a quadrature down converter which is also supplied with the output signal of the radio frequency generator 5 via the power divider 7 in addition to being supplied the received signal the base unit 1 also comprises a central clock generator 23 which can have for example a clock crystal having a resonant frequency ft of 32 768 khz the output signal of the central clock generator 23 is supplied on the one hand to a phase locked loop 25 which generates at its output the clock signal for the central evaluating and control unit 3 the clock signal can have a frequency of for example 4 u2013 10 mhz on the other hand the output signal of the central clock generator 23 is supplied to a divider 27 which can be constructed for example as a counter and divides down the output signal of the central clock generator at a particular ratio the output signal of the divider 27 is supplied to a counter 29 the information of which in turn is transmitted to the central evaluating and control unit 3 in this arrangement the divider 27 can be preferably adjusted with respect to the divider ratio by the evaluating and control unit 3 for example the divider 27 can be constructed as a loadable counter wherein the number of counting steps per counting cycle can be determined by the evaluating and control unit 3 in this manner the periodic output signal of the divider 27 can be set with an accuracy of 1 ft.