RF testing spins through transmission paths

Antenna engineering manager Mark Marden leads a team of engineers who characterize antennas for ground and air applications.

The company’s RF components include frequency converters, amplifiers, doublers, mixers, and switches. Many of these components work in subsystems such as line-replaceable units (LRUs) and electronics warfare (EW) systems. Tyco Electronics also manufactures a device that jams radio signals from improvisational explosive devices (IEDs) and helps save lives in Iraq.

When testing RF components and systems used in aerospace and defense applications, the Tyco Electronics engineers need to use a variety of techniques. Their tests must not only verify the accuracy of high-frequency signals but must also take into account the harsh conditions of the anticipated operating environment. Tyco Electronics designs products for aerospace and defense, which is not a high-volume business. For these products, a few thousand pieces is considered high volume. Testing ranges from 100% characterization for every unit, where each tests takes hours, to simple pass/fail tests that take just a few seconds.

Figure 1. Spiral antennas fit onto a launch vehicle for monitoring payload operation.Courtesy of Tyco Electronics.

Figure 2. Equipment in an anechoic chamber aligns the positions of a transmitting antenna and an antenna under test and articulates the antenna under test for data acquisition.

The engineers have devised a mechanism that lets them measure 360° radiating patterns of antennas in the chambers. They move the antenna under test 360° relative to a fixed antenna. Figure 2 shows the equipment layout for an anechoic chamber that engineers use to articulate the antenna under test’s azimuth angle and elevation angle. Typically, the positioner moves the antenna under test in 0.5° and smaller increments. Engineers can adjust the fixed antenna’s height because it resides on a movable mast, although its position remains fixed during a test. “The antenna under test becomes the center of the universe” said Marden.

Figure 3. A mechanical arm rotates an antenna under test around a pivot point.

Using an Agilent Technologies network analyzer, the engineers measure field amplitude and phase (not all customers require phase measurements), making some 30,000 measurements per frequency. Each test uses eight to 10 frequencies that cover the antenna’s frequency range. A test takes between 30 min and two hours to run. Engineers need about four hours to set up and calibrate for a test.

Before installing the antenna under test in the chamber, the engineers calibrate the setup using an antenna with a known gain as a standard. That provides a reference set of measurements to which the engineers can compare the response of an antenna under test.

Antennas used in aerospace and defense applications must operate over a wide range of temperatures, and they must withstand shock, vibration, and pressure. Prior to placing environmental stresses on an antenna, the engineers make baseline measurements, called “acceptance tests,” where they prove to the customer that an antenna meets specifications. After subjecting an antenna to environmental stresses, the engineers repeat the performance measurements, this time called “qualification testing.”

Figure 4. Engineers can adjust the height of a transmitting horn antenna to accommodate different sizes of transmitting antennas.

The environmental lab also contains several Thermotron temperature chambers, including a thermal-shock chamber. An elevator moves parts through hot and cold chambers to produce thermal stress. A temperature/altitude chamber stresses parts over wide temperature ranges and over altitudes to 100,000 ft. Humidity chambers simulate dry and humid climates.

Engineers in Marden’s group also support production testing. “The percentage of units tested for production per customer requirements varies widely,” he noted. “Mission-critical antennas such as those used in launch vehicles will get 100% pattern and gain tested on every unit in addition to VSWR measurements.”

Many of the company’s military antennas are US Government classified. To make sure that test data remains secure, engineers often store measurements in computers with removable hard drives that are stored in a safe.

Cable engineering manager Herb Pflanz and his team design and test cables and cable assemblies.

“We manufacture over 300 types of cables in Lowell,” said cable engineering manager Herb Pflanz. “We use raw wire, Teflon, Kapton, and Nomex to make cables for military and commercial aircraft.” (“Spinning lives in Lowell,” explains how Tyco Electronics manufactures cables.)

Cables range in length from 6 in. to hundreds of feet. Most operate from DC to 18 GHz, but some work in the range of 26.5 to 50 GHz.

The engineers test cables by measuring transmitted power, reflected power, VSWR, and insertion loss. Customers may specify insertion-loss measurements in dB/m or dB/100 ft. “Cables have a frequency-versus-loss tradeoff. The larger a cable’s diameter, the lower its insertion loss but the lower its frequency range,” said Pflanz. “Customers will specify loss, length, and frequency range, which dictate a cable’s diameter.”

Sometimes, a customer specifies sets of cables matched for time delay, insertion loss, or phase. Customers often specify phase matching to within ±10°. To meet that specification, Tyco Electronics engineers design cables with the highest possible velocity of propagation. According to Pflanz, a cable’s velocity of propagation typically ranges from 76% to 82%, with 100% being the speed of light.

