A frequency-independent phase shifter
Mohammad Madihian, Keita Watanabe, T Yamamoto · Journal of Physics E Scientific Instruments · 1979
The circuit described gives the frequency-independent phase shift for a band-limited signal, while keeping its amplitude invariant. The key component is a field-effect transistor which, driven by a frequency discriminator, provides a resistance which is inversely proportional to the frequency. A prototype circuit produces a phase shift flat to within +or-2 degrees over the frequency range from 20 to 40 kHz.