Hi,

I was just trying to get into Mathematica a little more but I've been stuck when trying to solve an equation. The H function models a low pass filter and I want to find out the cut off frequency.

Z[\[Omega]_] := 1 /(\[ImaginaryJ]*\[Omega]*c)

H[\[Omega]_] := Z[\[Omega]]/(r + Z[\[Omega]])

c = 1*^-6; r = 1000;

LogLogPlot[Abs[H[2 \[Pi]*f]], {f, 1, 1000000}, ImageSize -> Large, AxesOrigin -> {1, 1*^-3}, GridLines -> {{160}, {}}]

NSolve[Abs[H[2 \[Pi]*f]] == 1/Sqrt[2], f, Reals]

NSolve[Abs[H[2 \[Pi]*f]] <= 1/Sqrt[2], f, Reals]

f = 160; Abs[H[2 \[Pi]*f]] < 1/Sqrt[2]

Plotting the simple diagram worked. Now, when I try to numerically solve this (in)equation, I get this error:

NSolve::nddc: "The system ... contains a nonreal constant -500000\ I. With the domain Reals specified, all constants should be real."

Even though the Abs[] around it should get rid of the i. However, when I "manually" test the inequation, I get back true. What am I doing wrong?

Thanks for any help