Find the Right Signal Generator

This tutorial is part of the National Instruments Signal Generator Tutorial series. Each tutorial in this series, will teach you a specific topic of common measurement applications, by explaining the theory and giving practical examples. This tutorial analyzes the difference between the four main types of signal generators.

For additional signal generator concepts, refer to the Signal Generator Fundamentals main page.

Introduction

Signal generator come in many different forms. The most prevalent signal generator types include arbitrary waveform generators, function generators, RF signal generators, and basic analog output modules. The signal generator types vary in their features and functionality and are suitable for many different applications.

	Arbitrary Waveform Generators	Function Generators	RF Vector Signal Generator	Analog Output
General
Bus Type	PXI and PCI	PXI and PCI	PXI	PXI, PCI, and USB
Sampling Rate	Up to 200 MS/s	Up to 300 MS/s	n/a	Up to 1 MS/s
Bandwidth	Up to 80 MHz	Up to 105 MHz	Up to 2.7 GHz	< 1 MHz
DAC Resolution	Up to 16 bits	Up to 16 bits	Up to 16 bits	Up to 24 bits
Memory Depth	Up to 512 MB	Up to 32 kB	Up to 512 MB	n/a
Output Voltage Range	Up to ±6 V	Up to ±5 V	n/a	Up to ±10V
Number of Channels	1	1	1	Up to 8
Dynamic Performance	Best	Best	Best	Good
Clocking	Divide by N, High Res, or External	Internal or External	Divide by N, High Res, or External	Divide by N or External
Advanced Features
Waveform Generation Capabilities	Best	Better	Best	Good
Frequency Hopping and Sweeping	Good	Best	Good	-
Waveform Linking and Looping	Best	-	Best	-
Triggers	Best	Best	Best	Good
Markers	Best	-	-	-
Streaming	Best	-	Best	Good

See Also:
Advanced Aribitrary Waveform Generator Features

Arbitrary Waveform Generators

Arbitrary waveform generators (AWGs) generally provide deep memory, wide dynamic range, and high bandwidth to meet the demands of many applications including communications and semiconductor component and system tests. AWGs receive user-defined data from a PC and use this data to generate arbitrary waveforms. At its basic level, you can think of an AWG as an MP3 player. With an MP3, the user downloads a play list of songs into memory. The songs, and sometimes even the order in which they are played out, is stored on the device. These songs are then generated from the MP3 player. Similarly, an AWG user can download a “play list” of waveforms into the onboard memory of the AWG. Often, both the actual waveforms and waveform sequencing instructions to play those waveforms are stored onboard.

Let’s examine the basic architecture of an AWG:

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To generate a waveform from an AWG, the arbitrary waveform itself must first be created. Software tools such as the Analog Waveform Editor, the Modulation Toolkit, and NI LabVIEW help ease waveform creation. These waveforms and their waveform sequence instructions are stored in onboard RAM.

The generation sequence often begins with a TTL hardware trigger. Waveforms are built of individual samples, and the generation sample rate is determined by the onboard sample clock. There are modes for deriving the sample clock from the internal sample clock timebase (100 MHz VCXO) including DDS clocking an Div/N clocking, as well as modes to provide external clocks. You also have several choices for providing the frequency reference for the onboard phase-locked loop.

The waveform passes through memory to the digital-to-analog converter (DAC) which translates our digital samples into the desired analog output waveform. Before the DAC, samples are digital filtered, and after the DAC, the analog waveform is passed through an analog filter. These digital and analog filters greatly improve signal quality by increasing the effective sample rate through interpolation and removing spurious through harmonics low-pass filters. Most often, these filters are software programmable.

AWGs allow you to specify waveform segments that the AWG can repeat to construct complex waveforms. Because AWGs store waveforms in on-board memory, the length of the waveform is limited. Waveform looping helps generate signals with subcomponents that repeat many times. Looping a waveform segment improves memory efficiency and increases the potential duration of the waveform.

AWGs can also specify waveform stages that each consist of a waveform segment and looping information. The AWG generates each defined waveform stage sequentially. By combining sequencing and looping, you can construct highly complex waveforms using minimal memory. AWGs can specify different waveform segments for each stage, although the transitions 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, it will be created in software, downloaded to the AWG, and played. This is different from a full DDS technology described in the function generator section.

Let’s examine the breadth of arbitrary waveform generator solutions provided by National Instruments:

