<p>A frequency set-on oscillator comprises an input switch (12) an amplifier (22) and a delay line (16). A frequency divider (14) having two outputs is provided in line with the input switch (12). The two outputs of the frequency divider are connected to a mixer (20), one of the outputs containing the delay line (16). The output of the mixer (22) is fed to the amplifier (22) and the output of the amplifier is fed back to the input switch (12) via a bandpass filter (24). The device of the invention is a very simple network cnfiguration capable of sustaining a desired frequency for very much longer periods than has been possible with previous devices.</p>
