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Generating Realistic Signals For Testing

Jan. 3, 2013
Broadband active and passive components can play vital roles in RF/microwave measurement applications.

Test signals must be accurate, and they must also be stable. Generating RF/microwave test signals used to be a simpler endeavor, since the associated modulation was not as complex. But modern communications systems employ advanced digital modulation formats, and test signal generators must be capable of duplicating the carriers and modulation used in those systems.

In addition, with communications systems based on multiple-input, multiple-output (MIMO) antenna schemes, and with a growing need for creating multitone test signals for checking the linearity of active and passive components and systems, test signal sources must now be available in many forms—from traditional rack-mount units to smaller broadband sources that can be linked together with a common reference clock for multitone test-signal generation.

Modern communications systems attempt to transmit and receive increasing amounts of information through the use of digital modulation schemes, such as amplitude-shift-keying (ASK), frequency-shift keying (FSK), phase-shift keying (PSK), and quadrature amplitude modulation (QAM). The latter can be generated with at least two signal components that are out of phase—such as in-phase (I) and quadature (Q) components—with digital bits transmitted by different relative states or symbols formed by the signal components.

To characterize the many active and passive components in the receiving and transmitting portions of these modern communications systems, test-equipment manufacturers must recreate the types of digitally modulated signals used in the systems, in terms of the bandwidth of interest, amplitude range, and modulation schemes. In terms of test signal generators, output signals are also expected to be extremely accurate and stable, maintaining that accuracy over the operating lifetime of the signal source.

Traditionally, RF/microwave test signal generators have been housed in large enclosures which sat on a benchtop or were mounted in a 19-in. rack with other instruments. Such signal generators are still a large part of many test installations, and still available from some of the more trusted names in RF/microwave test. Among these are Agilent Technologies, Anritsu Co., Giga-tronics, Rohde & Schwarz, and Tektronix.

For example, the E8267D PSG signal generator from Agilent Technologies (Fig. 1) is available with a frequency range of 250 kHz to 44 GHz that can be tuned or swept with 0.001 Hz resolution. It can be set to minimum output power of -130 dBm, with output levels as high as +23 dBm to 20 GHz and +18 dBm to 40 GHz. It provides all the major digital modulation formats with a modulation bandwidth of 160 MHz. Harmonic levels are typically -55 dBc from 2 to 20 GHz and -45 dBc from 20 to 40 GHz. Spurious content is as low as -80 dBc, and single-sideband (SSB) phase noise is typically better than -100 dBc/Hz offset 20 kHz from carriers through 44 GHz, and specified at -124 dBc/Hz through 2 GHz. It is physically large and housed in a traditional rack-mount enclosure, meant to serve as the main (and only) test signal source in a test setup.

1. This signal generator is an example of the traditional benchtop housing. This unit operates from 250 kHz to 44 GHz with a host of modulation capabilities. (Photo courtesy of Agilent Technologies.)

Abandoning tradition, test laboratories and production test departments recently have sought more measurement functionality from a given space in a test rack. As a result, the popularity of smaller, modular test instruments communicating by means of LXI, PXI, VME, VXI, and USB interfaces has grown. By designing test equipment into modular housings that use one of these interface standards to communicate, test instrument suppliers can pack multiple functions into a space once occupied by a single function.

The modular approach makes it possible, for example, to assemble a rack of test equipment with multiple signal generators to generate two-tone or multitone test signals useful for linearity testing of components. These modular collections of instruments are typically controlled by an additional personal computer (PC) under the command of a dedicated measurement program.

One such series of modular signal generators is the NI 565x series from National Instruments, which is based on the PXI format. Several of these test signal modules (Fig. 2) can be slid into a PXI chassis with a control module to create. Signal generator modules are available through 6.6 GHz. By adding frequency upconversion modules, such as the firm’s model NI 5610 module, the frequency range can be extended further. The company also offers PXI arbitrary-waveform-generation (AWG) modules for creating complex modulation formats, as well as a single vector-signal-generator module (model PXIe-5673E) based on the high-speed PXI-Express (PXIe) interface that is capable of producing digitally modulated signals to 6.6 GHz. By combining modules within a PXI chassis, a complete system, rather than a single instrument, can be formed in the same space as the single instrument.

2. This compact module is representative of RF/microwave instrument functions in PXI form. It is one of a series of units capable of generating signals to 6.6 GHz. (Photo courtesy of National Instruments.)

In addition to the transformation from benchtop instruments to modules, modern RF/microwave signal generators are available increasingly as portable and even battery-powered units designed for use in the laboratory as well as for on-site testing. Some of these test signal sources are being developed by firms once thought of as “components suppliers,” which are branching out into other product areas and applying their component knowledge to the design and construction of compact test signal sources.

