Signal generators have a clearly defined task in a test system: to emulate the signals that a device under test (DUT) might see during normal operation. At one time, this meant simply sine waves, or pulses, or signals with analog modulation. But as communications formats have developed beyond simple amplitude modulation (AM) and frequency modulation (FM), the requirements for signal generators have become more demanding. With the increasing use of digital modulation formatsand nontraditional signal formats, such as frequency-hopping and ultrawideband (UWB) communicationsnew signal generators must often mimic multitone signals with sophisticated forms of digital modulation. And even the form of signal generators is changing, from traditional rack-mount or benchtop instruments to more compact modules, where a number of discrete sources can now fit in the space that was once occupied by only one signal generator.

Traditional rack-mount signal generators still perform their share of test duties, either as stand-alone instruments or part of an automated system, and most major test equipment manufacturers offer a variety of microwave signal generators, usually broken down by frequency range. Selecting a signal generator calls for matching a unit's performance levels to the needs of an application, with such parameters as frequency range, output power, frequency and level switching speed, and spectral purity often being the keys to making an educated choice.

Of course, no one signal generator can fit the needs of all test applications, and trying to provide enough bandwidth, output power, tuning speed, and spectral purity would result in a price tag that would exceed most (if not all) budgets. As a result, most commercial signal generators are sold as either broadband, general-purpose units (such as 10 MHz to 18 GHz) or "banded" units that cover certain frequency bands of interest (such as 3.7 to 4.2 GHz and 5.9 to 6.4 GHz for satellite-communications and telecommunications applications). This article will focus on more traditional broadband test signal sources; a follow-up installment will examine additional types of signal generators, including arbitrary waveform generators (AWGs) and vector signal generators (VSGs).

Among signal generators in the first category, with the broad tuning ranges, model 2550B (Fig. 1) from Giga-tronics covers a total span of 2 to 50 GHz in one instrument and one (2.4-mm) connector, with an option that extends the frequency range as low as 10 kHz. Other signal generators in the 2500B series are available for frequency ranges of 10 kHz to 2.5 GHz, 2 to 8 GHz, 2 to 20 GHz, 2 to 26.5 GHz, and 2 to 40 GHz, all with 0.001-Hz frequency tuning resolution. The signal generators provide many of the features valued for versatile test applications, including high output power, moderately fast switching speed, and low phase noise. The output power is better than +20 dBm through 20 GHz as a standard feature, and the time required to settle to a new frequency within 500 MHz or less is better than 550 s, with amplitude settling time of better than 500 s. The phase noise is -102 dBc/Hz offset 1 kHz from a 10-GHz carrier and -108 dBc/ Hz offset 10 kHz from a 10-GHz carrier, while harmonics are typically better than -30 dBc through 50 GHz and spurious are typically better than -50 dBc through 50 GHz.

With these numbers for spectral purity, it is important to note that performance levels are a function of frequency, especially when comparing signal generators from different manufacturers. For the model 2550B, for example, harmonics from 100 MHz through 39.6 GHz are typically -50 dBc, with the lesser performance occurring at the upper and lower bandedges. Similarly, spurious performance is as good as -66 dBc through 10.1 GHz and -54 dBc through 39.6 GHz. These performance levels are functions of not only the fundamental frequency oscillator, the reference oscillator, and the frequency synthesis approach used to generate the signal generator's microwave frequencies, but also the multiplication schemes used to produce its highest frequency bands.

Additional broadband benchtop signal generators include the MXG and PSG instruments from Agilent Technologies, the MG37000A series of signal generators from Anritsu, and the R&S SMF100A microwave signal generator from Rohde & Schwarz. All of these signal generators include versions reaching to millimeter-wave frequencies, and with generous output-power levels and outstanding spectral purity. The MXG signal generators, for example, are available in frequency ranges from 100 kHz to 20 GHz, 31.8 GHz, or 40 GHz, all with 0.01-Hz frequency resolution. Standard output power is +11 dBm to 20 GHz and +7 dBm to 40 GHz, with an option to boost these levels as high as +18 dBm to 20 GHz and +12 dBm to 40 GHz.

The frequency settling time or switching speed, which is a measure of the time required to move to within a certain resolution of a new frequency (and when comparing these numbers, it is important to realize that different manufacturers can use different definitions for settling time), is 5 ms or better when operating from a list of programmed frequencies (list mode). As an option, the switching speed can be accelerated to typically 600 s. As typical with many signal generators, the output level can be decreased to about -20 dBm in standard configuration, and to -90 dBm by including an optional internal step attenuator. The phase noise is typically -98 dBc/Hz offset 20 kHz from either a 6- or 12-GHz carrier. Harmonics are typically -60 dBc through 20 GHz and typically -56 dBc through 40 GHz, while spurious levels are typically -50 dBc through 24 GHz and typically -45 dBc through 40 GHz. The MXG signal generators are available with a host of modulation options, including amplitude modulation (AM), frequency modulation (FM), phase modulation, and pulse modulation.

