AKG Acoustics WMS 4000 manual Setting Up Multichannel Systems, Akg Wms Multichannel Technology

AKG WMS MULTICHANNEL TECHNOLOGY

SETTING UP MULTICHANNEL SYSTEMS

HOW TO DEAL WITH INTERMODULATION AND KEEP YOUR FREQUENCIES STRAIGHT

Whenever two or more signals are transmit- ted by a non-ideal system, undesired inter- modulation products will be created, cau- sing distortions (see also WMS 400, p. 31). An ideal system would deliver an output signal that is identical to the input signal over the whole frequency range even at lar- ger amplitudes, and no problems would arise.

In practice, however, ideal systems do not exist, as transistors in particular have only a relatively narrow linear gain range. This is why the transmission of several signals via nonlinear systems, such as transmitters and receivers, will result in unwanted arti-

facts generated by intermodulation. These intermodulation products have to be dealt with somehow in practice.

The order of intermodulation products depends on the nonlinearity of the system response curve; the amplitudes of intermo- dulation products will always grow in pro- portion to the product of the mathematical powers of the fundamental signals genera- ting a given intermodulation product. In reality, third-order intermodulation pro- ducts tend to be particularly troublesome because they rise much more rapidly than the fundamental signal, thus turning into real, i.e., audible noise.

Whenever the frequency of the desired sig- nal coincides with that of an intermodula- tion product the signal will be distorted. Moreover, the intermodulation product may activate the receiver's squelch function if the amplitude of the intermodulation fre- quencies exceeds the squelch threshold.

Obviously, the effective impact of intermo- dulation distortion also depends on the distance between transmitter and receiving antenna. In the case of wireless micropho- nes transmitting on an intermodulation fre- quency, the desired signal is often ruined by intermodulation distortion if you move the transmitter too far away from the receiver.

Ideal and real gain curves of ideal and real amplifiers

High audio input levels may overload the amplifier, so the peaks of the amplified signal are clipped as a result of saturation. The compression characteristic may be des- cribed by a polynomial (i.e., the sum of multiples of powers of a variable X). This polynomial includes all powers, with the odd powers (3, 5, 7, ...) responsible for intermodulation in multichannel systems. Because of its high coefficient, the third power term is especially important which is why third-order intermodulation products are dominant. The reciprocal value of the third-order coefficient defines the IP 3 Intercept (see below), which is the most important parameter for the intermodulation resistance of an RF amplifier. A smaller third-order coefficient of the trans- mission polynomial means a higher IP 3, which implies greater linearity of the RF amplifier and thus better resi- stance to intermodulation distortion.

IP 3 Intercept

The Intercept marks the intersection of the theoretical linear transfer curve for the wanted signal’s amplifier res- ponse curve and the theoretical linear transfer curve for the third-order intermodulation product. It is never actual- ly reached because the amplifier will compress the wanted signal before it reaches the IP 3 Intercept level.

The higher the Intercept of a radio transmission system, the lower the IM risk, and the more channels may be used within a given frequency band.

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