Abstract: A method and apparatus for generating an accurate, stable phase shift (b) in a sinusoidal signal employs fast analog multiplication to implement the trigonometric relationship sin(&ohgr;t+b)=sin(&ohgr;t)cos(b)+cos(&ohgr;t)sin(b). Cos(&ohgr;t) is generated by accurately shifting a signal sin(&ohgr;t) through 90° using a delay line, for example. Sin(b) and cos(b) are dc signals generated by digital to analogue conversion, using a demanded phase shift (b) whose sine and cosine are obtained from look-up tables. A controller for controlling a phase shift in an rf cavity is also disclosed and operates on the basis of the same trigonometrical principle. The amplitude of the signals in the rf cavity is also controllable; fast analogue multipliers are again employed to scale the signal amplitude to a nominal fixed value such as 1 volt.