The model E8257D is an example of Agilent's PSG signal generator line, with models covering from 250 kHz to 20 GHz, 31.8 GHz, 40 GHz, 50 GHz, and 67 GHz, all with frequency resolution of 0.001 Hz in CW operation and 0.01 Hz in sweep or list mode. The standard frequency settling time is less than 9 ms, with signal output levels to +20 dBm to 20 GHz. These levels can be decreased to -20 dBm in standard models and to -110 or -135 dBm with optional step attenuators. The phase noise is typically -107 dBc/Hz offset 1 kHz from a 20-GHz carrier, while harmonics are -55 dBc to 20 GHz and spurious content is typically -58 dBc to 40 GHz.

The R&S SMF100A microwave signal generator from Rohde & Schwarz features an easyto- follow front-panel display screen that shows menu choices as well as test system setups (Fig. 2). Standard models operate from 1 to 22 GHz, and can be extended to 100 kHz with an option, as well as to 43.5 GHz with another option. The frequency resolution is 0.001 Hz with 2 ms typical setting time. Output power is about +12 dBm to 20 GHz, and can be decreased to -20 dBm in standard units, and to -130 dBm with an optional internal step attenuator. The phase noise is typically -115 dBc/Hz offset 20 kHz from a 10-GHz carrier and -109 dBc/Hz offset 20 kHz from a 20-GHz carrier. Harmonics are typically -55 dBc to 22 GHz, while spurious levels are typically -56 dBc to 21 GHz.

Another broadband benchtop signal generator, the MG37020A from Anritsu, is designed for applications requiring faster switching speed, such as in automatic-test-equipment (ATE) systems. It spans 10 MHz to 20 GHz with 100-s switching speed and delivers output levels as high as +19 dBm to 10 GHz and +17 dBm to 20 GHz; these levels can be reduced to -110 dBm with an optional step attenuator. The phase noise is typically -92 dBc/Hz offset 1 and 10 kHz from a 10-GHz carrier and -86 dBc/Hz offset 1 and 10 kHz from a 20-GHz carrier. Harmonics are typically -50 dBc to 20 GHz and spurious content is typically -40 dBc to 20 GHz.

To save space on the benchtop, a number of manufacturers have developed more compact signal generators, such as the APSIN20G from AnaPico (www.anapico.com). It measures 4.21 x 6.77 x 8.66 in. (106 x 172 x 220 mm) and weighs just 6 lbs (2.5 kg), but is a full-featured signal generator (Fig. 3) with 0.1-mHz frequency resolution from 10 MHz to 20 GHz (an option extends the lower frequency limit to 100 kHz). The output power can be adjusted from -20 to +7 dBm with 0.01-dB resolution and as low as -90 dBm with an optional attenuator. The tiny signal generator, which includes a Universal Serial Bus (USB) port for connection to a personal computer, runs from +6 VDC and 2.5 A and is supplied with an AC-to-DC adaptor. It can be run by an optional battery for portable test applications.

The APSIN20G achieves phase noise of -100 dBc offset 1 kHz from a 10-GHz carrier and -108 dBc/Hz offset 20 kHz from a 10-GHz carrier. Harmonics are better than -50 dBc to 20 GHz while spurious content is typically -40 dBc to 20 GHz. The frequency switching speed is typically 100 s in list or sweep mode, and the signal generator includes a broad range of modulation formats as standard features, including AM, wideband FM, phase modulation, frequency-shift keying (FSK), phaseshift keying (PSK), frequency chirp, and pulse modulation.

April Instrument also has a reputation for providing large signal-generation performance in small packages through 26.5 GHz. The firm's model 8004, for example, provides 1 MHz resolution from 2 to 26.5 GHz with as much as +8 dBm output power to 25 GHz. The phase noise is -80 dBc/Hz offset 20 kHz from all carriers and the typical spurious content is -60 dBc.

Vaunix Technology Corp. has eliminated the signal-generator display screen by designing their Lab Brick synthesized test sources with a USB port and computer-based graphical user interface (GUI). The GUI software is shipped on a memory stick with each Lab Brick. The compact signal generators, which measure 4.90 x 3.14 x 1.59 in. and weigh less than 1 lb., are available in models for 4 to 8 GHz (model LMS-802), 5 to 10 GHz (model LMS-103), and 8 to 12 GHz (model LMS-123). All feature 100-Hz frequency resolution and 100-s frequency switching speed with +10 dBm output power. They offer pulse modulation and can perform phase-continuous frequency sweeps.

Finally, when it comes to making a signal generator small, and with multiple purposes, the NI PXI-565x line of signal generators from National Instruments are based on the PXI modular format so that a compact test system can be constructed in a rack-mount chassis the size of a traditional signal generator. The NI PXI-545x generators cover a frequency range of 500 kHz to 6.6 GHz with better than 2-ms switching speed and a host of modulation formats.