Cross modulation

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Cross modulation is a special form of intermodulation and denotes an impairment of the signal transmission in which two signals to be separated modulate each other due to non-linearities in the common transmission channel between several transmitters and receivers .

Cross modulation can occur when amplifying amplitude modulated signals ( amplitude modulation ). The best way to this at a theoretical example to illustrate: Suppose a high frequency - amplifier are simultaneously - with an information signal - modulated carrier signal s1 (t) and an unmodulated carrier signal s2 (t), respectively. If the amplifier now has a non-linear transmission behavior, which is usually the case at its performance limit, the previously unmodulated carrier signal s2 (t) is "cross" -modulated with the information signal contained in s1 (t). The amplifier behaves more and more like a mixer as the input voltage increases .

In practice this is of course undesirable because the carrier signal s2 (t) sensibly transmits its own information signal which would then be disturbed by the cross modulation. In order to avoid these non-linear distortions, the high-frequency amplifier should only be operated in its linear amplification range.

The so-called cross modulation distance is a measure of the suppression of the cross modulation. This is understood to be the difference between the level of the carrier signal and the level of the “external” information signal impressed on it by the cross modulation.

Cross-modulation also occurs with radio signals in the ionosphere and is known as the Luxemburge effect.

literature

  • HKV Lotsch: Overview of the nonlinear distortion in transistors, including cross modulation. Arch. Electr. Transmission . tape 14 , 1969, p. 204-216 .
    • Ralf Rudersdorfer, with the collaboration of Ulrich Graf and Hans Zahnd: Radio receiver compendium - understanding how it works, areas of application and international assignments, determining and interpreting parameters, optimizing receiving systems . 1st edition. Elektor International Media BV, Aachen 2010, ISBN 978-3-89576-224-6 .