Direct-conversion receiver
The direct conversion receiver, or homodyne receiver, a superheterodyne receiver with an oscillator frequency which corresponds to the receiving frequency ω. Technically, it is a synchronous or lock-in amplifier.
The mixture with the oscillator frequency will result in the low-frequency signal in this case. The gain and the selection (see selectivity ) of the desired signal carried in the LF amplifier. There is not an IF amplifier. The mirror spectra in this reception equipment, in contrast to the superheterodyne receiver, is avoided by the formation of the complex envelope. The formation of the complex results in the complex envelope signals that have from one real and one imaginary part, which are also referred to as I and Q component in the base band.
Generally, in this receiver structure, the high-frequency reception signal subjected to a Hilbert transform, and so formed, an analytical signal. This is multiplied by the complex carrier frequency of e - jωt, so as to obtain directly the complex baseband signal. Alternatively, and this equivalent, the received signal with the complex carrier frequency can also first be multiplied, and subsequently, the baseband signal in the form of two exact same low-pass filtering is obtained for the real and imaginary parts.
Direct conversion transceiver structures are in digital broadcasting in the field of digital signal processing, the vast receiver structure whereas, in particular digital modulation technique with high spectral efficiency as the quadrature amplitude modulation, this receiver structure common.
In classic analog receiver circuits, however, the homodyne receiver plays only a minor role, since it is difficult to achieve due to the limited accuracy of circuitry electronic components and circuitry is complex. In particular, the realization of analog Hilbert transformers or of precisely matched lowpass filters limit the practical usage of the application. Adapted analog direct conversion receiver were partially used in amateur radio.