	NI 5441	NI 5422	NI 5421	NI 5412
General
Bus Type	PXI	PXI	PXI and PCI	PXI and PCI
Sampling Rate	100 MS/s	200 MS/s	100 MS/s	100 MS/s
Bandwidth	43 MHz	80 MHz	43 MHz	20 MHz
DAC Resolution	16 bits	16 bits	16 bits	14 bits
Memory Depth	32, 256, 512 MB options	8, 32, 256, 512 MB options	8, 32, 256, 512 MB options	8, 32, 256 MB options
Output Voltage Range	±6 V	±6 V	±6 V	±6 V
Onboard Signal Processing	Digital Upconversion, On-the fly signal impairments, and more	-	-	-
Function Generator	Full	Emulated	Emulated	Emulated
Max Sine Wave	43 MHz	80 MHz	43 MHz	20 MHz
Max Square Wave	25 MHz	50 MHz	25 MHz	5 MHz
Max Triangle/Ramp Wave	5 MHz	10 MHz	5 MHz	1 MHz
Programmable Filters	Analog and Digital	Analog	Analog and Digital	Digital
Advanced Features
Frequency Hopping and Sweeping	Best	Good	Good	Good
Waveform Linking and Looping	Best	Better	Better	Better
Triggers	Best	Best	Best	Best
General Markers	Best	Best	Best	Good
Databit Marker Event	Best	Best	Best	-
Scripting	Best	Best	Best	Best
Streaming	Best	Best	Best	-

See Also:
NI Arbitrary Waveform Generators

Function Generators

Function generators create built-in waveforms, such as sine, square or triangle waves, at adjustable frequencies. Function generators do not require continuous input from the computer or large memory buffers, because the device dynamically generates waveforms.

Function generators can be either analog or digitally based. Analog based function generators use analog hardware to create simple functions and often are used when an application calls for a static sine or square wave at a specified frequency. Digitally based function generators use direct digital synthesis (DDS), a DAC, signal processing, and a one cycle memory buffer to dynamically create signals. DDS is a technique for deriving, under digital control, an analog frequency source from a single reference clock frequency. DDS produces high frequency accuracy and resolution, temperature stability, wideband tuning, and raid, phase continuous frequency switching.

Many signal generators create clock signals by dividing an internal timebase by an integer factor. This is called the divide-by-N method. Divide-by-N clocking, however, gives a limited set of clock frequencies. AWGs, and even several clock frequency generators, can use DDS to generate clock signals at very specific update frequencies not available by Divide-by-N clocking.

Let’s take a closer look at a typical DDS function generator:


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One complete cycle of the function waveform is stored in the memory lookup table as shown above. The phase accumulator keeps track of the current phase of the output function. To output a very slow frequency, the Δ phase, between samples would be very small. For example, a slow sine may have a Δ phase of 1 degree. Sample 0 of the waveform would be the amplitude of the sine wave at 0 degrees, sample 1 of the waveform would be the amplitude of the sine wave at 1 degree, and so on. All 360 degrees of the sinusoid, or exactly one cycle, would be output after 360 samples. A faster sine wave may have a Δ phase of 10 degrees. Here, one cycle of a sine wave would be output in 36 samples. If the sample rate were constant, the slow sine wave would be 10 times slower in frequency than the fast sine wave.

Furthermore, a constant Δ phase would entail a constant sine wave frequency output. However, DDS technology allows users to quickly change the Δ phase of the signal through a frequency list. Function generators can specify a frequency list containing stages that each consist of waveform frequency and duration information. The function generator generates each defined frequency stage sequentially. By creating a frequency list, you can construct complex frequency sweeps or frequency hopping signals. DDS allows function generators to make phase continuous transitions from one stage to the next.

Let’s look the breadth of function generators and clock frequency generators provided by National Instruments:

	NI 5406	NI 5402	NI 5401	NI 5404	NI 6653
General
Type	Function Generator	Function Generator	Function Generator	Clock and Frequency Generator	Timing and Multi-chassis Synchronization
Bus Type	PXI and PCI	PXI and PCI	PXI and PCI	PXI	PXI
Bandwidth	40 MHz	20 MHz	16 MHz	105 MHz	105 MHz
DAC Resolution	16 bits	14 bits	12 bits	-	-
Output Voltage Range	10 Vpp	10 Vpp	10 Vpp	2 Vpp	2 Vpp
Max Sine Wave	40 MHz	20 MHz	16 MHz	105 MHz	105 MHz
Max Square Wave	40 MHz	20 MHz	1 MHz	105 MHz	105 MHz
Max Triangle/Ramp	5 MHz	1 MHz	1 MHz	-	-
Frequency Resolution	.355 μ Hz	.355 μ Hz	9.31 mHz	1.07 μ Hz	1.1 Hz
Advanced Features
Frequency Hopping and Sweeping	Best	Best	Good	-	-
Triggers	Best	Best	Good	-	-
Arbitrary Function Memory	32 kB	32 kB	32 kB	-	-

See Also:
NI Function Generators

RF Vector Signal Generator

Vector signal generators offer a highly flexible and powerful solution for scientific research, communications, consumer electronics, aerospace/defense, and semiconductor test applications as well as for emerging areas such as software-defined radio, radio-frequency identification (RFID), and wireless sensor networks.

See Also:
NI RF Homepage
RF Vector Signal Generator

Analog Output

NI offers a host of other analog output products which utilize a DAC to generate analog signals. The basic architecture of an analog output board connects a small FIFO memory directly to a DAC. Most of the analog output boards are utilized to generate static voltage levels, and many of them can be used to generate low-frequency waveforms.

See Also:
NI DAQ Homepage
Dynamic Signal Acquisition and Generation
Compact FieldPoint
Compact RIO

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