Adding value to arbitrary generators

During the design and production of electronic products, complex circuitry or subsystems need to be tested and often require additional signals from a missing or difficult-to-obtain component or sensor.

These signals can be as simple as an audio frequency or clock signal or more complex like a serial data stream or the signal from an airbag sensor during a crash.

Signal sources to generate and simulate these signals have been around for many years, but their pace of development has been rapid in recent times.

Historically, the task of producing diverse waveforms has been filled by separate, dedicated signal sources, from ultra-pure audio sine-wave generators to multi-gigahertz RF signal generators. Users have often had to custom design or modify a signal source for the specific project.

The advent of digital sampling and digital signal processing have now led to a solution that answers almost any kind of signal-generation need with just one instrument: the arbitrary generator.

Arbitrary generators can be classified as arbitrary/function generators or arbitrary waveform generators. The arbitrary/function generator delivers classic sine, square or other regular waveforms with a high degree of precision and stability, along with basic arbitrary waveforms and sometimes also pulse generation capability.

By comparison, the arbitrary waveform generator can offer a wide range of even complex waveform variations. However, there is a gap between these two classes of instruments, that currently leaves users no alternative but to buy multiple products, not one of which meets their complete needs.

Applications for arbitrary waveform generators are numerous and span a broad range. One benefit of it is that it can replace a ‘real’ source that is unavailable during the test.

Sometimes, the real signal is acquired using an oscilloscope and downloaded into the generator’s memory from where it can be replicated. The arbitrary waveform generator also allows an ideal signal to be augmented with anomalies and impurities that the device under test is expected to encounter in real-world situations.

These generators are also suitable for generating long sequences of complex and/or high-bandwidth signals such as those required to test frequency-agile pulse compression radars. Users with a need for less complex signals have typically used arbitrary/function generators, however these users commonly express a significant number of frustrations with such equipment.

While not necessarily having a need for high-end arbitrary waveform generators, the users of arbitrary/function generators have some key needs that must be satisfied to enable them to perform their work in design, validation or manufacturing test.

A common frustration with existing arbitrary/function generators is that their operation is difficult to learn and easy to forget, causing poor productivity.

Ease of use is also valued by schools and universities where students often work with an instrument only once and are expected to learn from the nature of the experiment. This purpose is deflated if it takes too long to learn the operation of the instrument.

Recent industry surveys have identified a number of key user requirements, which can be summarised as:
• Must provide exactly the right signal and provide a graphical representation to confirm correct set-up;
• Easy to learn and easy to use;
• Easy to understand current status/set-up of machine;
• Reasonably affordable price based on cost/benefit;
• Large frequency range;
• Frequency stability appropriate connector layout;
• Low failure rate;
• Remote interface support of current standards such as USB and/or LAN.

A small footprint is also valued since users often have crowded benches and signal sources are generally used in combination with other test gear such as oscilloscopes, voltmeters, spectrum analysers and power supplies.

Performance requirements are driven by the advent of higher speed CMOS and DAC devices, so that signal generators have to have higher sampling rates and signal bandwidth, finer frequency setting resolution, flexible modulation capabilities, pulses with shorter rise and fall times and higher signal purity.

For example, most embedded processor applications use frequencies beyond 50 MHz and up to 200 MHz or higher.

Arbitrary/function generators have been available at ‘value’ price points for a number of years, putting them on a par with the portable digital oscilloscopes with which they are often used. However, there are some significant price/performance gaps in the market.

There are many arbitrary/function generators covering the frequency range from 10 to 100 MHz and with sampling rates of 100 MS/s to 200 MS/s, but above these ranges users have been forced to use more costly combinations of single-function products such as RF signal generators and pulse generators.

These shortcomings have now been addressed in a new range of instruments with better performance.

The Tektronix AGF3000 series is designed for engineers who test electronic circuits with one or more signal inputs and various waveshapes at size up to 240 MHz, generate arbitrary waveforms up to 2 GS/s, need up to 1 ppm clock stability and desire an intuitive, easy-to-use graphical user interface.

For applications requiring signals with sampling rates of 1000 MS/s or more, the series is claimed to save users up to 75% of the cost of current alternative high-performance gear.

The key to the combination of high performance, compactness and affordable price is a new ASIC known as the GOC (generator on a chip), which combines direct digital synthesis circuitry, a 2 GS/s digital/analog converter, waveform memory, modulation, burst and sweep circuits on a single 0.18 µm CMOS chip.

In addition to the high output frequency, fast sample clock and high-speed pulse waveforms, the instruments have independently adjustable leading and trailing edge speeds for pulse waveforms.

This is especially useful for characterisation of semiconductors and amplifiers possessing unsymmetrical slew rates.

A graphical user interface based on a large colour LCD makes the instruments easy to use – even for infrequent users. The relevant instrument settings are viewable at a single glance, along with a graphical confirmation of the desired waveshape.

USB connectivity provides convenient instrument control and waveform transfers via a USB memory stick between signal source, PC and oscilloscope, to extend the device’s waveform storage to a virtually unlimited capacity.

Users in the computer, communications and consumer industries often need two or sometimes more signals to simulate data and clock, analog and binary control signals, or multiple sensor signals.

These users now have a second channel in the same instrument. Featuring the shortest depth in this instrument category, the generators even fit on crowded test benches. Additional space savings are realised when two or more signals are needed because of the second channel in the same instrument.

Due to the large display and intuitive user interface, operation of the generator is easy to learn, enabling engineers to focus on their design tasks and improve time-to-market rather than spending time learning how to use the instrument.

ArbExpress 2.0 software extends the ease of use to create and edit arbitrary waveforms. The software transfers MatLab files and oscilloscope data. It also allows building equations, using interpreter-based logic and a wide range of functions.

TekMark Australia Pty Ltd
www.tekmark.net.au