RF and Microwave Testing Using the VXIbus Historically, the automation of RF and microwave test systems occurred more slowly than that of their lower frequency counterparts. Largely because the required instrumentation was unavailable with GPIB interfaces. The advent of RF and microwave instruments (e.g. network and spectrum analyzers) with computer-controlled interfaces facilitates the integration of high-performance microwave ATE. There are, however, still several issues that require close attention by the RF ATE designer: signal integrity, noise, and the long-term cost-performance ratio. The design of a universal automatic test system requires the routing of signals through switches and couplers, introducing signal loss--never an issue in the manual style of testing. The many feet of cable required for connecting sources and measuring devices in a rack-and-stack environment often result in induced noise from digital buses as well as external electro-magnetic fields. The effect of losses can be compensated through use of software, either in the instrumentation or the test program. Noise can be reduced through the use of screened cables and careful signal routing within the system rack. The rack often will be carefully screened and grounded. Beyond the technical areas is the cost issue. In the race to stay competitive, instrument manufacturers built more and more complexity and capability into their products. With this increase in sophistication follows an increase in cost. Synthesized spectrum analyzers are available with frequency resolution of 1Hz through 1GHz and the ability to measure SSB phase noise and frequency deviation. All of these features are unnecessary if, for example, only the level of a third harmonic must be measured. Compounding this issue is the question of performance upgrades. In many cases, instruments are purchased with more capability than needed for the initial application because the engineer wants to avoid obsolescence. Enter the VXIbus Test System The advent of the VXIbus has reduced or eliminated some of these problems and simplified solutions for others. A test system confined to a single VXIbus chassis improves signal integrity. Shorter cable runs mean less loss, with a resultant reduction of induced noise. It also is much easier to screen the system from high-level external fields. The modularity of VXIbus systems allows the system designer to specify the performance required for the current application and to upgrade performance when requirements change. A number of test systems for RF and microwave frequencies have been built using the VXIbus. These applications highlight how the VXIbus reduces and simplifies the problems faced by RF system designers. Digital Cellular Test Stand The European community uses the GSM Pan European cellular radio system. A new VXIbus tester was developed for this system which handles 900MHz sensitive receivers with no loss in performance as compared to stand-alone rack-and-stack equipment. Along with signal processing, the test system contains one receiver and two separately programmable transmitters to enable performance testing of a complete mobile phone via the RF connector. The unit is housed in a VXIbus chassis and consists of six, two-slot C-size VXI modules. It is controlled via a GPIB link from an IBM compatible PC which provides test sequencing facilities, display of test results and disk-handling requirements. System software is reprogrammable from the PC, enabling new software to be downloaded as it becomes available. This feature also permits modification of standard software to be carried out quickly and easily when specific applications are required. A block diagram of the system is shown in Figure 1. The transmitters and receiver are controlled by the main control processor, which contains the GPIB interface and generates all the test information. Transmit Data Processor 1 performs layer functions such as encoding and interleaving of the cell control channel data. Transmit Data Processor 2 performs the same functions for the traffic channel. Figure 1. Diagram of Digital Cellular Radio The two transmit synthesizers generate the required digital modulation. These outputs are combined in the RF combiner and transmitted to the mobile. The signal from the mobile passes through the RF combiner and is separated by a duplexer. This signal is down-converted to an IF digitizer by a flash A/D. The digitized samples are fed to both the raw data RAM and the demodulator. The raw data RAM acquires the raw samples for subsequent processing by the Receiver Data Processor 2, which performs the phase trajectory measurements. In addition, the digitized data from the A/D is demodulated and passed to the Receiver Data Processor 1 for subsequent processing. The ability to upgrade enhances the system by permitting installation of different synthesizers. Also, use of devices customized to a specific requirement eliminates redundant capability. Radar Test Stand The benefits of modularity are especially evident in a radar system for the testing of primary radars which can be housed in a single 13-slot VXIbus C-size chassis (Figure 2). The components in the system can be changed depending on the type of radar being tested. Figure 2. Radar Test System For instance, a Doppler radar will need a synthesized source with low close-to-carrier phase noise while, for a wideband system, a sweeper may be more cost-effective. The radars either can be fixed ground or airborne systems and frequencies from 900MHz to more than 20GHz can be accommodated. Figure 3. Radar Test System EMC Performance Capabilities A single system at an airfield could test the primary surveillance radar, surface movement radar and secondary radar. One system can be built to test almost all radars with only a change of plug-in modules and system software. Modularity allows individual instruments to perform several functions. For example, a signal generator used for receiver testing doubles as a local oscillator for down conversion and transmitter testing. This