These basic guidelines help in the selection of the test instruments and software needed to assemble an effective RF and microwave automated test equipment (ATE) system.

DENIS GABLE Director of Frequency Synthesizer Engineering Wide Band Systems, Inc., 389 Franklin Ave., P. O. Box 289, Rockaway, NJ 07866; (973) 586-6500, FAX: (973) 627-9190, e-mail: dgable@widebandsystems.com, Internet: www.widebandsystems.com.

Automated test equipment (ATE) systems with RF measurement capabilities are used throughout commercial and military facilities when there is a need to perform a large number of repetitive tasks. In commercial applications, for example, RF ATE systems are commonly employed for screening the performance of production semiconductors and integrated circuits (ICs) used in wireless communications devices and infrastructure equipment. In military applications, specially designed RF ATE systems can complete the many tests necessary for qualifying the performance of tactical radios, radar receivers, and electronic warfare (EW) systems. In either case, assembling an effective RF ATE system is a matter of understanding the capabilities and limitations of different function blocks in the system and how to make those blocks operate seamlessly under software control.

Constructing an RF/microwave ATE system (Fig. 1) involves addressing a number of key requirements, determined by the type of device under test (DUT). Although RF/microwave measurements can vary widely from commercial and military applications, most ATE systems share a list of requirements, including

• Types of tests to be performed
• Type and number of RF signals to be applied
• Power levels of the RF signals to be applied
• Expected power levels of the RF output signals from the DUT
• Number and type of power supplies needed (for active devices)
• Type of digital control to be used
• Expected throughout of the system
• Acceptable accuracy levels
• Budget constraints

The choices of hardware instruments comprising an ATE system will depend on how these requirements are met. For example, the types of RF tests required for commercial wireless devices can be considerably different that the RF and microwave tests performed on military electronics equipment. Tests for commercial wireless devices are generally established by the requirements of a particular standard, such as the various versions of IEEE 802.11 for wireless local area networks (WLANs) or IEEE 802.16 for emerging wireless metropolitan area networks (WMANs) including WiMAX systems. Tests for military equipment, on the other hand, can involve specially designed waveforms and specific frequencies. An example is the set of advanced waveforms used for testing tactical military software-defined radios (SDRs) such as those developed for the Joint Tactical Radio System (JTRS) program.

Selection of a signal source or sources for an RF/microwave ATE system is dictated by the intended test applications, with a wide range of suppliers and products currently available. Test signal sources vary widely in price and performance, depending upon the frequency range, modulation options, spectral purity, and other performance factors. As a starting point, a signal source or sources for an ATE system should exceed the operating range of the DUT, such as the 2-to-18 GHz span of many radar systems. In some cases, it may be desirable to exceed the DUT’s frequency range by at least a factor of 2 or higher, in order to generate and test DUT performance with second- or thirdharmonic signals.

An ATE test signal source should provide the resolution between frequency steps required to emulate the actual application for the DUT, such as switching among communications channels in a tactical radio. Frequency resolution ranging from 1 Hz to 1 MHz is common in modern RF/microwave signal sources, with even finer resolution possible in some digital synthesizers. The test signal source should also provide adequate dynamic range through sufficient maximum output power and through switchable attenuators. Such attenuators should be well designed, since they will impact the overall amplitude accuracy of the signal generator and, thus, the possible amplitude accuracy of the ATE system. The output power flatness and amplitude accuracy of a test signal source for ATE applications should be in the range of ±1 dB to ensure minimal contributions to amplitude measurement uncertainty. Inadequate amplitude accuracy can lead to sufficient measurement uncertainty to fail DUTs during production testing that might have passed during those measurements given a more accurate test system.

The spectral purity (phase noise, spurious, and harmonics) of an ATE test signal source should exceed that of the DUT to accurately determine the noise floor of the DUT, especially for active devices. Phase noise, for example, is generally specified as the amount of noise below the carrier level when normalized to a 1-Hz bandwidth at some offset from the carrier. The noise, of course, is spread across the full operating range of the synthesizer, so the 1-Hz convention simplifies the comparison of noise levels from one instrument to another. The frequency and amplitude switching speed of a signal generator for ATE applications should be sufficient to support expected levels of test throughput for a given DUT. Finally, the test signal source should provide the modulation formats required for fully testing the target DUT. If not available internally in an instrument, the signal generator should provide a modulation input port so that the required modulation can be supplied by an external source, such as a pulse generator or arbitrary waveform generator.

At one time, many of the RF/microwave signal sources found in ATE systems were known as free-running signal generators or swept-frequency oscillators. These were instruments based on YIG oscillators or voltage-controlled oscillators (VCOs) capable of wide frequency tuning ranges but lacking in high frequency stability. In modern ATE systems, most signal sources are now frequency synthesizers that can duplicate the tuning ranges and switching speed of free-running sources, but with much improved frequency stability and spectral purity.

ATE architects have a wide range of commercial frequency synthesizers to choose from, with some key differences in performance. The most common class of instrumentation frequency synthesizers are based on indirect synthesis techniques usually employing a phased-lock loop (PLL) and a reference oscillator to stabilize the phase/ frequency of a tunable wideband oscillator, such as a YIG or VCO, by synchronizing its phase with that of the reference oscillator. The approach can provide a stable output when using a low-noise crystal oscillator, such as a temperature-controlled crystal oscillator (TCXO) or oven-controlled crystal oscillator (OCXO), as the reference source. Unfortunately, the tuning speed of this synthesis approach is limited by the time needed for the PLL to acquire and lock when tuning to a new frequency and is generally in the range of milliseconds.

When speed is an issue, a second class of frequency synthesizers employs direct-synthesis techniques to produce output signals often with nanosecond switching speeds between frequencies and/or amplitude levels. Such synthesizers often employ direct analog approaches, in which a low-noise reference source is multiplied, divided, and mixed to create a comb of signals from which a desired signal is selected or “tuned” by means of switched filters. They may also make use of direct-digital- synthesis (DDS) signal generation. A digital phase step word is accumulated, where after, the instantaneous phase is mapped to a sinusoidal amplitude to be converted to the analog domain by use of a digital to analog converter (DAC).The internal bit resolution of these devices is especially critical to the spectral performance of a DDS generated signal. Limited resolution results in high spurious levels and a need for subsequent filtering or other techniques to avoid undesired spurious signals. A direct-analog synthesizer can provide extremely high levels of performance in terms of signal phase noise and switching speed, but is also associated with high cost due to the large component count in terms of required filters, switches, multipliers, mixers, and other RF and microwave components. A DDS-based signal generator offers improved resolution when used in a direct-analog-synthesis approach, but still relies on the best possible reference or clock performance for good spectral purity.

In assembling an ATE system, concerns in specifying the test signal source or sources often have as much to do with mechanical constraints as with electrical performance. Especially in ATE systems aimed at military test requirements, such as radar testing, where multiple-emitter environments must be recreated in the test laboratory or on the production line, the size and power consumption of a synthesizer may be key considerations.