hasn't been mentioned yet. Er isn't a constant value, it has to be determined either at defined fixed frequencies (e. g. 1 MHz, 1 GHz, 10 GHz) or as a function of frequency. Obviously the apparatus to use depends on the intended measurement frequency/frequencies.

Azulykit

Joined: 18 Feb 2007
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12 Jul 2008 0:20 Re: using microstrip line to measure the dielectric constant

Chapter 1 of "Microstrip Lines and Slotlines" K. C. Gupta spends some time discussing various dielectric constant measurement techniques.

It might be useful to see if someone has posted a copy here or even purchase a copy (eeeks!)

RF-OM

Joined: 03 Aug 2007
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Location: The Earth

12 Jul 2008 3:44 Re: using microstrip line to measure the dielectric constant

Thank you, I found it. There are a few pages (45-49) about this problem. The idea is to form the resonant structure and then measure the resonant frequency. There is even recommendation to cover the edges with metal foil in order to form the cavity and measure the cavity resonance. It is the same technology that we developed with professor that you saw yesterday. There is also some well known information about calculating effective Er from known Er. Please let me know if you want to see this chapter.