Immunity from Airborne Agents

Any radio system can be disrupted by interference. The end user naturally expects a reasonable degree of interference immunity in the design. In many cases, though, there is no definition of what is reasonable and no agreement on test methods. The interference performance of radio equipment sold to non expert customers can range from downright negligent to surprisingly good. This paper reports on development of standardised and automated test methods for short range telemetry links. Results obtained from several commercially available devices are presented, showing a wide variation in the ability to reject interference. Introduction A description of the performance of a radio system can run to many pages of information, whether it is a specification or a list of measurements. While many of the parameters relate to the normal operation in benign conditions, a surprisingly large proportion are in the list only because they matter when conditions are difficult, ie., when there is unwanted interference. Receiver parameters such as bandwidth, co-channel rejection, adjacent channel rejection, intermodulation performance, intercept points, blocking level and spurious response are routinely banded about, but they only reason they are relevant is that they give us clues as to the behaviour in the presence of interference. While it easy enough to write these parameters into a specification, it is somewhat more difficult to measure them in practice, and even then they do not give us a particularly good picture of the interference immunity of the receiver. This paper describes a way of bringing all these parameters together and showing the interference immunity of a receiver in one simple manner. The necessary measurements can be made manually, but they are well suited to being automated. The Well diagram The well diagram is so called because of its shape. It is the interference level that a receiver can tolerate plotted against frequency. The diagram for an ideal receiver would have the shape in Figure 1.