Waveform Generator

What’s a Waveform Generator?

Waveform Generators are electronic test equipment devices that are used to produce electrical waveforms. These waveforms can be either repetitive or single-shot (once only) in which case some kind of triggering source is required (internal or external). The resulting waveforms can be injected into a device under test and analyzed as they progress through the device, confirming the proper operation of the device or pinpointing a fault in the device.

Also known as signal generators, function generators, tone generators or frequency generators, these instruments contain either an internal oscillator or a processor combined with a Digital-to-Analog Converter that enables the waveform to be generated locally and then output.

Arbitrary Waveform Generators

Arbitrary Waveform Generators, usually abbreviated as AWG or ARB, and are distinguished from Waveform Generators in two ways. Instead of producing standard repetitive signals, such as sine, square, triangle waveforms, AWG are able to output complex repetitive or non-repetitive waveforms. Examples include replaying a previously recorded waveform, I&Q (In-Phase and Quadrature) communications waveforms, RADAR signals, speech or to direct position of lasers used to scan the retina in an eye.

The waveforms to be generated can be derived from previously recorded signals, or can be created by defining these as a series of voltage amplitudes on a time reference – in the same way as we constructed simple x/y graphs at school. Today the most popular method to create new waveforms is to use computer software such as MATLAB or LabVIEW.

Arbitrary Waveform Generators are available that are able to generate waveforms with sample rates (output rates) to GigaSamples per second, although Strategic Test specializes in products that are focused on sample rates between 1 kS/s and 125 MegaSamples/s, which means up to 125 million voltage output levels per second and that have deep memory so that longer waveforms can be generated.

An Arbitrary Waveform Generator consists of five main sections:

• An analog output section comprising of one or more Digital-to-Analog Converters (DACs), amplifiers and reconstruction filters. Reconstruction filters are low-pass filters that are used to remove the high-frequency steps on the waveform to produce a smoother signal. Remember that the original waveform is defined as a series of amplitudes against time, so as one point changes to the next there is a step function where the signal either increases or drops immediately similar to the edge of a squarewave. These edges can be a problem because they contain very high frequencies and so usually need to be filtered to reduce the possibility of these corrupting the final output signal. The DAC receives the waveform data from the AWG memory as a series of binary amplitude levels. The Digital-to-Analog Converter then converts these to an analog voltage waveform which is then fed to the output amplifier and reconstruction filter.
• A memory section used to temporarily store binary waveform data prior to transferring it value-by-value to the DAC. Depending on the functionality of the Arbitrary Waveform Generator, once the waveform file has been developed it can be downloaded to the local memory of the Arbitrary Waveform Generator and then output once, or continuously until stopped. This is referred to as the Standard Mode of operation on the Strategic Test AWG cards. If the duration of the waveform that you need to produce is longer than the available AWG memory, then an alternative method is to continuously transfer the waveform file from the host PC to the AWG card for output. This method is known as the FIFO Mode or Streaming Mode on the Strategic Test AWG cards.
• A bus section that is the electrical interface to the host computer. Its purpose is to transfer commands to the AWG to set up the output speed, choice of filter, etc.; to control the AWG during the output process and to receive the waveform data from host PC.
• A clock section that generates a clock signal that determines the sample rate of the DAC(s), provides internal bus synchronization and refreshes the onboard memory devices.
• A control section. This is used to control the previous four sections and transfer the data from bus to the onboard memory.

Arbitrary Waveform Generators – Modular Instruments

AWG’s are available as standalone instruments and as electronic cards that can be integrated with standard PC’s or cards that require an external industrial chassis based on the PXI, CompactPCI, VXI, VME or CAMAC bus standards.

Strategic Test supplies instrument cards based on the PCI Express, PCI-X, PXI and CompactPCI standards.

What about Multiple Instrument Card Synchronization?

Arbitrary Waveform Generators, such as those from Strategic Test are available with a choice or one, two or four channels. Each of these is fed with the sample clock to ensure that all of the DACs output the waveforms synchronously i.e. without time-skew errors between the channel outputs. Or course, each DAC is able to generate totally different waveforms but the sample or output rate must be identical. What would be the situation if more than four output channels were needed and multiple AWG devices had to be used? If the AWGs are PXI cards then these have the natural advantage that clock and trigger synchronization signals are contained on the bus for multiple card synchronization. However, the PCI Express, PCI-X and CompactPCI buses were not originally designed for instrumentation applications and so don’t have this feature.

Strategic Test solves this problem with the Star Hub option. This consists of a small daughterboard that is added to one of the PCIe, PCI-X or cPCI cards, which feeds the master clock and trigger synchronization signals to all the other cards via small flat cables. Star Hub enables up to 16 cards to be perfectly synchronized in one PC or chassis, while the System Star Hub option that is only available on the PCIe and PCI-X cards can also synchronize up to 17 Star Hub enabled PC’s equipped with up to 271 instrument cards.

What Are the Advantages of PCI Express, PCI, PXI or CompactPCI Cards over Standalone Instruments?

If you need the highest sample rates possible, then only standalone Arbitrary Waveform Generators have the capability to output at rates above 1.2 GSamples/s. Below this speed modular instruments based on one of the standard bus formats will usually be more cost-effective or the preferred choice when flexibility, memory depth, number of channels and system size are determining factors.

For example, consider configuring a mixed-signal system consisting of a Digitiser with 4 channels and an Arbitrary Waveform Generator also having 4 channels. Strategic test could supply two PCIe cards that were synchronized with the Star Hub option that would fit in any low-cost good performance PC that can also be used to analysis and report the measured data.

The alternative standalone system would consist of two large chassis that may have synchronization difficulties and would most likely cost more. In addition, these would be using older embedded processors that offer no possibility to upgrade and have a fixed use.