Even order IM products are far above the wanted signals and do not affect the performance.
However, odd order intermodulation products need to be considered. Luckily, the amplitude of IM products decreases for higher orders.
Therefore, you can generally ignore IM products above the 5rh order.
For large multi-channel systems where high quality equipment is used, often you can consider only IM3 products.
The following signals may be present at the output of a non-linear stage:
Fundamentals: Fl and F2
Second Order: 2Fl, 2F2, Fl±F2, F2-Fl
Third Order: 3Fl, 3F2, 2Fl+F2, 2F2±Fl
Fourth Order: 4Fl, 4F2, 2Fl+2F2, 2F2+2Fl
Fifth Order: 5Fl, 5F2, 3Fl+2F2, 3F2+2Fl
Additional higher orders….
For multi-channel applications such as those on Broadway (i.e., 30-plus channels), the intermodulation products can increase significantly and the calculation of intermodulation-free frequencies can be done by special software.
Intermodulation of third and fourth order produced by two transmitters. (click to enlarge)
By looking only at the third harmonic distortion in a multichannel system, the number of third order 1M-products generated by multiple channels is:
2 channels result in 2,
3 channels result in 9,
4 channels result in 24,
5 channels result in 50,
6 channels result in 90,
7 channels result in 147,
8 channels result in 225,
32 channels result in 15,872
Third Order 1M-products
As a result, the intermodulation frequencies should not be used, as those frequencies are virtual transmitters. The fundamental rule “never use two transmitters on the same frequency,” is valid in this case.
The RF level and the proximity define the level of the intermodulation product. If two transmitters are close, the possibility of intermodulation will increase significantly.
Intermodulation level of two transmitters with different distances. (click to enlarge)
As soon as the distance between two transmitters is increased, the resulting intermodulation product decreases significantly. By taking this into consideration, the physical distance between two or more transmitters is important.
If a performer needs to wear two bodypack transmitters, it is recommended to use two different frequency ranges and to wear one so that the antenna is pointing up and the other is pointing down.
Volker Schmitt is a senior engineer for Sennheiser US, and Joe Ciaudelli also works with Sennheiser US and has a history of providing frequency coordination for large multi-channel wireless microphone systems used on Broadway and by broadcast networks.