Radio-frequency (RF) system impairments can disrupt mission-critical communications. Deploying vehicles, aircraft, or portable radios for tactical applications requires a fast and nonintrusive test method to ensure that the radio system will fulfill its intended functions and under a wide range of adverse environmental conditions. Therefore, mission-critical RF system verification must be performed in situ, requiring a test system that is fast, simple to operate, full-featured, portable and usable in a variety of hostile environments encountered in military operations.
Verifying that an RF system is "mission ready" requires that various components within the system, as well as the sources for errors, are evaluated. These sources of error include signal attenuation, interference, distortion, as well as the items within the installation, such as the transmitter and receiver, power and control connectors, interconnect cables and coaxial connectors, antennas, and operator controls. Figure 1 shows a high-level block diagram for a simple radio installation and the major components within this system. If one or more of these items experiences serious impairment, the radio transceiver will not function.
Current in situ radio system test methods often involve the use of a measurement platform that is similar to the radio system to be tested. In this case, the test radio and the radio to be tested are used to communicate with one other to verify that each system is operating correctly, using a "talk test." While this approach provides a qualitative check of proper operation, it does not confirm if either radio can operate under adverse conditions or in fringe reception areas. Another issue with the "talk test" method is identifying which of the two radios is impaired when a problem is found. By design, some systems do not include a two-way communications path, precluding the use of a "talk test." An example of a one-way RF system is an aircraft navigation marker beacon, which is a receive-only radio within an aircraft.
Rather than a "talk test" using a radio platform that is similar to a system to be tested, a test solution is needed that can verify whether a radio system is "mission radio" or not, using actual operating conditions and test signals. The particular platform (vehicle, aircraft, hand-held) must be characterized so that a "mission-ready," over-theair test set of standards is established. In addition, appropriate test procedures and test instrumentation must be implemented to permit "mission-ready" evaluation and qualification for a fleet of these platforms.
When a radio communications platform is tested and found to conform to established standards, the platform is considered "mission ready." However, if the platform does not meet the standards, it is impaired and must be further evaluated and analyzed to isolate the faulty component or components for repair. This fault isolation and repair could be implemented as part of the "mission-ready" screening, or may be done in a parallel or off-line process.
Characterization of each RF system within a platform involves several steps in order to define what is to be tested (components and error sources), how the test will be performed (instrumentation and instruction), who will perform the test (the required operator skill level), and what results will be measured, monitored, and/or stored (pass/ fail criteria, data collection).
The first step determines which RF systems on a given systems' platform requires testing, including which components within each system are critical to the mobile radio being "mission ready" and which parameters are "mission critical." As an example, assume that the military platform to be tested is a rescue helicopter and the RF systems to be tested are its communications radios. As an example, a military helicopter is fitted with two radios: a VHF AM transceiver and a UHF FM/digital transceiver.
Table 1 lists the systems components to be tested in this example. To ensure that the entire communications system is operating properly requires that the radio is fully checked from the audio inputs and interconnections through the antennas. Table 2 lists the operational items that must be checked in this example. These items are just as important as the RF and audio components, since an error within one of the "soft" settings of the radio will prevent the radio from achieving "mission-ready" status just the same as a defective hardware component.
The ideal "mission-ready" test is fast, simple to operate, full-featured, portable, and usable in a variety of operating environments. However, if the RF system component and operational tests of step 1 are included, there would appear to be conflicting requirements. Nonintrusive testing that checks transmitter power, antenna efficiency, and coaxial insertion loss are at odds. What must be done is devise a "mission-ready" test for each of the component items identified, in a non-invasive way. The test method must account for the various signal path loss variables in the RF system, and then test the component items utilizing this signal path loss information.
