A system is described which produces a 90° phase shift for a wide range of single-frequency input signals. Thus, whilst it will not handle wideband input signals, it may be useful for such applications as N-path and quadrature modulation filters where the provision of variable-frequency quadrature carrier signals allows the position of the passband to be continuously altered. In practice the phase shift through the system can be maintained at 90° ±0.15° for a 10: 1 range of signal frequencies.