Microwave technology is the backbone of an ever-increasing array of products for personal, commercial, military, and aircraft applications. As an example, cellular telephony is, by far, the most expansive use of microwave communications technology in the last 40 years. The current personal communications services growth is showing no signs of slowing down as more and more microwave repeaters and base stations are being installed daily throughout the world. This includes developing countries where very little wire-line communications are installed, and where cellular radio will become the primary source of public communications.
What does all this mean to the microwave test and measurement industry? It means the need will be greater for microwave measurements made in the field. For this to be successful, test instrumentation must be simple to operate, have comprehensive automated test routines with built-in pass/fail limits, and store measurement results.
Microwave Measurements
The essential measurements conducted on microwave communications systems are the RF power output of the transmitter and the insertion loss and return loss of the transmission lines and antennas. The RF power measurement ensures that the system-power amp is operating at the correct level. The insertion and return loss measurements guarantee that the power being generated is not being lost or reflected by the transmission line and antenna.
When measuring these parameters on microwave systems, several basic rules must be observed and a number of pitfalls need to be avoided. Failure to take them into consideration can—and will—affect the measurement accuracy and repeatability.
Cable and Connector Care
Test cables and connection accessories can be major sources of measurement uncertainties. Meticulous cleanliness is essential. Before using them, inspect connectors for damage such as dents and scratches on mating surfaces.
Measure the connectors’ dimensions regularly to ensure that they are not worn beyond usable specifications. To avoid the costly replacement of connectors, connector savers are available. These should be of high quality to avoid degrading the source match voltage standing wave ratio (VSWR) of the test port.
Signal Separation Devices, Directivity, and Test-Port Match
The measurement of RF power, insertion loss, return loss (VSWR), and fault location on a microwave system requires the use of a signal-separation device such as a directional coupler, a return loss bridge, or a device referred to as an autotest accessory.
Directional couplers allow reflected power to be measured while forward power proceeds unimpeded through the coupler to the device-under-test (DUT). The return loss bridge performs the same function but uses a measurement-bridge arrangement. The autotest accessory is an integrated assembly containing a return loss bridge and a detector.
The autotest accessory makes it possible to simultaneously measure insertion loss and return loss. Recent circuit-design enhancements combine fault-locator and return loss front-end circuitry into one assembly. Insertion and return loss determinations and distance-to-fault measurements now can be performed without connecting and reconnecting the DUT.
Directional signal separators are key elements of reflectometers—instruments that determine exact reflected power levels. The degree to which directional signal separators prevent leakage of forward power from contaminating reflected power is referred to as directivity, and is specified in decibels. If the directivity of a reflectometer is poor, then the leakage will inhibit the analyzer from accurately measuring the reflected signal.
An ideal reflectometer with infinite directivity measures virtually any level of a reflected signal, limited only by the noise floor of the system. High directivity becomes particularly important when making accurate return loss measurements on low-level signals. Modern reflectometers offer directivities of 38 dB or better, which is adequate for most field measurements.
The accuracy of reflected power measurements also can be adversely affected by reflections caused by the measurement setup. Any impedance mismatch causes reflections. The mismatch introduced by a test port, referred to as source match, can reflect the DUT’s reflected signal back into the forward direction. To make matters worse, these signals may be re-reflected by the DUT, compounding the signal levels arriving at the reflectometer.
Measurement uncertainties may be minimized by keeping the source match of the instrument test port and the DUT as low as possible. For this reason, connector adapters are not recommended. It is essential that high-quality low return loss jumper cables connect to the DUT; and wherever possible, the test port should be connected directly to the DUT.
Test-port mismatch can cause a low-frequency ripple on the measurement trace, and introduce potential errors of 2 dB or more depending upon the magnitude of the mismatch. The effects on measurement uncertainty caused by source-match reflections between the test port and the DUT can be reduced by using high-quality attenuators with good return loss specifications. However, this is not always possible because it also reduces measurement sensitivity by the value of the attenuator.
Interfering Signals
Most microwave measuring instruments, with the exception of spectrum analyzers with preselection, are broadband. This makes measurements in a high RF field environment susceptible to the effects of interfering signals.
The effects can be significant, but may be eliminated on modern microwave scalar analyzers by using AC detection. This technique generates a square wave signal modulated on the synthesized source, causing the signal to be switched on and off at defined intervals. In the ON state, the analyzer measures both reflected and interfering signals. In the OFF state, the analyzer only measures interference signals.