Figure 5. Cables with the same physical length but with different propagation velocities will produce outputs with a difference in phase.

Tyco Electronics keeps a database of cable characteristics that engineers use to specify matched cables. Cables produced in the same lot will have nearly identical electrical lengths for a given physical length, but cables from different lots, even when manufactured using the same materials and processes, will have different velocities of propagation and thus different electrical lengths. The company will ship electrically matched cable of different lengths as long as those lengths meet customer requirements.

Customers may also specify amplitude-balanced cables. These cables are often matched for ±5% insertion loss, but may be tighter. A customer might, for example, specify an insertion loss of 20 dB ±1 dB.

Engineers at Tyco Electronics also design RF connectors with both standard and custom interfaces that the company manufactures in Lowell. Some connectors have complex designs that incorporate keying rings and self-locking mechanisms that prevent users from using the connectors incorrectly. Some are straight, while other are bent from 45° to 90°. Some connectors have field-replaceable ends.

Critical electrical measurements for connectors include peak-power handling, average power, and insertion loss. Average power ratings vary with frequency, and most range from 50 W to 500 W, although some connectors can handle several thousand watts. Engineers use power meters and network analyzers to make these measurements.

Connectors and cables carry signals to and from electronic devices, and Tyco Electronics manufactures amplifiers, filters, LRUs, and other devices that process signals. LRUs contain analog components such as amplifiers and frequency converters controlled by microprocessors.

Many RF assemblies have analog inputs that adjust parameters such as gain and frequency response. The microprocessor-based digital-control boards in LRUs have digital-to-analog converters that generate those voltages. Engineers use data-acquisition systems to measure and calibrate control voltages. Senior principal engineer Ravi Hans leads a group of eight engineers who develop digital control boards and test systems that test and calibrate RF assemblies.

Engineers in Hans’ group test control boards with oscilloscopes, logic analyzers, power supplies, and data-acquisition systems from Agilent Technologies. The engineers developed test boards that perform both digital and RF testing. Using field-programmable gate arrays (FPGAs), engineers can configure a test board for each digital controller. “We have unique interfaces to our boards and need a reconfigurable test board,” said Hans. The boards have RS-232 and USB computer interfaces for automated control and configuration.

To automate digital testing, the engineers have developed a reconfigurable test executive written with National Instruments’ LabWindows/CVI. “We can reuse code written for engineering evaluations, transferring them to production,” Hans added.

Engineers in Hans’ group worked with antenna engineers to develop a device that jams signals aimed at detonating IEDs. Engineers designed the software and firmware that soldiers use to program the jamming device. They also developed tests for RF signal power, frequency, spectrum performance, and DC power consumption.

RF components and systems, ranging in quantities from hundreds to thousands, must go through production testing. Test engineering manager Bill Kane oversees test development for RF component production facilities in Lowell and in San Jose, CA.

Test engineering manager Bill Kane leads a team of engineers who develop automated testers used for production and design verification.

The automated test systems typically provide savings of over 90% to unit cycle time. For example, one of the complex EW systems that used to require more than 40 hrs of manual test time can now be tested in 3 hrs over three temperatures. In addition, the test systems provide true “fire and forget” capability, which allows one operator to operate several stands at once; testing can also be conducted overnight with no operator necessary.

At Tyco Electronics, test engineers develop both government-funded and company-funded test systems. Government-funded systems are dedicated to testing a specific range of products, while company-funded systems may test a wide range of products. Test engineers often reconfigure company-funded systems to meet production needs on new programs using the latest equipment and software.

Four of the single-rack test stations test three types of high-power amplifiers. Bed-of-nails fixtures connect the tester’s power supplies to printed-circuit boards (PCBs). Network analyzers, spectrum analyzers, and digital multimeters (DMMs) measure most test parameters.

“Design engineers perform bench testing while we develop the automated test stations,” said Kane. “They use the automated testers during product development. We also run tests for design engineering, and we often write automation code that they use. We later reuse the code for production.”

Software engineers in Kane’s group have developed a library of test functions using Agilent Vee that they often reuse to test new products. Test software stores measurement results in a database, generating as many as 90 sheets of data per part, depending on the length of a test. Unclassified test data resides on an Oracle database; engineers can analyze the data, looking for trends that may take days of testing to appear, to improve manufacturing processes.

Engineers at Tyco Electronics extensively test antennas, components, and systems used for military and aerospace applications. “The ability to produce automated test systems in-house, alongside the project development team, provides a tremendous advantage in development cycle time and cost,” said John McGuire, manager of quality and operational excellence for the group.