The Digital Wireless Advantage – Interference and Intermodulation in Multi-Channel Systems

The A10 Digital Wireless System has a significant advantage over analogue FM wireless and other digital wireless systems in its ability to reject interference. This arises because of three main reasons.

  1. The A10 uses two cascaded SAW filters. These precision RF filters are excellent at rejecting out-of-band signals. The SAW filters used in the A10 Digital Wireless System can closely match the signal bandwidth of the system because the A10 topology is tolerance of the group-delay-distortion caused by these filters. FM wireless systems generally require wider filters than would be ideal for interference rejection because of their susceptibility to group delay and other effects.
  2. The A10 Digital Wireless System can operate in lower signal-to-noise and signal-to-interference environments than an FM wireless system. For some types of interference, this difference can be up to 10 dB or more.
  3. Perhaps most significantly, a digital system operates perfectly up to the point of signal loss, when it then mutes. When a digital system is working, it works with no signal degradation. FM systems, on the other hand, degrade gradually, from either noise or interference, and operate with audible artifacts present when in a degraded state. While this may be acceptable in a communications-only wireless application, it is not acceptable for critical production applications. Many forms of interference affect the recovered audio in a particularly objectionable way. This means that analogue systems require a much higher signal-to-interference ratio for acceptable operation than a well designed digital system.

What does this mean for the user? The most obvious advantage is that a digital system can successfully operate on or near a frequency that is exhibiting RF energy (whether noise, carrier, etc.) from an unrelated system in closer proximity than an analogue system. But even more important is its immunity from various non-linearities and spurious signals is greatly enhanced.

These spurious signals arise from imperfections in the transmitter and receiver design. For example, a transmitter placed close to a second transmitter will, to some extent, act as a mixer. The two signals will combine and result in a distorted "wanted" signal and radiation of "unwanted" signals on other frequencies. This is intermodulation. Multiple signals can also mix in the receiver. Depending on the receiver design, interference may present at frequencies when there really isn't any. A great deal of skill in designing transmitters and receivers is in minimising these effects, but they can't be eliminated completely.

The result is that, for analogue systems, it is necessary to publish tables (or a spreadsheet, or computer program) showing allowable combinations of frequencies. The more systems in simultaneous use, the more frequencies that become unavailable because of unwanted spurious signals.

Digital systems like the A10 have no consequential intermodulation and thus no IM products with which to contend. Because of it's much greater tolerance of interference, the A10 Digital Wireless System is immune from this problem. The interference is still there, but it has no effect on the transmitted audio. This results in more efficient use of available spectrum and greatly simplifies the deployment of multiple channel systems. A good approach to choosing frequencies for such a deployment is to simply spread them evenly amongst those available.

In summary, the A10 Digital Wireless System exhibits both enhanced performance compared with analogue FM systems, and is also much simpler to deploy in multi-channel environments.

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