Figure 2 shows a test setup for evaluating a communications radio system. The radio test set is used with an antenna to measure over the air transmissions from the radio under test, and also generates RF signals to the radio under test to test over the air receiver functions. Table 3 lists an example path loss rollup. For this example, 40 dB of RF loss must be added to the RF power readings in the radio test set, and likewise to the RF generator level settings to verify the platform operates according to "mission- ready" specifications. The component loss values may be obtained from vendor data, or calculated as in the case of the over-the-air path loss. If the RF output power at the transmitter for the radio under test transmitter is 10 W (+40 dBm), the over-the-air measurement will be attenuated by 40 dB, yielding a "mission ready" reading of 1 mW (0 dBm).
Similarly, the RF generator level for verifying the RF receiver sensitivity must also have the 40 dB loss added. If the RF sensitivity specification is -100 dBm, then the RF generator would be set to -60 dBm for the "mission-ready" over-the-air test. The RF transmitter carrier frequency for the radio under test, its modulation, and its fidelity are verified over the air using the radio test set to measure frequency, modulation, and distortion. If these items all pass, the antenna, coaxial system, microphone and transmitter are "mission ready." Likewise, if the receiver sensitivity and speaker audio pass over the air, the receiver and speaker are "mission ready."
The test method used to test the platform must be appropriate for the skill level for the intended operator. This also includes operator training. The requirements demand a simple yet powerful test capability within the radio test set, and that it contain a full suite of test instruments in support of the standards. If the radio test set supports scripted testing, the complete user instructions may be included onto the operational screens of the radio test set, reducing errors and paperwork. Although not specified in the example requirements, the test data that is collected from each platform could be saved and archived for future evaluation for studying the test system, failure trends, and failure rates. This would be an additional requirement upon the radio test set.
The model 3500A portable radio test set developed by Aeroflex provides all the features and functions needed to implement the testing schemes mentioned earlier. This portable instrument runs on batteries for in-field use, and includes a full-featured RF signal generator and receiver as well as all the instruments and meters required to implement fast and accurate in-field measurements. Table 5 lists the matrix of required items for the example communications platform detailed in this article, along with the test items that fulfill the requirements. In any cases where a single test instrument is required, and a custom script application was implemented, the procedure could be fully contained within the instrument.
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The 3500A is a self-contained, 1-GHz test system that weighs less than 8 lbs. It operates from 2 MHz to 1 GHz and includes a spectrum analyzer with -136 dBm noise floor. The test set can be supplied with a tracking generator as an option, and can perform AM/FM receiver and transmitter tests. It is also available with an audio oscilloscope option for evaluating the quality of audio signals from a radio under test.
The portable test system, which can operate for as long as 7 hours on a battery charge, was designed for overthe- air field testing. It can display disance to fault information, return loss, VSWR, and cable loss. Distance to fault information is shown on a graph as a function of return loss versus distance, while VSWR, return loss, and cable loss are shown as amplitude as a function of frequency. It performs a large number of measurements by integratng a large number of tools into a relatively small housing. These functions include an AM/FM modulation meter, an audio frequency generator and modulator, an audio frequency counter, an audio level meter, and an RF frequency error meter.
The portable test set supports infield test software from outside suppliers, such as the Field Test 6 software from Survey Technologies, Inc. The software can be downloaded from the Survey Technologies web site and added to the test system to simplify the acquisition, analysis, and display of signalstrength data across a given terrain. The software is also useful for analysis of emitters within a building. The 3500A itself includes a number of functions that ease testing, including a frequency find feature.
In short, the use of a full-featured portable radio test instrument, augmented with the required test methods and procedures, can facilitate fast and reliable testing for management of mission-critical radio communications systems. This integrated radiotest application offers future opportunities for troubleshooting and repair of impaired systems, custom applications for directed tests, and data collection for process control and quality improvements.
FOR FURTHER READING
1. J. Banks, J. S. Carson II, B. L. Nelson, and D. M. Nicol, Discrete-Event System Simulation, fourth edition, Pearson Prentice Hall, Englewood Cliffs, NJ, 2005.
2. Advancing Wireless Test-RF Datamate, Aeroflex Booklet, #46891/883.
3. 3500A Portable Radio Communications Test Set. Aeroflex Product Brochure, #46891/329, Issue 1, October 2008.