SMD Capacitor's ESR measurement problem with BODE100 Vector Network Analyzer

Thread Starter

Ander De Marcos

Joined Oct 17, 2019
Hi everyone,
My name is Ander and I have one question about a SMD capacitor's ESR or serial resistance. I'm using BODE 100 vector network analyzer and here there are the results:
Red color: Capacitors Impedance in OHMs in a logaritmic scale. It's correct because descreasing until the self resosnant frequency. Later it increases.
Green color: Phase in degrees(º). It makes sense because until the self resonant frequency it is a capacitor (-90º) and then it gets an inductive behaviour.
Blue color: Real part of the capacitors impedance (resistance) in OHMs. It has not sense because in low frequencies it is negative. Later, it makes sense that the at higher frequency, the higher resistenace it has due to the skin effect.

My questios are about the Real part of the impedance ( the resistive part, blue curve). Why have I measured negatives values, does it make any sense? Values after 10MHz are reliable? how can I measure it?