eliminates paying for redundant features normally inherent in traditional rack or bench products. The system provides several transmitter parameter checks: pulsed RF frequency, pulse width, pulse rise time, chirp characteristics, power output, PRF and Doppler shifts. Tested receiver characteristics include sensitivity, scintillation (weather radars), receiver bandwidth, sidelobe interference and transponder reply codes (with resolutions to 200ps). Measurements have been made in an anechoic chamber to establish the EMC performance of this system. Figure 3 demonstrates the low-level radiated emissions of the complete system housed in an RF and microwave VXIbus chassis. The modular approach to radar testing also means users merely add to or replace specific cards to accommodate new equipment or technology. For example, a system for testing DME could be expanded to test SSR transponders. The extra investment would be small since only one additional instrument card, a time interval analyzer, would be required. Satellite Communications Test Stand Many of the functionality features and benefits of the digital cellular and radar test systems apply to a VXIbus test stand dedicated to parametric measurement of satellite communications (SATCOM) systems and subsystems. The system consists of a microwave frequency counter, power meter, a down converter and a time interval analyzer used in combination to analyze SATCOM transmitter signals. For receiver testing, the system includes a microwave synthesized signal generator, an arbitrary waveform generator and RF synthesized signal generator modules used in combination to stimulate calibrated SATCOM signals. Depending on the number and scope of tests to be performed, a complete, dedicated VXIbus SATCOM test system can be integrated in one 13-slot VXIbus C-size chassis. SATCOM Transmitter Testing The quality of SATCOM transmitters can be determined by comparing the frequency, power and modulation characteristics of the output signals to expected norms. The microwave frequency counter module can measure up-and down-link carrier frequencies from the L though the Ku band, and analyze common IFs to 70MHz and below. The counter's YIG-tuned preselector acts as a tunable filter on the counter input, allowing individual channel frequencies to be measured without requiring other channels to be taken of the air. The preselector also provides enough FM tolerance to enable accurate measurements of carrier frequency even while the channel-under-test is carrying traffic. Carrier signal power level can be measured directly at frequency by the microwave power-meter module. For modulation analysis, the transmitter signals with traffic can be down converted to a lower frequency IF for analysis. The microwave synthesized signal generator provides the LO signal to the down converter module. The microwave synthesizer provides LO signals for down converting S, C, and Ku band carriers and enough output power to ensure adequate LO level (+10dBm) even after reasonable cabling and switching losses. The low close-in single-sideband phase-noise performance of the synthesized signal generator is also important. This prevents modulation information close to the carrier from being masked out by the LO signal during down conversion. The modulation quality can be analyzed at the IF frequency by a signal analysis module. SATCOM Receiver Testing To test SATCOM receivers, the microwave synthesized signal generator module generates calibrated transmitter signals. The synthesizer module generates S, C, and Ku band carrier frequencies to 1Hz resolution and sets levels to 0.1dB resolution down to -90dBm for sensitivity tests. The synthesizer module has a linear up-conversion input that places a modulated subcarrier from a system-under-test directly onto the microwave carrier. This up-conversion input has a 50MHz instantaneous bandwidth, wide enough to preserve common BPSK, QPSK and 16QAM modulation formats (typically less than 35MHz bandwidth). Subcarrier input frequencies range from 300MHz to 1GHz. With this capability, a known modulated IF (e.g., the 700MHz IF for DSCS 3) can be connected to the synthesizer module's up-conversion input, providing a real-time simulation of the up- or down-link microwave carrier. This simulated signal, in turn, can be adjusted by the synthesizer module's attenuator or AM capability to quantify a receiver's operational limits. For example, the arbitrary waveform generator module can drive the synthesizer's AM input at a low rate to analyze the receiver-under-tests's susceptibility to signal fading. Multiple IF signals can be applied to the up-conversion input to simulate adjacent channel interference. The level of the interfering signals can be adjusted while measuring the receiver-under-test's BER in real time, hence its tolerance to intersymbol interference. For direct measurement of the receiver's third order intermodulation (TOI) performance, two or three signals from RF synthesizer modules with outputs up to 1GHz can be applied to the up-conversion input. This allows two- or three-tone TOI testing at the microwave carrier frequency without requiring multiple microwave generators. The result is a large cost saving using the VXIbus SATCOM test system approach. Whether simulating adjacent channel interference, performing TOI tests or doing LO substitution, a key performance characteristic of the synthesizer is its low single-sideband phase spurious noise. This ensures that the simulated signals used to test the SATCOM receiver are not being corrupted by unintentional noise from the test stand. Compared to GPIB rack-and-stack ATE systems offering equivalent measurement performance, the RF VXIbus test stands described offer several benefits: Size and portability Ease of updating capabilities by swapping or adding new measurement modules as requirements change. Cost reduction by utilizing individual modules in multiple measurement configurations, eliminating hardware redundancy. TOC 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13