Signal generation often precedes signal analysis and, as signals used in electronic products grow more complex and even higher in frequency, generating test signals becomes more critical for evaluating those products. Except in analyzers with built-in signal sources, creating test signals is usually the task of a signal generator or arbitrary waveform generator (AWG). Testers currently have a wide choice of test equipment, from small, economical units to more sophisticated, larger signal sources. Selecting the best signal source for a job usually depends on the job. By mapping out test signal performance requirements for an application, the challenge of finding an optimal test source can be greatly simplified.
One goal for any test signal source is to provide signals that emulate those normally processed by an electronic product or system to be tested, whether it is an audio amplifier, a cellular telephone, or a radar. A suitable test signal source should provide signals with accurate, repeatable characteristics that can properly exercise the equipment under test (EUT), such as an amplifier, antenna, or filter. Depending upon what is being tested, the requirements for the test signals will differ, such as frequency and power levels.
Which Generator Type?
Modern signal and function generators build upon a variety of technologies, including analog frequency conversion, frequency multiplication, and digital signal synthesis, to create test signals. Signals are generated in different forms, including continuous wave (CW), pulsed, and differential pairs that form phase-modulated signals. Generators may provide single-frequency outputs, sweep signals across a range of frequencies, or even “hop” quickly from one frequency to the next to create patterns of mixed signals or waveforms to emulate a communications channel. In AWGs and digital waveform generators, higher sampling rates and greater numbers of digital bits yield higher precision when the bits are converted to analog waveforms. More memory will also be needed for each channel to store representations of complex waveforms.
Essential performance parameters for signal and waveform generators include frequency range, frequency resolution, output power, and tuning or switching speed. Additional parameters include swept bandwidth and sweep speeds for swept signal generators and pulse width and pulse rise/fall time for pulse generators. The importance of each parameter will depend upon how a test signal source is used. For example, a signal generator with fast frequency tuning may be ideal for high-volume production testing but it may lack the phase-noise performance needed for testing phased-array antennas and radar systems. Low noise may be desired for some test applications whereas a digital waveform generator may be programmed to provide white noise as the desired test signal in some applications.
Since measurement requirements for CW test signals extend from DC through millimeter-wave frequencies, pico-second pulses, and multi-Gb/s digital and optical waveforms, many different signal sources are required for modern test applications. For example, at audio frequencies (20 Hz to 20 kHz), the Stanford Research Systems DS360 function generator, covers 10 mHz to 200 kHz. It features a direct-digital-synthesis (DDS) architecture with analog and digital elements for high linearity and low distortion. It has typical frequency accuracy of 25 ppm and less than 0.001% total harmonic distortion (THD). The test source generates sine waves, square waves, pink or white noise, and even two-tone signals for intermodulation distortion (IMD) testing. It can perform linear or logarithmic sweeps anywhere within its frequency range.
To demonstrate the capabilities of digital signal generation, the Keysight/Agilent/HP 33622A AWG uses its company’s Trueform technology to transform a 1-GSamples/s sampling rate to a 120-MHz bandwidth with 1-μHz frequency resolution. With as much as 64 MSamples memory available per channel as an option on one- or two-channel versions, the signal source can generate complex waveforms defined in MATLAB, Excel, and other software programs with low jitter of typically 1 ps.
The Rohde & Schwarz SMBV100A vector signal generator (VSG) uses a hybrid combination of analog and digital signal generation to synthesize multiple test signals from 9 kHz to 3.2 GHz, in particular multiple satellite signals such as Global Positioning System (GPS) signals. The VSG can synchronize the timing of multiple signals in amplitude and phase to create complex waveforms over its frequency range. Over a similar frequency range, from 15 MHz to 3.35 GHz, the Keysight/Agilent/HP 81134A pulse pattern generator is tailored for creating complex pulsed waveforms with tight timing precision, achieving transition times of 60 ps or less between the shortest pulses (at the highest frequencies). The timing of the pulses can be set with 1-ps resolution, which translated to a phase resolution of a mere 0.01 deg.
The Keysight/Agilent/HP E8257D PSG signal generators rely on analog frequency synthesis to generate low-noise signals over standard frequency ranges from 250 kHz or 10 MHz to 20 GHz and optionally as high as 67 GHz. They provide single-frequency or swept-frequency outputs with low phase noise and high output power (typically better than +15 dBm through 20 GHz) and level accuracy of typically better than ±1 dB.
Also using analog frequency synthesis, the Anritsu MG3696A microwave signal generator covers a frequency range of 2 to 65 GHz. It delivers as much as +3 dBm output power across the full frequency range with single-sideband (SSB) phase noise of -69 dBc/Hz or better offset 1 kHz from all carriers in its range. It offers impressive frequency tuning resolution of 0.01 Hz and can perform linear and logarithmic frequency sweeps with fast tuning speeds. In CW tuning mode, for example, the frequency switching speed is typically less than 40 ms to be within 1 kHz of the final tuned frequency.
When generating higher frequencies from digital waveform generators, higher sampling rates are required. The single-channel Tektronix AWG70001A AWG operates at sampling rates from 1.5 kSamples/s to 50 GSamples/s to produce pulses and waveforms at bandwidths from DC to 15 GHz. It is possible to generate high-data-rate modulated optical carriers and orthogonal frequency division multiplex (OFDM) modulation used for high-data-rate wireless communications by synchronizing several of these digital signal sources with a common controller and the company’s RFXpress software. The AWG incorporates a variety of interfaces, including GPIB, USB, Ethernet, and VXI, for control with an external computer. With multiple AWGs, not just test signals can be produced but realistic versions of multiple-signal operating environments to better understand the effects of interference and competing signals.
For more information on these units and to check out a variety of equipment from different manufacturers available for sale or rental, visit the Axiom Test Equipment (www.axiomtest.com) website.