Arbitrary function vs waveform generators

Traditional function generators (FG or SFG) are analogue generators based on voltage-controlled analogue triangle-wave oscillators that deliver limited standard waveforms.

Various other waveforms may be created by shaping the base triangle.

For example, a square wave is created by driving the analogue triangle through a comparator which is switched at the triangle's midpoints.

Another example is distorting the triangle wave via a diode-shaping network to provide a reasonably pure sine wave.

Although analogue generators can output all the standard waveforms throughout the entire frequency range, they have their shortcomings.

Some are associated with poor resolution and accuracy.

Others relate to their limited waveform capabilities which prevent users from capturing, creating and playing real-life waveforms.

Arbitrary/function generators (AFG) are digital generators based on a direct digital synthesis (DDS) system, characterised by a fixed sample clock rate (that allows the use of only one output filter) and fixed lookup table length.

The addressing scheme used in this system is known as a phase accumulator.

An increase in frequency is achieved by skipping addresses in the lookup table, using fewer samples and completing the cycle in fewer steps using fewer addresses.

Subsequent scans through the lookup table rarely duplicate the same precise address pattern, typically producing amplitude modulation effects in the output signal as well as jitter and phase noise problems.

Furthermore, given this architecture, the AFG will always have a very small memory and no memory partition capabilities to enable long real-life waveform or sequencing repetitive patterns.

Arbitrary waveform generators (AWG) are digital memory-based generators with the ability to output through the DAC every shape of waveform, including hand-sketched, or recreate a captured waveform.

With its numerous features and capabilities, an AWG allows users to increase or decrease the amplitude, increase or decrease the frequency, repeat the signal as frequently as needed or modify the waveform in many complex ways.

The AWG features variable sampling rates that can produce perfectly repeatable output wave shapes when generating complex periodic signals.

The frequency of the waveform is determined by the frequency of the sample clock and the number of points in the lookup table.

The basic role to determine the frequency at the output is: sample clock divided by memory points equals frequency.

Either the sample clock frequency, the length of the lookup table or both may be adjusted to obtain the desired output frequency.

With the true AWG, each waveform is repeated precisely.

Being memory based, the AWG enables the user to program its memory by dividing it into segments of data and use any segment individually.

Furthermore, a true arbitrary waveform generator is usually equipped with a sequencer allowing it to link and loop the segments in any manner the user chooses.

Several advance modes are available that enable various ways to output the signal, such as continuous, stepped, single and mixed mode.

Unlike AFGs, multiple AWGs can be synchronised in order to enable multi-channel solutions.

Tabor's Wonder Wave series.

In its Wonder Wave series, Tabor has managed to combine the best of both worlds, incorporating the most outstanding features and capabilities of each generator type.

While being a true arbitrary waveform generator, memory-based with all the memory management capabilities to create complex waveforms, Tabor's innovative design implements a DDS to create all the standard modulation formats and frequency agility capabilities.

The Wonder Wave series's patent pending design is based on a very high level of integration enabling a single generator to perform multiple generator applications among them, signal, function, pulse train, pattern, modulation, sweep, noise and even dedicated video generators.

Because it is a true arbitrary waveform generator it can recreate virtually anything.

While most generator manufactures such as Agilent, Tektronix and others attempt to convince that DDS-based, arbitrary/function generator (AFG) technology with very high sampling rate is the best way to go, they are actually selling unnecessary features.

"Sampling rate is not the most relevant spec - again, this spec does not tell us everything", says Joan Mercade, application engineer at Tektronix.

The fact is that AFGs have only fixed sample rates and their architecture results in 'rarely duplicating the same precise address pattern' resulting in a 'lack of precision in the output'.

Henry Reinecke, former president of Pragmatic Instruments.

It should be understood that these are not traditional function generators at all, nor are they true arbitrary waveform generators.

When we refer to function generators, it is implied that the waves will be output from analogue circuits and allow all the waveforms to be generated throughout the entire frequency range.

In that sense, all these AFGs are actually digital impersonations of true function generators.

In other words, you won't see 20MHz AFG specifying a 20MHz triangle wave simply because it is a digital instrument that must calculate the frequency from a fixed sample clock divided by the number of points.

So being a digital instrument, does it mean that it is a true arbitrary waveform generator? The answer is an unqualified no.

Why? Because having a fixed sample clock and DDS-based architecture prevents it from implementing the large memory and memory management (sequencer) that is the basic and most vital feature for generation of real-life waveforms.

In a true arbitrary waveform generator the clock drives the memory in a loop and every single point in the memory is sampled and the waveform is consistently reproduced exactly the same way for every cycle.

Because there are no skipped or repeated wave points, the jitter and phase noise are minimal.

According to Joan Mercade, application engineer at Tektronix: "in AWGs every sample in the waveform memory is the result of a synthesis process.

"The capability to replicate a real-world signal is limited by the available record length" which is why having a big memory and memory management is critical.

Tabor's conclusion.

The question arises: Are AWGs the 'dream' instruments that the market has been waiting for? Unfortunately no.

The fact is that AWGs have been around for years and although they can create any real-life waveforms, they invariably lack the ability to generate standard waveforms and modulation.

And, they are usually very high-priced.

So what's the solution? Bob Buxton, Tektronix: "Arbitrary waveform generators are ideal for generating long sequences of complex and/or high bandwidth signals, such as those required to test frequency-agile pulse compression radar".

"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".

Until now, customers were forced to settle for a variety of generators that delivered nearly, not quite perfect.

However, with the introduction of Tabor's Wonder Wave series, they won't have to settle for anything less than perfection.

Featuring seven different models, Tabor's Wonder Wave series is designed to solve a myriad of applications - from the simplest need for a standard function to the most demanding requirement to create long, complex and challenging waveforms - all this at an affordable price tag similar to the cost of simple low-end arbitrary/function generators.

The challenge is to first understand the limitations of AFGs and AWGs available in the market.

Only then will you realise that the best and most necessary features are already built in to Tabor's Wonder Wave models.

It is important to remember that customers are not buying an instrument for a single application.

They expect versatility and the ability to meet future needs.