I have found a technique detailed on page 34 & 35 of this thesis.
In this, the author states a complex bandpass filter can be derived by first making a prototype real valued IIR filter, then setting the complex conjugate poles equal to each other. He then states the resulting complex filter will have a quality factor half that of the real valued one but the same centre frequency. The resulting filter is then more efficient than a combination of bandpass filter + Hilbert transform.
I can find no reference to this technique anywhere! Can anyone point me towards a reference explaining the mathematical concepts of this or an example of it's implementation so I can understand it better? How would you rate this technique compared to a bandpass+hilbert transform, or is there an even better way? I am only looking to achieve a quadrature signal for further processing, in this case frequency shifting.