AKG Acoustics WMS 4000 manual Microphone a carrier frequency, Spectrum analyzer trace, Frequencies

AKG WMS MULTICHANNEL TECHNOLOGY

Microphone A carrier frequency	Intermodulation of carrier frequencies A and B	Microphone B carrier frequency
Spectrum analyzer trace	The trace clearly shows that third-order intermodulation	Spectrum analyzer trace
	products are only 38 dB lower in level than the carrier
	frequencies.

Intermodulation trace on a spectrum analyzer display Third-order intermodulation products from three carrier frequencies.

The received signal level declines in pro- portion to the square of the distance bet- ween transmitter and receiver, and the intermodulation level produced in the receiver declines in proportion to the third power of the received signal level. This implies that intermodulation declines expo- nentially, in proportion to the sixth power of the distance between transmitter and recei- ver. If the distance is longer than 66 feet (20 m), receiver intermodulation is drow- ned out by noise. What remains is another important type of intermodulation distor- tion that has not yet been mentioned: trans- mitter intermodulation. In this case, the intermodulation products are not generated in the receiver, but in the transmitters, and are radiated by them along with the desired

carrier frequencies. This will only happen, however, if there is enough crosstalk of car- rier frequencies between two neighboring transmitters that intermodulate with each other. In this case, the antenna of one transmitter receives the carrier signal of a neighboring transmitter. If this signal makes it into the non-linear output stage of the transmitter, the first harmonic of the desired signal will transform it into a signal whose frequency is indistinguishable from the receiver intermodulation. The same happens in the other transmitter that will generate a mirror-image intermodulation product. Curiously, a love duet close to the receiving antennas may lead to intermodu- lation distortion caused by the nonlinear receiver. If the two singers move away, the

intermodulation remains unchanged, but is now produced in the transmitters. In large multichannel systems, reducing the RF output of the transmitters is a way to bring down transmitter intermodulation by mini- mizing the nonlinear response of the trans- mitter's output stage. The RF output of WMS 4000 transmitters, for example, can be reduced from 50 mW to 10 mW (ERP –Equivalent Radiated Power).

Here are some hints on how to minimize receiver intermodulation:

Always lay out the antenna system so as to ensure reliable transmission from every point on the stage. Moreover, be sure to use only the types of cables recommended in the user’s manual. The distance between

Intermodulation trace on a spectrum analyzer display Intermodulation products from 2 carrier frequencies, 3rd-order IMD and 5th-order IMD.

transmitters and active antennas should be at least 15 feet (5 m) (see also Antenna Position Check Applet on p. 45). Increasing the input attenuation of the antenna system helps, as does reducing the transmitter RF output to 10 mW. The latter has proved particularly useful for hand-held transmitters in situations where range is not an important consideration; generally, the RF output level used should always be just high enough to ensure adequate trans- mission. Systems with a higher transmitter RF output (ERP) are more prone to inter- modulation problems, but this is compen- sated for by their better resistance to elec- trosmog.

When multichannel systems are used on Broadway, for instance, only the strongest will survive.

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