Frequency Selection for Wireless Microphones

Frequency coordination is essential to the proper function of wireless systems. Not only must channels be sufficiently spaced apart, but they must also be checked to ensure that they will not interfere with each other if the RF signals become distorted. This phenomenon, commonly referred to as \emph{Intermodulation distortion}, occurs when signals pass through nonlinear devices such as amplifiers and receiver circuits. Finally, channels must be selected to avoid television stations (both analog and digital), and also two-way radio systems operating in certain cities.

RF and Audio Signals Compared

It is worth noting that RF signals are just like audio signals. They are waveforms that oscillate up and down in much the same way that an audio signal does. The primary difference is that because of their extremely high frequency, additional phenomena occur such as reflections. However, just like an audio signal, an RF signal can clip and distort, and this can can cause all kinds of problems.

The best way to avoid this problem is to ensure that all signals entering an RF amplifier or receiver are approximately the same power. This can be done by using transmitters of the same power rating at roughly the same distance from the receiver antenna. Additionally, it is important to select receivers that don’t overlap with nearby, very high power transmitters, such as television stations. FInally, this problem can be avoided by using systems with a limited bandwidth, or by adding filters to the antenna input to reject undesired signals.

Channel Spacing

Wireless microphones operating in the UHF band have an FCC-mandated maximum channel deviation of +/-75 kHz. While many systems use a much lower deviation in the range of 30-40 kHz, it is important to space channels sufficiently. It is best that channels be spaced by over 1 MHz where possible, but the minimum spacing should be no less than 600 kHz.

If two channels have spacing less than 500 kHz, then it is possible that the filters in the receivers will be unable to discriminate between the two transmitters. In this case, the result will be occasional popping and possibly bleedover from one microphone to the other receiver. This is the exact same effect that occurs when one is traveling, and they hear another station cutting into their FM radio. Generally speaking, more expensive wireless systems have tighter filters and can reject interference better.

Intermodulation Interference

Intermodulation occurs when two or more RF signals pass through a device, such as an amplifier or a mixer, and harmonics are generated. In a practical wireless system, the most likely place for intermodulation interference is an active antenna splitter or an antenna amplifier (or booster). During normal operation, these devices do not cause intermodulation – but if one or more signals are too strong, they clip and the amplifier generates harmonics–just like a microphone preamplifier does when it is overloaded.

The resulting interference generated by such a device is predictable, and it is possible to avoid its effects in many cases. Generally speaking, amplifiers distort according to a power law – that is, the square, cube, and so forth appear at the output of the amplifier in addition to the original signal. In most amplifiers, the odd powers are much stronger than the even ones. With this in mind, we can predict with a fair amount of accuracy what products \emph{may} appear at the output of the antenna amplifier.

There are a number of programs that have been written to predict these products. The most popular program at the moment was written by Professional Wireless Systems, and is called the Intermodulation Analysis SYstem. In addition to making these calculations (as well as adding ‘secret sauce’), IAS can also download the FCC’s television database and cross-reference it with a location to generate an accurate picture of the RF landscape. IAS is very expensive, however.

There are free programs available as well. Sennheiser’s SIFM program performs much the same calculations as IAS, but it requires the user to know what television channels are strong at a specific location.

Television and Broadcast Interference

Wireless microphones operate in the television broadcast bands. In the United States, there are two types of television broadcasts–analog and digital. While the vast majority of analog transmission ceased on June 12, 2009, a number of low-power television stations continue to broadcast in analog. Therefore, it is important to understand how analog stations will affect wireless operation, as well as digital stations.

White Space Devices

In the fall of 2008, the Federal Communications Commission approved the use of low-power, personal communications devices that will operate in the television braodcast bands. These devices, called “white space devices,” will use the same radio spectrum that is currently used for wireless microphone operation.

These devices are required to use two separate forms of protection to ensurt ehtat they do not interfere with broadcast television stations or with wireless microphones. FIrst, these devices must cross-reference their location with a database of licensed users of the spectrum. Second, these devices must use spectrum-sensing technology to determine if a television channel is occupied prior to transmitting and potentially causing harmful interference. In addtion to these requirements, portable devives (such as the ones patrons might bring into a theatre) will transmit with a maximum power of 100 mW – roughly the smae power as a wireless microphone. However, unlike a wireless microphone, which confines its power to approximately 150 kHz, this 100 mW will be spread out over 6 MHz. Over the 150 kHz window that a wireless microphone uses, this noise will be 16 dB lower than the wireless microphone signal. In conclusion, this means that these white space devices are unlikely to cause interference unless they are very close to the recieve antennas.

2 Comments

  1. samson isebhore says:

    i ve a problem with with my wireless microphone, the problem is that any time am using it my neighbour turn on his own, anything speech he makes, interfere with my own. I need a solution on how to solve this problem.

  2. Mike says:

    I recommend that you change the channel your wireless microphone operates on.

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