1. The NI PXIe-5186 and NI PXIe-5185 PXI digitizer modules offer signal-capture bandwidths of 5 and 3 GHz, respectively. National Instruments.>
Modular instrumentation provides a great deal of flexibility, whether on the test bench or in production. Rather than a rack of full-sized instruments, a modular approach attempts to fit the same functionality into a mainframe of function modules designed for compatibility and ease of programmability. The idea is not new, but has been promoted by numerous test-equipment manufacturers for more than three decades. Current market offerings include a wide range of instrument functions, from microwave signal sources to high-speed digitizers, using numerous different instrument busses. Modules may differ in appearance, and all aren't necessarily meant for mainframes, but the concept of mixing and matching measurement functions is the same.

Control busses for RF/microwave instrumentation comes in many forms, including simple computer-aided interfaceslike local area network (LAN), peripheral component interconnect (PCI), and Universal Serial Bus (USB)along with interfaces that leverage these busses, such as LAN extensions for instrumentation (LXI), PCI extensions for instrumentation (PXI), and VME extensions for instrumentation (VXI). The standards are designed to foster interconnectivity of instruments under computer control. For example, PXI-based systems can also include instruments equipped with USB and General Purpose Instrument Bus (GPIB) interfaces, just as LXI instruments can communicate with instruments that are not LXI compliant. PXI was introduced by National Instruments in 1997 and now features the PXI System Alliance (PXISA) behind it. That group's membership boasts such notables as Aeroflex, Agilent Technologies, Giga-tronics and, of course, National Instruments.

Additional open standards, such as AXIe, leverage these existing standards for improved performance in high-performance test systems. AXIe includes PCIe and LAN interfaces so that AXIe can act like virtual PXI or LXI instruments, fostering true universal control in a modular automatic test system. LXI, for example, includes rigorous guidelines for timing and synchronization, including the IEEE-1588 precision time protocol for effective synchronization within 100 ns over a LAN. This standard is ideal for controlling instruments with low acquisition rates over long distances.

Historians may remember that, before they were Agilent Technologies, Hewlett-Packard Co.'s test and measurement engineers developed the HP 70000 modular measurement system (MMS) based on the firm's proprietary Hewlett-Packard Modular System Interface Bus (HP-MSIB). Introduced as a microwave spectrum analyzer, it was essentially a receiver with key function blocks housed in separate modules, connected in a rack-mountable mainframe by the high-speed bus. The MMS, still in use at key US government agencies and with documentation still available online, represents a sterling example of the high performance levels possible with modular test equipment.

More to date, the model M8190A arbitrary waveform generator (AWG) from Agilent Technologies is an instrument module with proprietary silicon germanium (SiGe) BiCMOS digital-to-analog converter (DAC), built into an AXIe module. It can generate analog signals with complex modulation at bandwidths to 5 GHz. A single M8190A AWG can fit into a two-slot AXIe chassis, or a pair of the AWGs can be mounted in a five-slot AXIe chassis together with a controller (see Defense Electronics, June 2011, p. 36). Less than two years ago, Agilent, Aeroflex, and Test Evolution Corp. proposed the new AdvancedTCA Extensions for Instrumentation and Test (AXIe) open standard based on the AdvancedTCA (ATCA) standard, with the goal of providing modular instruments designed with a horizontal, rather than vertical, profile for maximum scalability in rack-mounted systems. The standard is also designed for ease of integration with existing modular test standards, such as PXI and LXI.

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On the signal-analysis side, National Instruments recently announced a pair of PXI digitizers co-developed with Tektronix. The models NI PXIe-5186 and NI PXIe-5185 PXI digitizer modules (Fig. 1) offer signal-capture bandwidths of 5 and 3 GHz, respectively, with single-channel sampling rates to 12.5 GSamples/s and dual-channel sampling rates to 6.25 GSamples/s. The high-density PXI format allows not only a great deal of measurement power in a limited space, but the flexibility to change functionality quickly by changing test modules. Each digitizer module occupies a 3U PXI Express housing and provides high resolution by merit of 8-b analog-to-digital converters (ADCs).

