Frequency conversion between baseband and a radio-frequency radio channel is normally required both in mobile radio transmitters and in mobile radio receivers. A frequency mixer is used in homodyne transmitting arrangements, for example, to convert a baseband modulation signal to a radio frequency in the Gigahertz band. A carrier frequency which corresponds to the desired transmission frequency is required for this purpose.
The carrier frequency is normally provided by a voltage-controlled oscillator which does not, however, oscillate at the transmission frequency but rather at a frequency which is different than the transmission frequency. Said frequency is selected in such a manner that it can be converted to the transmission frequency with as little effort as possible. Selecting an oscillation frequency other than the transmission frequency diminishes the effects of the transmission frequency on the oscillator. The performance of the voltage-controlled oscillator may additionally be improved if the oscillator frequency is selected to be lower than the transmission frequency. The oscillator may oscillate, for example, at half the transmission frequency in order to ensure a low power consumption and good phase noise characteristics. A frequency-doubling circuit which converts the oscillator frequency to the desired carrier frequency—corresponding, in this example, to the transmission frequency—is therefore required.
Similarly, a frequency-doubling circuit may also be used, in receiving arrangements for converting a received radio frequency to baseband or an intermediate frequency, to provide a radio-frequency so-called local oscillator signal from an oscillator signal.
The frequency-doubling operation described is normally effected by means of a radio-frequency mixer having two inputs which are both connected to the oscillator and square the oscillator signal at the output of the mixer. For this purpose, the oscillator signal is split, by means of a phase shifter, into two signal components (which are orthogonal to one another) and is up-mixed to the desired transmission frequency. An output signal at twice the oscillator frequency is then available at the output of the mixer on account of the signal-squaring operation described. In addition to a radio-frequency mixer, the known frequency-doubling circuit therefore requires a phase shifter for producing orthogonal signal components.
The document JP 10-004683 A specifies a rectifier circuit for processing a single-ended signal. An operational amplifier which forms the output of the circuit is provided. The output is fed back to the input connection via resistors. In addition, an external reference signal is supplied.