TEST: Analog ASIC fuels waveform generator

Beaverton, Ore. — The D/A converter at the heart of Tektronix Inc.'s new AFG3000 series of arbitrary/function generators (AFGs) is a custom analog IC. The "generator-on-a-chip" (GoC) will create the data points for a complex waveform in its memory and then spit them out at 2 Gsamples/second.

Electronics engineers in a broad array of industries are working with increasingly higher speeds, observed Bob Buxton, product line manager within Tektronix's "value line instruments" group. System design and debug often require designers to emulate a specific input signal from a missing component or sensor. In the automotive world, Buxton noted, the waveform generator may emulate the engine bursts detected by anti-knock sensors or the signal generated by an airbag sensor during a crash. In the medical world, the function generator could be called upon to emulate heartbeats.

For such applications, engineers need a general-purpose instrument that's easy to configure and use but doesn't cost an arm and a leg. At less than $10,000, the programmable Tektronix AGF3000 is said to duplicate the many signal patterns of more-expensive, dedicated-purpose instruments.

The six models in the Tektronix AGF3000 series address a variety of needs: The models at the high end of the line will appeal to designers developing high-performance computing, communications equipment or video products. The more basic models will attract users in colleges and trade schools, as well as designers of consumer electronics, medical and automotive systems.

All of them make use of the GoC, a direct synthesis waveform modulator, feeding a 2-Gsample/s 14-bit D/A converter. The device is fashioned in 0.18-micron CMOS.

The choice of a custom ASIC solved many problems of space and heat dissipation (as well as EMI), said Mark Albert, product-marketing manager for the waveform generators. An off-the-shelf D/A converter would offer a 12-bit, 1-Gsamples/s conversion rate but not go much faster, he believes. An off-the-shelf synthesizer, moreover, would likely generate only sine waves.

In operation, arbitrary waveforms are the product of a synthesized square or triangular wave beat against a 250-MHz sine wave. The modulation controller adjusts the phase and frequency of the triangle. A separate CPU chip provides input to the modulator, which gets its data points (for a waveform) out of an on-chip lookup table. Leaving the memory off-chip would have made the generator too slow to support a 2-Gsamples/s conversion rate, Albert said. Having on-chip lookup allows a 250-nanosecond interleave (with clock stability of 1 ppm).

The modulation controller, one of eight on the GoC, generates a 250-MHz output. When the modulator's output is multiplexed with other modulators, the output of the D/A is 2 Gsamples/s. The generator can operate in continuous-, burst- or swept-frequency mode. Its built-in modulation schemes include AM, FM, PM, FSK and PWM.

For applications requiring signals with sampling rates of 1 Gsample/s or more, the AFG3000 can save up to 75 percent of the cost of alternative high-performance products, according to the company.

Tektronix did not describe the break-even points it needed to justify a custom ASIC design. But Buxton said the company has had a lot of experience with that.