For example, Hittite Microwave Corp. has developed the portable model HMC-T2270 synthesized signal generator with a range of 10 MHz to 70 GHz–impressive considering it fits in a housing measuring only 12 x 8 x 3 in. (305 x 203 x 76.2 mm) and weighing only 8.25 lbs (3.7 kg). The portable source delivers +29 dBm output power at 1 GHz and +3 dBm output power at 70 GHz. In spite of the small size, it features laboratory-grade performance, with SSB phase noise of -118 dBc/Hz offset 10 kHz from a 1-GHz carrier and -79 dBc/Hz offset 100 kHz from a 67-GHz carrier. The signal generator includes a carrying handle and runs on AC power. It has GPIB, Ethernet, and USB interfaces and is supplied with measurement software for a PC.

The model SSG-4000HP synthesized signal generator from Mini-Circuits lacks the carrying handle, but is similarly compact and provides output signals from 250 to 4000 MHz with a 70-dB dynamic range of -50 to +20 dBm. It is powered by an external +24-VDC supply, uses a USB interface, and is supplied with a compact disc (CD) containing user-friendly graphical user interface (GUI) software for controlling the signal generator with a PC.

Two companies offering small-signal generators that are not components suppliers—AnaPico AG and Vaunix—boast small sources that are suitable for creating multitone sources for such applications as MIMO testing and linearity measurements. The model APSIN20G from AnaPico, for example, is a broadband unit measuring just 172 x 220 x 106 mm but capable of outputs from 9 kHz to 20 GHz (Fig. 3). It generates output levels from -20 to +10 dBm. It includes an internal rechargeable battery module and is powered by an external +15-VDC supply. Although many of these small signal generators are limited in modulation capabilities, the APSIN20G features AM, FM, pulse modulation, FSK, and PSK. It has Ethernet, GPIB, and USB control interfaces and exhibits phase noise of -108 dBc/Hz offset 20 kHz from a 10 GHz carrier.

3. The model APSIN20G signal generator can be controlled by numerous interfaces and provides clean outputs from 9 kHz to 20 GHz. (Photo courtesy of AnaPico.)

Vaunix is one of the few signal generator supplies that offers sources that can be not only controlled by a USB connection, but powered by it as well. Measuring just 4.90 x 3.14 x 1.59 in. (124 x 80 x 40 mm) and weighing less than 1 lb. (0.45 kg), the firm’s Lab Brick LSG and LMS lines of signal generators can also operate by means of a battery or external power supply for non-USB applications (Fig. 4). The LSG series includes models in bands from 20 MHz to 6 GHz while the LMS series is comprised of a number of units in bands from 0.5 to 20.0 GHz.

4. The Lab Brick LSG and LMS families of compact signal generators operate to 20 GHz and can be controlled and powered by USB. (Photo courtesy of Vaunix.)

These higher-frequency Lab Brick sources, while lacking digital modulation capabilities, can be supplied with optional pulse modulation for radar testing. They offer excellent electrical performance in spite of low power consumption. A model LMS-402D, which operates from 1 to 4 GHz, has phase noise of -98 dBc/Hz offset 10 kHz from any carrier, while a model LMS-203, with output signals from 10 to 20 GHz, has phase noise of -75 dBc/Hz offset 10 kHz from any carrier. Both provide fast switching speed of better than 100 μs between frequencies to enhance production-line testing.

A relative signal-generator newcomer, Pronghorn Solutions, has developed a compact, low-power source suitable for using in groups to generate multitone test signals. The firm’s model PHS-3000 operates from 150 MHz to 9 GHz in a housing measuring just 3.5 x 5.5 x 1.25 in. and weighing less than 2.5 lbs. It can be powered by means of 110 VAC using an external power supply and can also run 4 hours on its internal battery. The signal generator can be controlled via its front panel or by built-in USB port and an external PC and software. Frequency resolution is 100 kHz from the front panel and 1 kHz with USB control. Although the PHS-3000, like many of the smaller signal generators, is limited in modulation capabilities, it is available with an option for pulse modulation. It delivers +5 dBm output power through 9 GHz.

These “pocket-sized” signal generators provide clean, basic output signals that can be readily combined through an N-way power divider (where N is the number of tones needed in the test signal), with each signal generator tuned to the desired offset to create a wide range of multiple-tone signals. While generating such signals with traditional benchtop/rack-mount signal generators would be cost-prohibitive (in addition to occupying several full racks in a test environment), these smaller signal generators can often accommodate such multitone testing at the cost of one or two traditional test signal sources.

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