Tower-mounted amplifiers (TMAs) are a cost-effective alternative for wireless carriers that want their base stations to receive signals more clearly in a wider coverage area and reduce transmitted power requirements. TMAs also have presented component manufacturers with tremendous business opportunities. For a major RF and microwave component manufacturer in Europe, however, the anticipation of increased business was tempered with the realization that reducing TMA test time posed quite a problem.
Analyzing integrated products such as TMAs, which combine a low-noise amplifier (LNA) and bandpass filters, is difficult because the multiple elements of the product require different tests to assure their performance. The European manufacturer was looking for a test system that could accurately measure S-parameter, out-of-band attenuation, intermodulation distortion (IMD), and noise figure (Table 1). The company also wanted this capability without manually changing any connections during the entire tuning and testing process.
Developing such a test system in-house proved to be too much of a burden on the company’s resources. Partnering with a test-equipment manufacturer would be more advantageous because it would allow the TMA manufacturer to use internal resources more efficiently, minimize time-consuming and expensive software development, and reduce the time to market by improving time-to-volume capabilities. A single supplier also would make procurement simpler and improve operator training.
Conducting analysis of TMAs usually is a two-step process: a tuning stage and a final testing stage. The TMA manufacturer wanted this custom test station on the manufacturing line to conduct both stages without disconnecting the device under test (DUT). By doing so, tuning and final test time would be decreased and calibration simplified.
The manufacturer also wanted the test system to be a scalable solution so stations could be added when production increased. This type of solution also could be upgraded to test future TMA-like products.
Armed with such definitive guidelines, the TMA manufacturer evaluated test companies that could satisfy all these requirements. After thorough and careful investigation, the TMA manufacturer partnered with Anritsu to develop a solution.
At the center of the test system is the Scorpion® Vector Network Measurement System (VNMS), which is combined with a custom multiport, multifunction test set (Figure 1). By using a vector network analyzer (VNA)-based solution, the tuning speed was increased so real-time tuning was possible. Integration of a VNA within the system also assured that the demanding accuracy requirements were met.
Finally, the VNA allowed tuning and final tests to be performed at the same test station without changing any connections. In this way, the manufacturing process would be more efficient (Figure 2).
Software Development
The test system is controlled by a PC via GPIB commands packaged in an ActiveX library. An ActiveX software interface was selected because it allows rapid and easy integration into a variety of higher-level test-executive platforms. By using industry standards as a basis, the custom software could be developed within the tight time constraints set down by the TMA manufacturer.
The custom software improved measurements and analysis. An automated tuning procedure and a final testing procedure were developed to cover all TMA models. When a specific TMA product is tested, the appropriate input and criteria parameters are accessed through the test executive, and the test procedure is executed.
The test-procedure variations then are limited to only the mechanical connection procedure, input parameters, and test criteria. Such a setup improves throughput and reduces operator training requirements.
Software allowed viewing of the results as well as making measurements. Intuitive and real-time graphical displays were written into the system, enabling the results to be displayed immediately on the PC screen for fast analysis capability.
Automated Calibration
Simplifying the calibration was accomplished by more than reducing the number of connections. Anritsu included calibration as an automated step in the test sequence, and it is executed based on a system timer set to a specific number of days. When the timer goes off, the user is prompted to perform a calibration (Figure 3). The entire calibration procedure can be accomplished quickly by following the illustrated prompting from the PC.
The test system uses an AutoCal® Module that further eases calibration by automatically switching in an open, short, load, and through line. The same calibration procedure is followed regardless of the specific TMA model being tested. As a result, a single test station may be used for a wide variety of TMA models. This flexibility is particularly important since the manufacturer produces TMAs for a variety of cellular standards that incorporate a number of architectures to support multiple frequency ranges.
Summary
TMAs are a perfect example of how integrated products affect the wireless landscape. Because TMAs may increase coverage as much as 40% by reducing system noise and improving uplink sensitivity, they are an efficient and economical method for carriers to improve signal performance.
Because of their complexity, however, TMAs create tremendous challenges in production test. By using a VNA-based solution, a test system has been developed that measures S-parameters, out-of-band characteristics, IMD, and noise figure from a single connection. TMA manufacturers now can meet volume and quality demands on these products to improve their business.
About the Author
David Judge is a systems engineer at Anritsu. He has 12 years of experience in the test and measurements industry and received a B.S.E.E. from Lehigh University and an M.S. in computer science from Oregon State University. Anritsu, 1155 E. Collins Blvd., Richardson, TX 75081, 800-267-4878.
Published by EE-Evaluation Engineering
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December 2000