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Acoustics / Audio
September 2007
RF OUTPUT POWER:
Choose the Right Amount
By Joe Ciaudelli & Volker Schmitt
Using radio frequency (RF) wires mic 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.
Transmitter power is a rating of its potential RF signal strength. This specification is measured at the antenna output. The actual transmitted power is influenced by the efficiency of the antenna. Therefore, power specifications are of only limited use in assessing a transmitter’s range, considering largely variable antenna and reception conditions. Also, battery life is associated with RF output power. Increased power will reduce battery life with only a moderate increase in range. Transmitters could be designed to house additional batteries. However, in a body pack transmitter this defeats the purpose in providing a small, unobtrusive device.
The applied RF output power must fall within the limit allowed by each country’s legislation. In the US, the maximum RF output power for wireless microphones is limited to 250mW. In most of the countries in Europe, this figure is 50mW, while in Japan it is only 10mW.
Despite the 10mW 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. 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 5th order. For large multichannel 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: F1 and F2
Second Order: 2F1, 2F2, F1±F2, F2-F1
Third Order: 3F1, 3F2, 2F1±F2, 2F2±F1
Fourth Order: 4F1, 4F2, 2F1±2F2, 2F2±2F1
Fifth Order: 5F1, 5F2, 3F1±2F2, 3F2±2F1
Additional higher orders....
For multi-channel applications such as those on Broadway (i.e. 30+ channels), the intermodulation products can increase significantly and the calculation of intermodulation-free frequencies can be done by special software. By looking only at the third harmonic distortion in a multi-channel system, the number of third order IM-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 IM-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.
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.
Once again, intermodulation can also occur in antenna boosters and in the first stage (front end) of the receiver if they are exposed to RF levels of sufficient strength. A quality receiver only needs a moderate level to achieve full “quieting”, providing optimum signal-to-noise ratio. You want the RF captured by your receiver to be in the window between the minimum field strength required for full quieting and the point where intermodulation begins to occur. To prevent the receivers from getting unacceptably high input levels, the receiving antenna must be installed at a minimum distance to the transmitters. Figure 4 shows what happens if a 30 mW bodypack transmitter range comes close to a receiver antenna. The receiving antennas should be positioned at a minimum distance of about 15 feet from the transmitters. This condition is of high importance for good operation of large multichannel systems.
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