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Differences Between Function Generators, Arbitrary Function Generators, and Arbitrary Waveform Generators

Primary Software: LabVIEW Development Systems
Primary Software Version: 2016
Primary Software Fixed Version: N/A
Secondary Software: N/A
Hardware: Signal Sources (AWG/FG/AO)

I am looking at NI's options for signal generators, and I noticed that there are three different options: Function Generators, Arbitrary Function Generators, and Arbitrary Waveform Generators. What are the differences between these modules?

Function Generator
Function Generators allow you to generate periodic standard functions such as sine, square, triangle, ramp up/down, DC, and noise. If the function generator has Direct Digital Synthesis(DDS), a single period of the standard waveform is loaded from a look-up table, and the device chooses samples from memory to achieve the periodic signal at precise frequencies instead of generating all samples of the waveform. Because of this, a DDS capable card can achieve very precise frequencies and can create a phase-continuous signal with frequency hopping. In Figure 1, the Configure Standard Waveform VI is used to select which type of standard waveform is going to be generated.

Figure 1. Configuring a Standard Waveform

Arbitrary Function Generator
Arbitrary Function Generators refer to function generators that can generate a periodic, user-defined waveform which typically consists of exactly 16,348 points using DDS.

This waveform can be defined by selecting User as the Standard Waveform (Figure 1) and uploaded to memory using the Define User Standard Waveform VI, seen in Figure 2. Because this waveform is defined as a Standard Waveform, Arbitrary Function Generator should still be configured with the Standard Function output mode when using the Configure Output Mode VI, seen in Figure 3:

Figure 2. Creating a User-Defined Standard Waveform

Arbitrary Waveform Generator
Arbitrary Waveform Generators allow you to generate a user-defined waveform of any size, unlike an Arbitrary Function Generator which can only generate a custom periodic waveform. AWGs operating in Arbitrary Waveform mode only generate each sample of the waveform that is stored in memory in order. While AWGs allow a user to precisely define the waveform to generate in arbitrary waveform mode, they are limited in the frequency precision they can achieve, particularly at high frequencies. Discontinuities may also occur if the frequency or the waveform is modified on the fly.

Some AWGs have DDS capability when in standard function mode, such as the NI 5441, NI 5442, NI 5450 and NI 5451. These AWGs, when programmed for Standard Function mode using the Configure Output Mode VI, have all the benefits of DDS. Other AWGs such as NI 5412, NI 5421 and NI 5422 have a standard function mode that does not utilize DDS. These devices will generate a standard function that is loaded onto the board and generate them in the order that they are stored in memory, rather than specific samples from DDS. These devices are not ideal for applications where a standard function's frequency needs to change on the fly.

Figure 3. Configuring the Output Mode of an NI-FGEN Session

Related Links:
KnowledgeBase 468G7IA0: Frequency Resolution of the FGEN/AWG Devices(NI-54XX Series)


Report Date: 12/26/2013
Last Updated: 09/29/2017
Document ID: 6GPDS1BD

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