"Tone generator" redirects here. For an electronic musical instrument, see Sound module
A signal generator is one of a class of electronic devices that generates electrical signals with set properties of amplitude, frequency, and wave shape. These generated signals are used as a stimulus for electronic measurements, typically used in designing, testing, troubleshooting, and repairing electronic or electroacoustic devices, though it often has artistic uses as well. [1]
There are many different types of signal generators with different purposes and applications and at varying levels of expense. These types include function generators, RF and microwave signal generators, pitch generators, arbitrary waveform generators, digital pattern generators, and frequency generators. In general, no device is suitable for all possible applications.
A signal generator may be as simple as an oscillator with calibrated frequency and amplitude. More general-purpose signal generators allow control of all the characteristics of a signal. Modern general-purpose signal generators will have a microprocessor control and may also permit control from a personal computer. Signal generators may be free-standing self-contained instruments, or may be incorporated into more complex automatic test systems.
History
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In June 1928, the General Radio 403 was the first commercial signal generator ever marketed. It supported a frequency range of 500 Hz to 1.5 MHz.[2] Also, in April 1929, the first commercial frequency standard was marketed by General Radio with a frequency of 50 KHz.[3]
General-purpose signal generators
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Function generator
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A function generator is a device which produces simple repetitive waveforms. Such devices contain an electronic oscillator, a circuit that is capable of creating a repetitive waveform. (Modern devices may use digital signal processing to synthesize waveforms, followed by a digital-to-analog converter, or DAC, to produce an analog output.) The most common waveform is a sine wave, but sawtooth, step (pulse), square, and triangular waveform oscillators are commonly available as are arbitrary waveform generators (AWGs). If the oscillator operates above the human hearing range (>20 kHz), the generator will often include some sort of modulation function such as amplitude modulation (AM), frequency modulation (FM), or phase modulation (PM) as well as a second oscillator that provides an audio frequency modulation waveform.
Arbitrary waveform generator
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An arbitrary waveform generator (AWG or ARB) is a sophisticated signal generator that generates arbitrary waveforms within published limits of frequency range, accuracy, and output level. Unlike a function generator that produces a small set of specific waveforms, an AWG allows the user to specify a source waveform in a variety of different ways. An AWG is generally more expensive than a function generator and often has less bandwidth. An AWG is used in higher-end design and test applications.
RF and microwave signal generators
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RF (radio frequency) and microwave signal generators are used for testing components, receivers and test systems in a wide variety of applications including cellular communications, WiFi, WiMAX, GPS, audio and video broadcasting, satellite communications, radar and electronic warfare. RF and microwave signal generators normally have similar features and capabilities, but are differentiated by frequency range. RF signal generators typically range from a few kHz to 6 GHz, while microwave signal generators cover a much wider frequency range, from less than 1 MHz to at least 20 GHz. Some models go as high as 70 GHz with a direct coaxial output, and up to hundreds of GHz when used with external waveguide multiplier modules. RF and microwave signal generators can be classified further as analog or vector signal generators.
Analog signal generators
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An analog RF signal generatorAnalog signal generators based on a sine-wave oscillator were common before the inception of digital electronics, and are still used. There was a sharp distinction in purpose and design of radio-frequency and audio-frequency signal generators.
RF signal generators produce continuous wave radio frequency signals of defined, adjustable, amplitude and frequency. Many models offer various types of analog modulation, either as standard equipment or as an optional capability to the base unit. This could include AM, FM, ΦM (phase modulation) and pulse modulation. A common feature is an attenuator to vary the signal’s output power. Depending on the manufacturer and model, output powers can range from −135 to +30 dBm. A wide range of output power is desirable, since different applications require different amounts of signal power. For example, if a signal has to travel through a very long cable out to an antenna, a high output signal may be needed to overcome the losses through the cable and still have sufficient power at the antenna. But when testing receiver sensitivity, a low signal level is required to see how the receiver behaves under low signal-to-noise conditions.
RF signal generators are available as benchtop instruments, rackmount instruments, embeddable modules and in card-level formats. Mobile, field-testing and airborne applications benefit from lighter, battery-operated platforms. In automated and production testing, web-browser access, which allows multi-source control, and faster frequency switching speeds improve test times and throughput.
RF signal generators are required for servicing and setting up radio receivers, and are used for professional RF applications.
RF signal generators are characterized by their frequency bands, power capabilities (−100 to +25 dBc), single side band phase noise at various carrier frequencies, spurs and harmonics, frequency and amplitude switching speeds and modulation capabilities.
Audio-frequency signal generators generate signals in the audio-frequency range and above. An early example was the HP200A audio oscillator, the first product sold by the Hewlett-Packard Company in 1939. Applications include checking frequency response of audio equipment, and many uses in the electronic laboratory.
Equipment distortion can be measured using a very-low-distortion audio generator as the signal source, with appropriate equipment to measure output distortion harmonic-by-harmonic with a wave analyser, or simply total harmonic distortion. A distortion of 0.0001% can be achieved by an audio signal generator with a relatively simple circuit.[4]
Vector signal generator
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A vector signal generatorWith the advent of digital communications systems, it is no longer possible to adequately test these systems with traditional analog signal generators. This has led to the development of the vector signal generator, which is also known as a digital signal generator. These signal generators are capable of generating digitally-modulated radio signals that may use any of a large number of digital modulation formats such as QAM, QPSK, FSK, BPSK, and OFDM. In addition, since modern commercial digital communication systems are almost all based on well-defined industry standards, many vector signal generators can generate signals based on these standards. Examples include GSM, W-CDMA (UMTS), CDMA2000, LTE, Wi-Fi (IEEE 802.11), and WiMAX (IEEE 802.16). In contrast, military communication systems such as JTRS, which place a great deal of importance on robustness and information security, typically use very proprietary methods. To test these types of communication systems, users will often create their own custom waveforms and download them into the vector signal generator to create the desired test signal.
