The present invention concerns an electronic mixer for generating a mixed signal by mixing a local oscillator signal with a useful signal comprising at least one field effect transistor which has at least one gate, at least one source and at least one drain, a useful signal input for feeding in the useful signal with a useful frequency, a local oscillator signal input for feeding in the local oscillator signal, which is so designed that in operation of the mixer it receives a local oscillator signal whose local oscillator frequency is equal to an integral fraction of the useful frequency which is reduced or increased by a mixing frequency, and a signal output at which the mixed signal is present in operation of the mixer.
The present invention further concerns a method of generating a mixed signal by mixing a local oscillator signal with a useful signal in at least one field effect transistor having at least one gate, at least one source and at least one drain, comprising the step of generating the local oscillator signal with a local oscillator frequency, wherein the local oscillator frequency is equal to an integral fraction of the useful frequency which is reduced or increased by the mixing frequency.
For efficiently acquiring electromagnetic signals beat receivers, in particular heterodyne receivers, have long been used, in which the electromagnetic signal to be acquired, which is referred to hereinafter as the useful signal, is mixed with a signal which is generated locally, that is to say at or in the mixer itself, which hereinafter is referred to as the local oscillator signal. The mixed signal then has a mixing frequency which is equal to the differential frequency between the frequency of the useful signal, this is referred to hereinafter as the useful frequency, and the frequency of the local oscillator signal, this is referred to hereinafter as the local oscillator frequency. The amplitude of the mixed signal is a measurement in respect of the amplitude of the useful signal. The signal identified as the mixed signal with the mixing frequency in the present application as the output of the mixer is frequently also referred to in the literature as the intermediate frequency signal with the intermediate frequency.
The terahertz frequency range or submillimeter wavelength range which is roughly defined by between 100 gigahertz (GHz) and 10 terahertz (THz) is one of the last ‘dark’ areas of the electromagnetic spectrum.
Technically usable detectors are not commercially available in that frequency range or are commercially available only at low frequencies. In particular the mixing efficiency of field effect transistors which are frequently used for mixing a useful signal with a local oscillator signal drops off severely towards higher useful frequencies.
In that respect however the efficiency of the mixers also depends on the available power of the local oscillator signal. Powerful local oscillator sources are not available at high terahertz frequencies, or are so available only at a high level of technical and financial complication and expenditure. Therefore it is known from the state of the art to use a local oscillator which generates a local oscillator signal whose local oscillator frequency is equal to an integral fraction of the useful frequency increased or reduced by the mixing frequency. Such a mixing method is referred to as subharmonic mixing.
In subharmonic mixing, the non-linear properties of the mixer component, thus also in the case of the field effect transistor, are put to use. A mixed product is generated with the difference of an integral multiple of the local oscillator frequency and the useful frequency.
It will be noted however that the efficiency of subharmonic mixing is markedly lower than the mixing efficiency upon coupling the fundamentals involving the same power into the mixer element, in particular the field effect transistor.