Engineering a voltage signal with an asymmetric power spectral density
First think about this picture: -



This is Euler's equation and is all about single rotating vector (\$e^{ix}\$) that moves in and out of both real and imaginery planes. Next consider what a sine or cosine is: -



In other words each are composed of a positive rotating vector and a negative rotating vector. However, as a sum of rotating vectors they only occupy a fixed position - the two rotating parts that make either are contra rotating and this makes sine or cosine fixed and free from rotation.

Here is what they look like: -



Cos(x) exists on the real axis and iSin(x) exists on the imaginery axis.

I think what you are asking for is a method of creating an imaginery voltage like the path taken by the red line: -