2. The USB power sensor/meter is a model LB480A performing pulse-profiling measurements 100 MHz to10 GHz. LadyBug Technologies LLC.>

Aeroflex, with its 3000 Series of RF Modular Instruments, provide complete PXI-based solution for complex wireless communications testing, with modules available for vector signal generation and analysis. The modules, which are supported by the firm's PXI Studio application software for ease of creating automatic measurements, can be specified at frequencies to 3 and 6 GHz. They are capable of supporting multiple wireless standards, including GSM/EDGE, UMTS/HSUPA, LTE FDD, CDMA 2000 and 1xEVDO, TD-SCDMA, WiMAX, WLAN, and Bluetooth wireless technology.

The use of the Universal Serial Bus (USB) can simplify connection of test equipment to a personal computer (PC), in some cases eliminating the need for instruments with their own controls or display capabilities. Especially with data transfers to 480 Mb/s possible per the USB 2.0 standard, many test-equipment manufacturers are now offering RF/microwave test functionality in the form of compact USB devices, including power sensors/meters, signal generators, receivers, and even digital storage oscilloscopes (DSOs).

For example, the model GT-8888A USB power sensor from Giga-tronics measures the power levels of continuous-wave (CW) signals from 10 MHz to 8 GHz over a dynamic range of -60 to +20 dBm. It can make 2000 readings per second and provide fast and economical measurements of power in production, automatic-test-equipment (ATE) environments. For higher-frequency measurements, the models MA24108A and MA24118A power sensors from Anritsu can measure the power levels of both CW and modulated signals from 10 MHz to 18 GHz over a 60-dB dynamic range.

And the model PWR-6GHS USB SmartPower Sensor from Mini-Circuits stresses the simplicity of using a USB-equipped computer for high-frequency power measurements. With a frequency range of 1 MHz to 6 GHz and dynamic range of -30 to +20 dBm, it also comes equipped with measurement software and is a plug-and-play, ready-to-go USB power sensor with no driver needed. In addition, no calibration is required prior to making a measurement.

When pulse profiling is important, one of the widest USB-based power-measurement ranges comes from the model LB480A USB power meter/sensor from LadyBug Technologies LLC. In addition to the wide dynamic range, it offers a wide frequency range of 100 MHz to 10 GHz (Fig. 2) and can be used to measure peak, pulsed, and average power levels. The firm also offers USB power sensor/meters for CW/average power measurements with frequency ranges as wide as 10 MHz to 26.5 GHz.

Telemakus offers a variety of USB-based test instruments and components, including power meters/sensors, synthesized signal generators, digital attenuators, and switch modules. The model TED6000-50 power meter/sensor is a true root-mean-square (RMS) power meter with dynamic measurement range of -40 to +10 dBm at frequencies from 50 MHz to 6 GHz. It draws about 150 mA at +5 VDC from a USB port.

Vaunix Technology Corp. provides the test signals via USB control, with its Lab Brick line of compact USB test instruments. Its LMS-203 and LMS-163 model signal generators cover 10 to 20 GHz and 6 to 16 GHz, respectively, with 100-s switching speed ad 100-Hz frequency resolution. The two signal generators provide at least +10 dBm output power with typical harmonic levels of -40 dBc and spurious levels of -80 dBc.

That workhorse of all test instruments (next to the voltmeter), the oscilloscope, has also benefitted from the USB connection, such as the DSO-8000 series of USB oscilloscopes from Link Instruments. The instruments are capable of single-shot sampling rates to 500 MSamples/s and repetitive sampling rates to 20 GSamples/s. Model DSO-8500 features a 100-MHz bandwidth and can also function as a 250-MHz fast-Fourier-transform (FFT) analyzer.

Novatek Instruments also offers a wide range of USB-based scopes, including its 40-MHz, two-channel model DSO-2090 USB and its 200-MHz model DSO 5200 USB. The former provides 8-b resolution with sampling rates to 100 MSamples/s, while the latter has 9-b resolution over two channels with sampling to 200 MSamples/s and equivalent-rate sampling to 50 GSamples/s. Both use the computer's screen to show the familiar scope display.