A receiving unit including an operational amplifier having a fixed gain receives an electromagnetic signal, from which a voltage divider with a variable resistance generates an input voltage for the amplifier. The amplifier's output voltage is compared with a reference voltage. The variable input resistance is adjusted to vary the amplifier's input voltage, until the amplifier's output voltage lies within a defined range around the reference voltage. This adjusted resistance value is compared with nominal resistance values previously stored in a value field and respectively allocated to field strength values, to determine a field strength of the received signal. A circuit arrangement includes an operational amplifier, a memory unit connected to a processor unit, a first voltage divider, a second voltage divider, and a control element that controls the second voltage divider to provide the controlled variable input resistance. The processor unit reads the control state of the control element to determine the adjusted input resistance, and therefrom determines the field strength value of the received signal.