May 01, 2012, by Volker Schmitt & Joe Ciaudelli
Using radio frequency (RF) wireless microphone transmitters with the right amount of RF output power is important to ensure total system reliability.
There is a common misconception that higher power is better.
However, in many applications high power can aggravate intermodulation (IM) distortion, resulting in audible noises.
First, the applied RF output power must fall within the limit allowed by each country’s legislation. In the U.S., the maximum RF output power for wireless microphones is limited to 250 mW. In most of the countries in Europe this figure is 50 mW, while in Japan it is only 10 mW.
Despite the 10 mW limitation, many multi-channel wireless microphones are operating in Japan. This is achieved by careful attention to factors like antenna position, use of low loss RF cables and RF gain structure of the antenna distribution system.
There are indeed some applications in which more RF output power is an appropriate measure; a perfect example would be a golf tournament; as the wireless system needs to cover a wide area. There are usually only a few wireless microphones in use at this type of function, and those microphones are generally not in close proximity to each other.
If transmitters with high RF power are close together, a process called intermodulation usually occurs. As soon as a non-linear component like a transistor is exposed to two or more Signals with sufficient strength, the non-linear device will be driven into saturation, resulting in harmonics that can interfere with your wanted wireless mic frequencies.
Intermodulation calculation. (click to enlarge)
This kind of effect can be observed in every amplifier, including output amplifiers in a transmitter, antenna boosters and receiver input amplifiers.
At the same time, the RF noise floor in the medium - which in this case is the air - is increased. As a matter of fact, a transmitter in close proximity to another transmitter will not only transmit its own signal, but it will also receive the signal and add this to the RF amplifier stage.
The mathematical calculation of this phenomenon is (with two transmitters):
For the second harmonic:
Frequency 1 + Frequency 2 = Lower 2nd Intermodulation (IM2)
For the third harmonic:
2x Frequency 1 - Frequency 2 = Lower 3rd Intermodulation (IM3)
2x Frequency 2 - Frequency 1 = Upper 3rd Intermodulation (IM3)
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.