Useful Wireless Test System Architectures Using Programmable Step Attenuators
By Brett Chermansky, Chief Engineer, JFW Industries

There are two types of commonly used test system architectures that use programmable step attenuators. The first type is the “hand-over” system. It is designed to test cellular handsets in a controlled lab environment. It allows cellular network providers a way of evaluating newly designed handsets on their existing networks. It is also used to verify the performance of new firmware and software revisions for base stations. Programmable attenuators are used to simulate real world signal loss due to distance, trees, walls and other factors. Since the connections from the base-station to the phones are cabled, no RF is radiated into the air, minimizing interference with other equipment in the lab. This allows for several hand-over systems to occupy the same lab and several evaluations to be performed simultaneously.

The hand-over system is constructed with a power divider connected to each input port. This input divider fans an input to all of the outputs in the system (See Figure 1). In practice, this allows one sector of a base-station, to be available to multiple test phones simultaneously and at different power levels. This architecture is also used on the output ports. The power dividers on each output port combine the input signals, attenuated at different levels, to a single output. This allows one test phone to see signals from different cell sites simultaneously and allow it to perform the hand-over. Using 0-127dB x 1dB step programmable attenuators allows the user maximum flexibility when adjusting signal levels. With attenuators set to 127dB the connection between cell sites and mobiles is completely broken. The use of solid-state programmable step attenuators in the hand-over system gives two distinct benefits. The first is that there is no drop-out of signal when changing dB value as you would see in a typical electro-mechanical programmable attenuator. The second benefit is that the unit is not limited to a certain number of cycles as with electro-mechanical units. This is extremely important in automated testing environments where some tests require the unit to perform hand-overs continuously for extended periods of time.

The second popular type of test system using programmable step attenuator is the transceiver test system. The construction of a transceiver test system is very similar to the hand-over test system in the fact that it is built with step attenuators, power dividers and it allows several devices to be hooked up to each other in an interference free environment. The main difference is that the transceiver test system does not have dedicated input and output ports. This configuration provides the user maximum flexibility because it makes each device visible to every other device connected to the system (see Figure 2). As in the hand-over system, the transceiver test system uses in-line programmable step attenuators to allow for the adjustment of signal levels between each transceiver pairs. The transceiver test system can be used to simulate a virtual battlefield or working environment where several radios in the field all need to communicate with each other. The speed of the step attenuators allow the user the ability to simulate radios moving around within the environment by increasing or decreasing the dB value proportional to the rate of a moving vehicle.

While there are many uses for programmable step attenuators in modern RF communications systems and testing applications, the hand-over system and transceiver test systems are pushing to the forefront as the dominant topologies using these attenuators. These types of systems have been built for years within cellular network companies for their own use in many different configurations. It is only recently that companies are providing full turn-key solutions to these testing needs.

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