Making the right choice in your selection of a signal generator - Embedded.com

Making the right choice in your selection of a signal generator

Signal generators come in many different forms. The most prevalenttypes of signal generators include arbitrary waveform, function and RF signal generators, as well asbasic analog output modules. These types vary in their features andfunctionality, and are suitable for many different applications.

Arbitrary waveform generators (AWGs)generally provide deep memory, wide dynamic range and high bandwidthto meet the demands of applications such as communications and ICcomponent and system tests. AWGs receive user-defined data from a PCand use this data to generate arbitrary waveforms. An AWG user candownload a list of waveforms that they want to generate into theonboard memory of the AWG. Often, both the actual waveforms and thewaveform sequencing instructions that play them are stored onboard.

To generate a waveform from an AWG, the arbitrary waveform itselfmust first be created. The generation sequence often begins with a TTLhardware trigger. Waveforms are built of individual samples, and thegeneration sample rate is determined by the onboard sample clock.

Figure1: Waveform passes through memory to a DAC, which translates digitalsamples into the desired analog output.

There are modes for deriving the sample clock from the internalsample clock timebase (100MHz VCXO), including direct digital synthesis (DDS)and Div/N clocking, as well as modes to provide external clocks. Thereare several choices for providing the frequency reference for theonboard phase-locked loop.

The waveform passes through memory to a DAC (Figure 1 above ), which translatesdigital samples into the desired analog output waveform. Before theDAC, samples are digitally filtered, after which the analog waveform ispassed through an analog filter. These digital and analog filtersimprove signal quality by increasing the effective sample rate throughinterpolation and removing spurious signals through harmonics low-passfilters. Most often, these filters are software-programmable.

Waveform looping
AWGs allow you to specify waveform segments that the AWG can repeat toconstruct complex waveforms. Because AWGs store waveforms in on-boardmemory, the length of the waveform is limited. Waveform looping helpsgenerate signals with subcomponents that repeat many times. Looping awaveform segment improves memory efficiency and increases the potentialduration of the waveform.

AWGs can also specify waveform stages that each consist of awaveform segment and looping information. They generate each definedwaveform stage sequentially. By combining sequencing and looping, youcan construct highly complex waveforms using minimal memory. AWGs canspecify different waveform segments for each stage, although thetransitions are not necessarily phase-continuous.

Finally, many AWGs have an emulated function generator capability.In this case, when asked to output a standard function waveform, itwill be created in software, downloaded to the AWG and played. This isdifferent from a full DDS technology. Function generators createbuilt-in waveforms such as sine, square or triangle waves at adjustablefrequencies. They do not require continuous input from the computer orlarge memory buffers because the device dynamically generateswaveforms.

Function generators can be either analog- or digital-based.Analog-based function generators use analog hardware to create simplefunctions and are often used when an application calls for a staticsine or square wave at a specified frequency.

Digital-based function generators use DDS, a DAC, signal processingand a one-cycle memory buffer to dynamically create signals. DDS is atechnique for deriving under digital control an analog frequency sourcefrom a single reference clock frequency. DDS produces high-frequencyaccuracy and resolution, temperature stability, wideband tuning, andraid, phase-continuous frequency switching.

Figure2: In a DDS function generator, one complete cycle of the functionwaveform is stored in the memory LUT.

'Divide-by-N'
Many signal generators create clock signals by dividing an internaltimebase by an integer factor. This is called the divide-by-N method.However, this gives a limited set of clock frequencies. AWGs, and evenseveral clock frequency generators, can use DDS to generate clocksignals at very specific update frequencies not available bydivide-by-N clocking.

One complete cycle of the function waveform is stored in the memorylookup table (Figure 2 above ).The phase accumulator keeps track of the current phase of the outputfunction. To output a very slow frequency, the Delta phase, betweensamples would be very small. For example, a slow sine may have a deltaphase of 10 .

Sample 0 of the waveform would be the amplitude of the sine wave at 00 ,sample 1 of the waveform would be the amplitude of the sine wave at 10 ,and so on. All 3600 of the sinusoid, or exactly one cycle,would be output after 360 samples. A faster sine wave may have a ?phaseof 100 . Here, one cycle of a sine wave would be output in 36samples. If the sample rate were constant, the slow sine wave would be10 times slower in frequency than the fast sine wave.

Table1: Signal generator types vary in their features and functionality, andare suitable for many different applications.

Furthermore, a constant Delta phase would entail a constant sinewave frequency output. However, DDS technology allows users to quicklychange the Delta phase of the signal through a frequency list. Functiongenerators can specify a frequency list containing stages that eachconsist of waveform frequency and duration information.

They generate each defined frequency stage sequentially. By creatinga frequency list, you can construct complex frequency sweeps orfrequency- hopping signals. DDS allows function generators to makephase-continuous transitions from one stage to the next.

Vector signalgenerators offer a highly flexible and powerful solution for scientificresearch, communications, consumer electronics, aerospace/defense andIC test applications as well as for emerging areas such assoftwaredefined radio, RFID and wireless sensor networks.

Cindy Ong is Marketing Engineer at NationalInstruments Corp.To read a PDFversion of this story, go to “ Choosethe right signal generator for your apps.”

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