Mischstufen

Mischstufen are mixer stages used in radio frequency and signal processing to perform frequency conversion by heterodyning an input signal with a locally generated oscillator signal. In a typical arrangement, one or more Mischstufen convert a received radio frequency (RF) to an intermediate frequency (IF) that can be more easily amplified and filtered in subsequent stages. When multiple Mischstufen are used, they may progressively shift frequencies or adapt to different bands.

The operation of a Mischstufe relies on mixing two signals, producing sum and difference frequencies. The desired output is usually the difference frequency, defined by f_IF = |f_RF − f_LO|, while the sum frequency is typically rejected by filtering. Image frequency concepts describe a potential spurious path where an unwanted signal can also produce the same IF, requiring preselection filtering and good isolation between ports. Practical designs balance conversion gain, noise figure, LO leakage, and port isolation.

Architectures vary: active mixers using transistors or diodes, and passive mixers based on diodes or resistive networks. Double-balanced and ring mixers are common because they offer better LO isolation and image suppression. Mischstufen can be implemented as discrete circuits or integrated into MMICs (monolithic microwave integrated circuits). Modern implementations include Gilbert-cell topologies, which enable broadband operation and good linearity.

In practice, Mischstufen are central to superheterodyne receivers, where one or more stages downconvert RF to IF for selective amplification. They also appear in radar, communications systems, and signal-processing applications. Key performance criteria include conversion gain or loss, noise figure, linearity, isolation between RF, LO, and IF ports, and overall dynamic range.