Arbitrary waveform generators (AWGs) are invaluable tools for creating a wide array of signals for testing RF and power electronic circuits. Modern AWGs are typically based on direct digital synthesizer (DDS) technology to convert digital input words into high-speed output waveforms by means of a high-resolution digital-to-analog converter (DAC). A basic AWG can provide a variety of standard waveforms, including pulse, square, ramp, triangle, and sine-wave signals. An AWG with somewhat more functionality and processing power can produce controlled noise outputs, cardiac waveforms, signal bursts, and even advanced modulation formats, including amplitude modulation (AM), frequency modulation (FM), frequency-shift-keying (FSK), and pulse-width-modulation (PWM) formats. The most sophisticated AWGs can also provide digital pa ttern outputs.
While AWGs have a reputation for being expensive and di fficult to use, a number of less costly instruments and programming so ftware have made arbitrary waveform generation much more affordable. Although they are among the most versatile of test signal sources, many engineers avoid AWGs since they may not know how to use them in their application. Many web sites offer the instruments for sale, but without also offering advice on how to specify an AWG. For applications that require a modulated waveform, or a standard waveform function, a low-cost function generator may provide good results, but without the versatility of an AWG. In some cases, commercial AWGs may even integrate a basic function generator for simple waveform generation (see sidebar).
In the example case that follows, it will be shown that an AWG can provide an inexpensive test solution while also helping to expedite the design cycle. The example is based on the use of a low-cost model G5100A AWG from Picotest. The combination AWG and function generator is based on direct-digital-synthesis (DDS) technology. It can produce 50-MHz sine waves, 25-MHz square waves, and 10-MHz arbitrary waveforms (see figure). The engine is a 14-b, 125-MSample/s DDS. The instrument is supported by the company's Wavepa tt Waveform Editor Software, which allows users to create, edit, and download complex waveforms. The software also allows users to retrieve waveforms using a model MSO 8104 oscilloscope from Agilent Technologies.
The test instrument proved invaluable in developing a customer's requirement for a low-cost sine-wave power source. Power sources can be designed for a variety of uses, including providing 115 VAC at 400 Hz for aircraft systems as well as at 50, 60, and 800 Hz for a variety of commercial applications. While there are many possible solutions for such a design, the goal is to find the lowest cost solution that meets or exceeds the performance requirements.
A pulse-width-modulated (PWM) power source was ruled out due to cost and complexity. In order to evaluate the feasibility of four other power source approaches (see table), test waveforms for the four contenders were programmed into the AWG using the supporting software. Each of the waveforms was captured on an Agilent DSO60104A Digital Storage Oscilloscope. The oscilloscope's Fast Fourier Transform (FFT) capabilities were used to evaluate the harmonic content of each waveform, so that the output filter requirements could be evaluated for each of the technologies considered.
The full-bridge approach provided the cleanest output power of the four power sources considered, while requiring only four MOSFET power devices. These MOSFETs are lower-voltage devices with very low drain-source resistance in the on state (RDSon) and very low cost, since they were developed to serve the large, 55-V automotive market. The full-bridge approach also allows the smallest output filter and, in this case, allowed the output inductor to be created from the leakage inductance of the low-frequency transformer. All of the candidate power-source circuits were simulated, and four were constructed for evaluation. The results of the evaluation are summarized in the table.
The full-bridge power stage was the preferred circuit, based on overall performance and cost. The output is very clean and the simulation and measurements are in very good agreement with the results measured using the AWG and the oscilloscope. The AWG file pa ttern was programmed into a read-only memory (ROM) in order to provide the control pulses to the four MOSFETs in the full-bridge source design. The ROM provided 4 b of control, with 1 b for each transistor; the AWG could easily generate the 4-b pa ttern required for evaluating the full-bridge power source.
For the customer's application, the output loading on these inverters is resistive, so that the dynamic output impedance is not a major consideration. In the event that the load was not resistive, PWM technology would perform better, as would a power source based on a pulse-code-modulation (PCM) approach.3 Either of these technologies requires a sine-wave reference signal, which can be provided by using either a clipped triangle wave or the stepped waveform.
As testing revealed, the half-bridge power source is be tter used from a high-voltage isolated bus; in the case of this application, a +350-VDC bus was available. It was found that the half-bridge source does generate significant ninth-harmonic output content, although these harmonics could be further reduced by means of a more complex output filter.
The stepped waveform was achieved by using a digital counter and five resistors, in the process forming a crude digital-to-analog converter (DAC). A commercial DAC integrated circuit (IC) could also be used for the same purpose, although this is a more expensive option. A single operational-amplifier (opamp) filter was used to reduce the harmonics to -50 dBc for a very clean output. The clock frequency was 12 times the output frequency, limiting the upper frequency capability of the circuit.
The AWG proved to be an invaluable design aid, greatly reducing the time required for the trade study phase, while providing a high degree of confidence in the selected approach. The AWG quanti fied the Fourier components so that the output filter requirements could be assessed, prior to the development of the SPICE model and the first power-source prototypes. All of the power-source prototypes that were built were in excellent agreement with the original AWG results as well as the SPICE so ware simulation results.
For more information on the design of this power source and the evaluation of the other power-source options, including schematic diagrams, AWG program files, a brief tutorial on programming different waveforms on the AWG, and additional information about the low-cost G5100A AWG, please visit h ttp://www.picotest.com/articles/AWG-Aids-Design-of-Power-Electronics.html.
References
1. Fink and Christiansen, Electronics Engineers' Handbook, McGraw-Hill, New York, 1982.
2. Steve Sandler and Charles Hymowitz, SPICE Circuit Handbook, McGraw-Hill, New York, 2006.
3. Steve Sandler, Switchmode Power Supply Simulation with PSpice and SPICE 3, McGraw-Hill, New York, 2006, Chapter 7.