I have tested my dielectric sample using LCR meter in Cp-Rp mode for different temperatures. At particular temperature, as the frequency increases, some of the values are negative. what could be the reason for this?
Does these negative capacity measurement at high frequency correspond to the temperature with the higher capacity measurments at low frequency? If it's the case, it could be effect of series parasitics inductuctances on the tested sample or on the measurement circuit.
Thankyou Jeremie Chastagnier for your answer.
The values of negative capacitance varied. For e.g. in one case, if the temperature is low, the value of capacitance is negative for higher frequency. While on the other hand, if the temperature is high, the value of capacitance is negative for lower frequency.
It could be the effect of series parasitic inductances? If yes, Do I need to take absolute values in case of ac conductivity calculations?
In the case there is a parasitic series inductance coming from the measurement circuit or from on the tested device, when you pass the series resonance frequency the measured load become inductive. That can be a reason why RLC metter in cp - rp mode give negative values of capacitance.
I expect that this parasitic series inductance is not temperature dependant (unless you use ferromagnetic materials). So for the temperature which give the highest capacitance at low frequency measurement, I expect lower series resonance frequency, leading to "negative" capacitance measurement at lower frequency. In order to confirm this series parasitic inductuctance assumption, you could control that your capacitance measurement at low frequency increase with the temperature.
If it's the case, you should calculate the parasitic inductuctance in order to obtain the real capacitance value. The measured capacitance far below resonance frequency are correct, but the measured values at resonance and above need a correction.
The parasitic inductance value can be calculate from the resonance frequency (capacitance sign change) and low frequency capacitance measurement. For dielectric with intrinsic frequency dependent permittivity it could be little more difficult.
Jeremie Chastagnier Thankyou so much for your answer.
If you can give me some reference to this note it will be very helpful.
Also, I am still confused how can I calculate the ac conductivity of my sample ? According to the equation
σac = ωεoε”, where εo = free space permittivity and ε”= imaginary part of dielectric constant
σac = (2*3.14*f) εo ε’ Tanδ, where ε’= real part of dielectric constant and Tanδ=dissipation factor
as (ε”/ε’ = tanδ) and ε’=CpL/ εo a
σac = (2*3.14*f) (CpL/a) Tanδ
So in this case, Do I need to take absolute values of Cp. If not then how ?
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