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Detectors-Slide17

There is an important sub-class of RF detectors called RMS responding detectors. Non-RMS responding RF detectors which are commonly called log-amps work well in applications where the characteristics of the input signal such as bandwidth and peak to average ratio do not change. If one expects that the signal being measured will be changing from say, QPSK to QAM64 or if this is not the type of signal being transmitted, then it makes sense to choose an RMS responding detector. The plot on the right shows how the transfer function of an RMS-responding detector does not change as the input signal’s modulation changes. It is important to note that there is significant overlap in functionality between RMS-responding and non-RMS responding RF detectors. Apart from their immunity to different signal types, the other main difference in functionality between these devices is that RMS-responding detectors tend to have a slower response time than non-RMS responding devices. It is worth noting that using the term “Log Amp” to describe non-RMS responding RF detectors, while popular, is not strictly correct. In the case of the plot shown here of the transfer function of the AD8362 RMS detector, it can clearly be seen that the transfer function has the same characteristic that is associated with Log Amps. So this RMS detector is also a Log Amp. However, the terms Log Amp and RMS Detector are widely used to distinguish between RMS and non-RMS responding devices.

PTM Published on: 2010-12-10