How to measure the capacitance to get dielectric constant?

capacitance of a dielectric

I'm working on a pcb substrate and i need to know the properties like the characterisctic impedance and the dielectric constatnt the material is FR4 two copper layer.
Is there any instrument to measure the capacitance?
Can i measure that with a network analyzer with some configuration?

Thanks

how to get dielectric constant from capacitance

A method that seems to work pretty well is to print on a very loosely coupled resonator, like a λ/2, and see its resonance frequency. From that, you can compute effective εr

measure capacitor esr with network analyzer

Thanks but need i to know the dielectric constant? and I don't know

biff44 said:

A method that seems to work pretty well is to print on a very loosely coupled resonator, like a λ/2, and see its resonance frequency. From that, you can compute effective εr

Click to expand...

how to measure di-electric constant

To roderick81,

I don't really know if this idea is what you want, or in fact whether it will work at all.

But then maybe what you can do is etch away the copper cladding on your PCB, use two horn antennas connected to your network analyser with the bare substrate slotted between them, take the transmission loss results and compute your εr from there. There are literature on the net about this, people who do EM materials should know about this. However, I'm not one such people.

There IS however an equipment to measure capacitance off a sample of the substrate, I don't know what it is called but I'm sure you know what it is.

Do let me know if the idea is sound enough.

Hope this helps,
Dave

measuring capacitance and the dielectric constant

Thank you for you answer,
Once, i heard about a device that measure capacitance called qmeter but i `ve never used that device.
Finally, i will try to make two microstrip line and measure the phase difference and then to obtain the effective dielctric constant.

best regards

how to measure dielectric constant

Hello all.
It is very easy to get the dielectric constant at low frequencies.
Just take a non-etched 2 layer square peace of your PCB material. Use a simple multimeter (like Fluke) which is able to measure capacitance. Measure the capacitance of your peace of PCB and then use the standard parallel plate capacitor formula to find the dielectric constant of your substrate.

C= er e0 A/h

Where:
C is the capacitance in F
er - dielectric constant of your material
e0 - 8.85E-12 F/m
A - Capacitor area in square meters
h - Dielectric thickness in meters

Unfortunately you will have a small error in this calculation because this formula doesn't include the fringing capacitances from the edges. But this method will give you a very good estimate of your dielectric constant anyway.

If your capacitor is circular you will be able to find an approximate closed formula to compute the fringing capacitances. This will give you a more exact value of the capacitance and hence the dielectric constant of your substrate.

Depending on the substrate you will see that the dielectric constant can be dependent on the frequency. This will make the process of finding your er more difficult etc...

Hope it helps.
S.

Re: How to measure the capacitance to get dielectric consta

There is also a device called impedance analyzer which basically is a network analyser with fixed purpose: to measure the impedance.

It basically sends a fixed frequency(which can be varied in steps, depends on the model) and then measures the attenuation of the output signal. You can test your element in either series or parallel resonance modes. It gives out various parameters like ESR, quality factor and ofcourse the capacitance/inductance(at that particular frequency)

If you do have access to a network analyser, then the above mentioned thing can be done manually but you need standard(known) valued inductor to form a resonance network.

Note: Just remembered that the other name for an impedance analyzer is a LCR meter.