# Two port impedance, negative inductance?

Hi
I'm doing simulations in preparation for experimental measurements of decoupling capacitor parasitic inductance. I've done simulations of two port measurements, in mentors hyperlynx, of parallel capacitors on small power planes. These have given me s2p files that I've converted to normalized z-parameters in matlab.
So, I want to extract inductance from the complex impedance. To do this I use
$L = \frac{\Im(z12)}{2\pi f}$
Taking a measurement in f=1.326e8Hz, where inductance dominates, gives me L = -0.3982/(2*pi*1.326e8)=-4.78e-10H. So, negative inductance! What am I missing here? Figure 1 and 2 shows |z| and re(z) and im(z).

#### Attachments

• z.png
2.1 KB · Views: 444
• z-ir.png
1.9 KB · Views: 450