Intermodulation

Intermodulation (IM or IMD) designated in communications technology and electro-acoustics, the formation of unwanted spectral components by non-linear transfer functions of circuit elements, if at least two different frequencies to be processed.

In communication engineering

Basics

In communication engineering, the intermodulation products caused by non -linearities in components such as diodes, voltage-dependent reaction capacity in transistors or voltage-dependent resistors. These are desired in the additive mixers to produce combination frequencies.

Simplified for memoryless systems ( that is, the history does not matter here ), the non-linear transfer function as

Are described.

In this case, g is an arbitrary non-linear function such as a quadratic function. This causes the superposition theorem is no longer valid because the various input signals affect each other. Mathematically, the intermodulation by a Taylor series describe, if the nonlinearities are not too strong.

Approximation with Taylor series

The larger the coefficients k2 and k3 are, the smaller is the intermodulation 2nd and 3rd order.

The potencies are available for intermodulation 2nd order, 3rd order intermodulation etc. Mathematically, this can be by means of addition theorems described as follows:

And

If the access is Zweitonanregung with the frequencies f1 and f2, as intermodulation products obtained at the output of the circuit part described on the frequencies

Wherein m and n are natural numbers, the sum of which corresponds to the order of intermodulation.

In the signal transmission, the intermodulation 3rd order (or ) are undesirable since they fall within the useful frequency range and difficult or can not be filtered out.

Also 2nd order intermodulation (2 * f1, 2 * f2, f1 f2 and f2 - f1), depending on the system configuration, can cause malfunctions.

Mostly, the intermodulation products are strongly attenuated with the order > 3, or can simply be filtered out by the vast distance to fundamental frequency, so that they do not play a major role in transmission technology.

Measures against the intermodulation are, for example, the use of steps with high quiescent currents in battery-powered devices which is not always possible, however. By reducing the modulation of the receiver or transmitter stages, an improvement is achieved at the expense of sensitivity, or the input of the transmission power.

Measuring IM interference sources

Scalar measurement

The majority of the performances of the intermodulation signals (IM ) signals are measured purely scalar. That is, there are two signal generators as a transmitter and a spectrum analyzer is used as a receiver. The (DUT ) is located between generators and receivers. In contrast to the vectorial case Netzwerkanalysatormessungen no mismatches are eliminated. Furthermore, there is no phase information of the IM signals.

In these measurements must always be taken to ensure that no IM signals from the transmitter and / or receiver are generated. In particular, must be used to prevent the powerful transmit signal generated on the sensitive receiver due to its own non-linearities in signals at the receiver for narrow band-pass filter. Whether a measured IM signal is generated by the DUT or in the receiver, it can be checked on the receiver for the duration of the measurement by increasing the input attenuator. All signal amplitude is reduced by the same factor, the ON - signals from the DUT. Takes place is dependent on the order of the intermodulation signal amplitude change, it is a signal generated by the receiver in the signal.

Vectorial measurement

Alternatively, the scalar measurement can be performed based on a vector, the vector Netzwerkanalysatormessung IM measurement. This usually has to much lower measurement error because mismatches are eliminated automatically with a previous calibration. The key advantage of vector measurement is to model the interference sources. Even the location of the defect can now be calculated. To perform this measurement vector IM, it is necessary absolutely to calibrate the network analyzer. For the two additional calibration standard power sensor and the comb generator are required. The calibration Without Thru helps to calibrate two virtual goals on only one physical target.

In the speaker technology

For speakers, the intermodulation is the typical distortion that is immediately audible when overloaded - the voice coil leaves while the linear region of its deflection.

In multi- way systems, intermodulation can usually be effective only in the frequency ranges that are emitted with the same chassis. (Bass ( frequencies below 100 Hz ) and mid frequencies 500: Since distortions in the range 500 Hz -4 kHz are most audible and intermodulation most by the excessive membrane of low frequencies is triggered to build 3-way speakers Hz -4 kHz) are emitted with separate chassis, the midrange is carefully protected from the air movement of the bass loudspeaker. 3-way systems are therefore clearly superior to one -way and two -way designs in this regard. Nevertheless, the bass speaker a 3- way box must have a wide range of linear displacement, not his part to generate harmonics.