The analyzer is able to distinguish these signals, so it can be used in the most congested environments. This is particularly useful when making feeder and antenna measurements in the field.
Calibration
When making any type of RF measurement, the test-system configuration must be calibrated. All test cables, connectors, and accessories will have some contribution to uncertainties, and these must be minimized by calibration before making measurements on the DUT.
Factors, such as the insertion loss, return loss, and frequency response of test connectors, devices, and cables, must be considered. This is achieved by a path calibration or normalization and short/open calibration on the return loss test port.
Additional components, adapters, or cables must not be installed into the test configuration once calibration has been performed. To do so impacts the calibration and may introduce unknown errors into subsequent measurements.
Modern Instruments
To meet the demand for more flexible, portable, and easier-to-use microwave test instrumentation, manufacturers are taking maximum advantage of component miniaturization and surface-mount technology, fast efficient processors, and digital signal processing. The ideal instrument is a single integrated package containing an RF power meter, a frequency counter, a scalar or network analyzer, and a synthesized signal source/sweeper. The instrument should be supported by a range of accessories such as power sensors, return loss bridges, autotesters, scalar detectors, and fault locators to cover the frequency range of 10 MHz to 46 GHz.
Ease of Operation
The instrument must be easy to operate. A large graphical display with a mix of hard and soft menu-driven keys enhances ease of operation. A dual-channel color screen with the capability to display two measurements per channel (up to four separate traces) aids in interpreting results.
You can compare a live trace with a reference response recalled from memory. The menu-driven structure helps you break down the setup procedure for a measurement into easily managed and understood blocks. Once a particular measurement mode is selected, the soft keys change annotation to display the test options and facilities available within that menu.
In addition to the soft keys, hard keys should allow the fast selection of commonly used functions such as source modes, calibration, measurement modes, display modes, measurement formats, scaling, and marker functions. Selecting any hard key should bring up a new soft-key menu indicating the functions available in that mode.
In this manner, the task of setting up the multifacets of the instrument to conduct a particular measurement is very simple. There also should be hard keys for ancillary functions such as memory facilities, hard-copy print, user help menus, and utilities.
The instrument should incorporate display facilities for dual-channel measurements of insertion loss, return loss, and fault location. This helps all of the essential measurements of transmission lines and antennas to be conducted in a single pass.
Instrument Settings Stores
Even though the operation of modern instruments has been simplified, the task of setting up a measurement mode can be a lengthy procedure if conducted each time the same type of measurement is made. It also may be subject to repeatability errors if the procedure is not documented and faithfully followed.
For these reasons, the instrument should have storage facilities so commonly used test setups can be stored and retrieved. Then you only must conduct the calibration procedure before making the measurement.
Guided Measurements
Guided measurements are a series of macros written to perform the more commonly used tests, such as cable or waveguide return loss, insertion loss, and fault location. They use the macro facility to guide you through each of the measurements with on-screen messages, prompts, and graphics, and a navigator to guide you into various menus.
Test-Results Storage
Making microwave measurements in the field requires the temporary storage of test results so that they may be transported back to the office environment. Once there, the results can be put onto a more permanent storage medium.
Storage of results may take one or all of these formats: Graphical Screen Dumps (.bmp), Comma Separated Variables (.csv), and EXCEL Spreadsheet (.xls). With the spreadsheet formats, the test data can be reviewed, mathematically manipulated, inserted into word-processing documents, or printed. The .bmp format allows measurement graphical traces to be imported into word documents or graphics presentations.
Conclusions
The task of making microwave measurements still is very complex and can be plagued by uncertainties. However, if a few simple precautions are taken, meticulous cleanliness is practiced, and basic rules obeyed, they can be made easily and successfully. Couple these with the capabilities available in modern microwave test instrumentation and you have a test system that can be operated by almost anyone. The test results may be stored in the field and quickly extracted for analysis and documentation in the office.
About the Authors
Chris Rix, a senior engineer at Marconi Instruments, has more than 30 years experience in telecommunications. In 1989, he joined Marconi Instruments in the United Kingdom as a senior sales specialist, and has been working in the United States for the company since 1996. Mr. Rix was educated through the British Royal Air Force colleges.
Fernando Torrelio is an applications engineer at Marconi Instruments. He received a B.S.E.E. degree from California State University.
Marconi Instruments, 2301 Horizon Dr., Fort Worth, TX 76177, (817) 224-9200.
Copyright 1997 Nelson Publishing Inc.
June 1997
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