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Low Intermodulation Loads for Outdoor Applications


Friday, July 11, 2014 by SPINNER

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Loads are essential key components for mobile communications systems. In addition to metrological applications, these components are needed primarily for the termination of directional couplers or splitters if circuits such as hybrid combiners or attenuators are configured. In addition to an adequate thermal power rating, VSWR as well as intermodulation values as low as possible are of special importance for these applications. Whereas insufficient VSWR results in isolation problems in connection with directional couplers, undesired intermodulation products (generated by mixing individual transmit signals) can cause interference and masking of receive signals. Thus, conventional thick-film loads cannot be used in most demanding telecommunications applications due to their unsatisfactory intermodulation behavior.

In order to meet the requirements specified, SPINNER has been offering the well-known BN 534279, BN 534280 and BN 534277 models with the IP20 degree of protection for indoor applications for a long time.

Recently, more and more customers have approached SPINNER with requests for universal applicability under harsh environmental conditions (for example for outdoor applications, applications in tunnel systems or in subway stations), and so the product range has now been supplemented by five models which are briefly presented below. All models are based on a 50O basic module containing the actual load and, depending on the requirement, are combined with individual or multiple different heatsinks. The robust housing consists of aluminum alloy resistant to sea water and has an IP67 degree of protection. In order to achieve stable and optimum intermodulation values, the connection was designed as a corrosion-resistant plated 7-16 coupler.

Many applications, for example the combination of up to four input signals using a hybrid coupler matrix, involve the use of several individual loads. If only a single coupler output is to be used, three loads are required for the termination of the remaining outputs. On the other hand, multiple attenuators are often needed for the connection of repeater systems to the outputs of base stations, consisting of tappers and loads. For these applications, the BN 534268 model containing three identical individual modules in a joint heatsink is available. The compact three-module design allows a relatively clear mechanical structure of the specified system. Thanks to the larger heatsink, each of the three inputs can be loaded with a maximum power rating of 150 W. All other electrical characteristic values are identical to those of the individual modules described above. This also applies to the IP67 degree of protection.

One version of this three-module load is BN 534267. In general, this design also consists of three absorber modules which are mounted on a joint heatsink and, in this case, are interconnected by means of a power splitter. With this combination, a high-performance load with IP67 degree of protection can be achieved, which can be permanently loaded on the input with a power rating of up to 400 watts. Thus, this model is particularly suited for all applications requiring a heavy-duty measurement absorber with optimum intermodulation values. Examples of further applications include the termination of unused coupler outputs or the attenuation of combined high power sum signals by means of directional couplers or tappers and loads.

While this model also comes up with a guaranteed VSWR of less than 1.15, this distributed arrangement results in guaranteed intermodulation values of better than -160 dBc over the entire frequency band (IM3 at 2x 43 dBm). Both models (BN 534267 and BN 534268) are equipped with mounting brackets for easy wall mounting.