Technical Insights > RF + Microwave

Common Signal Sources and Their Differences

2018-11-21  |  11 min read 

Written by Tit Bin Teo, Technical Engineering Manager

What is the Difference between an RF Signal Generator, a Function Generator, an Arbitrary Waveform Generator, and Pulse Generator?

Signal sources are an important component in many test systems and are designed to provide a source of “clean” or near ideal signals so that distortions do not influence the test results. Signal sources are also used to simulate real-world stimuli to test a device’s performance in a realistic environment (Figure 1). There are many types of signal sources, each with unique capabilities and limitations. Choosing the right signal source can be confusing because signal sources have overlapping capabilities and features. For example, can an arbitrary waveform generator (AWG) replace an RF signal generator? Can you use a function generator instead of a pulse generator? We will answer these questions in this post. 
 

The stimulus-analysis test model

Figure 1: The stimulus-analysis test model

Types of Signal Sources 

There are several types of signal sources that generate electrical signals for testing. There is the RF signal generator, commonly referred to as a signal generator, for wireless testing and function generator, for low frequency applications. In addition, the AWG is helpful for general purpose applications and the pulse generator is ideal for high-speed digital communications. Let’s take a closer look at each of these signal sources.

RF Signal Generator

An RF signal generator, or signal generator, is a source that outputs an RF signal (loosely defined as 20 kHz to 300 GHz). The RF signal is often a sinusoidal waveform at a determined frequency and amplitude. The signal generator can modulate this sinusoidal waveform (i.e., carrier signal) to carry information using frequency modulation, amplitude modulation, and phase modulation schemes. Advanced vector signal generators are capable of complex IQ modulation schemes for various digital modulation formats. 

Arbitrary Waveform Generator

AWGs first appeared in the market during the 1980s when available data conversion, memory, and instrument communication technologies made them possible. The main application areas were IQ modulation, radar, and magnetic storage media, and device test. Waveforms in an AWG are usually defined as a series of waypoints across time. Therefore, there is no limit to the type of waveforms an AWG can generate. Due to improved Digital-to-Analog Converter (DAC) performance, AWGs can be used in place of signal generators where power and spectral purity is a not major concern. AWGs have frequency bandwidth ranging from 500 MHz to 25 GHz.
 

Basic block diagram of an AWG

 

Figure 2: Basic block diagram of an AWG

Function Generator

Traditionally, function generators have been the most popular signal sources. They are capable of generating a limited set of waveforms (sine, square, triangle, etc.) and users can adjust many of their characteristics such as frequency, amplitude, DC offset, duty cycle, and symmetry. Many function generators implement basic internal or external modulation such as Amplitude Modulation (AM), Pulse Modulation (PM), and Frequency Modulation (FM), and some of them can even sweep the output frequency in a range of interest. Unlike the AWG, a function generator’s frequency bandwidth is in the hundreds of MHz range. 

Pulse Generator

Pulse generators can provide a train of pulses with controllable parameters such as Pulse Repetition Frequency (PRF), pulse duration, and “high” and “low” voltages. Edge location and rise/fall time can be controlled, sometimes independently in some instruments, so that jitter or Duty Cycle Distortion (DCD) can be emulated. Modern pulse generators are not limited to a repetitive train of pulses, but they can generate a pre-defined sequence of “1s” and “0s” to become genuine serial and parallel data generators. Traditionally, some timing parameters of the pulses such as rise/fall time or jitter were controlled using analog circuitry.

Which Signal Source Should You Use?

Selecting the right signal source depends on the type of application you are using. With technology improvements, the boundaries between instrument capabilities are starting to blur. There are many instances where several types of signal sources could be used. In cases like this, the decision comes down to ease-of-use, allocated budget, and future applications. Let’s look at several common applications to get an idea of the considerations that go into selecting the right instrument.

High-Speed Digital

Pulse generators are used for generating clock, pulse, data, pattern, and pseudorandom binary sequence (PRBS) sources for all applications up to 3.35 GHz. Pulse generators enable you to generate the long data patterns required to test high-speed interfaces, like PCI Express or Serial ATA and many more. The jitter-insertion capabilities enable you to conduct jitter tolerance tests. Target applications of pulse generators include physical layer characterization, signal integrity, and jitter tests. While an AWG is capable of generating binary pulses, it cannot match the pulse generator’s capability in generating long data patterns, precise jitter control, and precise rise/fall time control. 

The pulse generator is the ideal source for the following applications:

  • Simulated eye diagram measurements
  • Cross-talk measurements
  • Compliance tests
  • Jitter tests
  • Signal integrity measurements
  • Stress tests for receivers

Pulse parameters of a pulse generator

Figure 3: Pulse parameters of a pulse generator

Wireless

RF signal generators are used when spectral purity is important. Although many generators include the capability to sweep the output frequency and provide basic analog modulation, they are not capable of generating bipolar modulation used for functional testing of modern radio communication systems. Advanced vector signal generator (VSG) permits bipolar modulation of two quadrature carriers (90º phase difference) so that any amplitude/phase modulation state can be achieved through two baseband signals, known as the I (In-Phase) and Q (Quadrature) components. 

With advancements in digital-to-analog converter technologies, AWGs with sample rates close to 100 GSa/s are now commercially available. With their high sampling rates, wide analog bandwidths, and high spectral purity performance, AWGs have successfully replaced RF signal generators in some application areas.

However, RF signal generators still have several advantages over modern AWGs:

  • Better dynamic range and out-of-band spur suppression
  • Better phase noise performance
  • More accurate and wider output power range
  • Ability to adjust carrier frequency easily without waveform re-calculating
  • External modulation inputs for custom I/Q signals
  • Automatic level control circuit to achieve higher amplitude accuracy

AWGs can generate RF signals in different ways

Figure 4: AWGs can generate RF signals in different ways. Traditional vector signal generation relies on two-channel AWGs to generate quadrature baseband signals (I and Q). Modern high-speed AWGs can directly generate one or multiple dissimilar modulated signals

Low Frequency

Function generators, also known as waveform generators, are versatile signal sources. They are simple to use and quick to set up. Function generators are ideal for generating low frequency signals where you need  a “good enough” source for your lab prototypes. Function generators can also create arbitrary waveforms such as cardiac pulses, random noise, mechanical vibrations, and audio signals. Couple that with a low price point and you get the right amount of performance for the right price. If you need low frequency signals of 100 MHz or less, you will not go wrong by using a function generator.
 

Simulation of a noisy signal by superimposing noise on a clean signal with a function generator

Figure 5: Simulation of a noisy signal by superimposing noise on a clean signal with a function generator

Real-life Simulations

AWGs are used in many applications to simulate real-life situations such as radar testing and optical communications. Even though AWGs are versatile signal, they are not the easiest signal to use. Waveform creation tools are required when using an AWG, and their complexity can be overwhelming. Ease-of-use is a key consideration if you are planning to use an AWG instead of the typical signal generator in your test.

Conclusion

The distinctions between signal sources are starting to blur with improvements in Digital-to-Analog (DAC) technologies and software. Selecting the right signal source for your application is no longer a straightforward affair. Specifications are an important consideration when selecting the right instrument. However, ease-of-use, cost, and software type should also inform your decision.

Want to learn more about the key specifications of signal generators and why they matter? Download the Essential Signal Generator Guide and 9 Best Practices for Optimizing Your Signal Generator.