Abstract: Jamming arrangements comprising an oscillator including a number of circuits each comprising a surface acoustic wave filter feeding an amplifier responsive only to signals above a predetermined threshold level, the filter characteristics being chosen so that each circuit is responsive to a different range of frequencies, the circuits being connected mutually in parallel and a switch being provided via which the output circuits of the terminals are coupled to the input terminals to define a feedback loop when the switch is closed, the arrangement being such that when the switch is closed and a signal to be jammed is injected into the loop at a frequency within the pass band of at least one of the filters so as to produce a signal which is fed to its associated amplifier above the threshold, oscillation is maintained around the loop at the frequency of the signal to be jammed.