Even most of the common instrumentation (spectrum analyzers or E.M.I. receivers) are equipped with automatic tracking prelectors, for some special applications such field strenght measurements, an external active prelector would be desirable. This may happen when many high amplitude signals are observed and some of  them are very close each other, by generating I.M.D. products (2nd, 3nd order , etc.). This phenomenon is quite known when the measurments are conducted in a real environment full of E.M.I. pollution (mobile cells, broadcastings, microwave links, two way repeaters, etc.).
This issues can be normally avoided by adding a pass band filter or more than one, in order to reject the undesired signals. Often a cascaded low noise amplifier need to recover the filters losses and cable losses, if the preselector is remotely installed. The cascaded low noise must be appropriate selected for high IMD use, to ensure an good linearity in the measure.
In this section we describes outdoor active prelectors with switched filter banks and high IP3 amplifiers. In another section of this website, we describe tunable frequency preselectors (see www modules and special design/ frequency preselectors with tunable filters (YIG). Picture 1 shows a typical schematic architecture with mono filter, while picture 2 shows a schematic block diagram with 4 switched filters.
Please consider this are just examples of some frequency available. We could design preselectors with filter banks up to 40 GHz on demand.

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MIL 1300-1700