Abstract: The circuit arrangement described measures the time when a frequency F.sub.o, changing towards a desired frequency value, differs by not more than a predetermined amount (e.g. 100 Hz) from the desired value. Initially F.sub.o is mixed with the sum of 10 MHz and the new desired value for F.sub.o. The lower sideband is selected and thus produces a frequency F.sub.1 which is thus always 10 MHz when F.sub.o reaches its desired value (which may be variable). The circuit therefore has to measure when F.sub.1 comes within 100 Hz of 10 MHz. Initially F.sub.1 is multiplied by a predetermined multiplication factor (.times.100) and the product subtracted from a reference frequency. The latter is the sum of a fixed frequency of 300 KHz and of 10 GHz (i.e. 10 MHz multiplied by 100). This therefore produces an output frequency F.sub.2 whose value differs from 300 KHz by 100 Hz.times.100 (or 10 KHz) when F.sub.1 differs from 10 MHz by 100 Hz.