Design Article
An Ultra Wideband wireless microphone system for DTV applications
Jackie Green, Audio-Technica
10/3/2007 12:30 PM EDT
Sometimes, the most revolutionary ideas are those that in hindsight appear quite obvious and simple. The technology behind the SpectraPulse ultra wideband wireless microphone system is just that " revolutionary and simple in concept. Consider that something as simple as a miniature switch is part of what was behind the technology revolution of integrated circuits. The engineering skill and effort it took to implement this simple idea was extremely difficult, but this simple idea changed the world. Once it worked, it became ubiquitous.
The Audio-Technica U.S., Inc. SpectraPulse development has been such a project. Many companies have tried to implement ultra wideband (UWB) wireless into products, but we have been the first to succeed in incorporating UWB into the world's first commercially viable audio product.
Ultra wideband wireless is a naturally simple idea. It consists of very rapidly pulsed signals, each one a short burst over a very wide instantaneous bandwidth of frequencies. In early conception dating back to Marconi in the late 1800's, a UWB signal was a spark-gap transmission that was used for long range communications. These early communications systems were replaced by more efficient, longer pulse duration, narrowband techniques.
Short pulse technology was only revisited nearly 80 years later in the mid 1960's. At this time, as technologies, methods and materials allowed engineers to work in much higher frequency ranges, UWB began to emerge as a novel form of communications due to its low probability of detection characteristics. The successful use of UWB by the U.S. government and military led to the FCC's groundbreaking rule-making that permitted unlicensed commercial use of the technology for the very first time. They set the definition of an ultra wideband signal as having an instantaneous bandwidth of at least 500 MHz (where the spectrum is defined as the 10 dB down points from the center of the transmission) " or " a signal with a >20% fractional instantaneous bandwidth. (500 MHz is approximately 20% of a 2.4GHz center frequency signal. 2.4GHz is a popular wireless frequency with many available parts, and many engineers have experience working at this frequency.)
SECURE
The FCC has mandated both peak and average output power limits for unlicensed UWB products in order to ensure that they will not interfere with other wireless and security devices. The UWB signal must be lower than the spectral power density emission limit of - 41.3 dBm/MHz, which is also the limit applied to unintentional radiators in Part 15 of the FCC codes. We, however, are intentionally radiating at these very low levels and must still maintain reliable operation and range. Although higher power has advantages, the method in which the SpectraPulse system works allows us to accomplish extremely reliable and safe communication that is also inherently secure.
The nanosecond duration burst signal is one of the keys to UWB operation. The pulse for the SpectraPulse system is approximately two nanoseconds in duration and occupies a bandwidth of about 500 MHz during the limited transmit duration. The detection of this limited-duration signal is extremely difficult even with expensive test equipment, since these very short pulses are so close to the ambient noise floor. In comparison, traditional FM signals are always on, meaning that the carrier is always present and signals are easily detected and decoded. Traditional FM microphone systems transmit about 50 milliwatts of average power. The SpectraPulse system transmits 40 nanowatts average power. This is approximately one million times lower than an FM system.
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