Digital pattern generator
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A logic signal generator or data pattern generator or digital pattern generator produces logic signals—that is, logical 1s and 0s in the form of conventional voltage levels. The usual voltage standards are LVTTL and LVCMOS. It is different from a "pulse/pattern generator", which refers to signal generators able to generate logic pulses with different analog characteristics (such as pulse rise/fall time, high level length, ...).
A digital pattern generator is used as stimulus source for digital integrated circuits and embedded systems - for functional validation and testing.
Special purpose signal generators
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A pitch generator and a probe for locating a specific pair of wires amongst many, for example in a punch block.In addition to the above general-purpose devices, there are several classes of signal generators designed for specific applications.
Pitch generators and audio generators
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A pitch generator is a type of signal generator optimized for use in audio and acoustics applications. Pitch generators typically include sine waves over the human hearing range (20 Hz to 20 kHz). Sophisticated pitch generators will also include sweep generators (a function which varies the output frequency over a range, in order to make frequency-domain measurements), multipitch generators (which output several pitches simultaneously, and are used to check for intermodulation distortion and other non-linear effects), and tone bursts (used to measure response to transients). Pitch generators are typically used in conjunction with sound level meters, when measuring the acoustics of a room or a sound reproduction system, and/or with oscilloscopes or specialized audio analyzers.
Many pitch generators operate in the digital domain, producing output in various digital audio formats such as AES3, or SPDIF. Such generators may include special signals to stimulate various digital effects and problems, such as clipping, jitter, bit errors; they also often provide ways to manipulate the metadata associated with digital audio formats.
The term synthesizer is used for a device that generates audio signals for music, or that uses slightly more intricate methods.
Computer programs
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Computer programs can be used to generate arbitrary waveforms on a general-purpose computer and output the waveform via an output interface. Such programs may be provided commercially or be freeware. Simple systems use a standard computer sound card as output device, limiting the accuracy of the output waveform and limiting frequency to lie within the audio-frequency band.
Video signal generator
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A video signal generator is a device which outputs predetermined video and/or television waveforms, and other signals used to stimulate faults in, or aid in parametric measurements of, television and video systems. There are several different types of video signal generators in widespread use. Regardless of the specific type, the output of a video generator will generally contain synchronization signals appropriate for television, including horizontal and vertical sync pulses (in analog) or sync words (in digital). Generators of composite video signals (such as NTSC and PAL) will also include a colorburst signal as part of the output. Video signal generators are available for a wide variety of applications and for a wide variety of digital formats; many of these also include audio generation capability (as the audio track is an important part of any video or television program or motion picture).
See also
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References
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Written by Tit Bin Teo, Technical Engineering Manager
November 1940 News Flash. Disney releases Fantasia with “Fantasound”, a new audio stereophonic sound system.
The HP 200B audio oscillator, one of the earliest applications of a signal generator, was used to calibrate Disney’s breakthrough stereo sound system installed in theaters that showed Fantasia. This ensured audiences were able to enjoy the sounds and music as it was intended, with minimal distortions. But what is a signal generator? Simply put, a signal generator is a source that outputs a signal. This signal can be a basic sinusoidal wave, a pulse, or a modulation signal. Signal generators are also often called signal sources or simply, sources. A signal generator allows you to output signals with various frequencies, amplitudes, and time durations. Many signal generators even allow you to modulate frequency, amplitude, and phase signals.
Another application where signal generators are used is in RF power amplifier testing. In this test, the signal generator outputs fixed RF power across a range of frequencies to the amplifier. The output of the RF power amplifier is measured to determine the output flatness.
Signal generators are also used in modern high-speed communication systems like 5G and 802.11ax. Powerful software is used with the signal generator to generate complex orthogonal frequency division multiplexing (OFDM) signals for transceiver testing.
When testing designs, you want to have certainty in your measurements. When a signal generator outputs a sinewave, you want a sinewave that is as near to ideal as possible. A non-ideal sinewave will carry high levels of phase noise, harmonics, and spurs. High phase noise obscures low-level signals. A high-performance signal generator gives you a sinewave that is nearly ideal, with low phase noise, harmonics, and spurs. Figure 1 shows the fundamental signal (marker 1), harmonics (marker 2 and 3), spurs (marker 4 and 5).
Figure 1: Fundamental signal, harmonics, and spurs.
A signal generator also allows complex signals to be generated from a single, integrated instrument without complex hardware add-ons. Used together with powerful software, complex signals such as Orthogonal Frequency-Division Multiplexing could be generated by a single high-performance signal generator with high fidelity.
A signal generator also has the ability to output accurate power levels. This capability is often used in power amplifier, filter, and attenuator testing.
There are several types of signal generators; they can be classified based on their form factor and capabilities. You can save money by getting the right type of signal generator with just the right performance for your application.
The most common signal generator form factor is the benchtop form factor. These are traditional box instruments that we normally find on benches and in racks. Benchtop signal generators are well-suited for R&D, where analysis and troubleshooting benefit from direct interaction with the instrument via the front panel. Benchtop models range from RF to microwave, and from analog to vector.
More recently, signal generators using the modular PXIe form factor have become available. Modular PXIe signal generators are compact signal generators that occupy only several slots in a PXIe chassis. This compact form factor is ideal for applications that require multi-channel measurement capabilities, fast measurement speed, and a small footprint. They also offer scalability and flexibility to configure solutions with a shared processor, chassis, and other modular instruments. The PXIe vector signal generator uses the same software applications as the benchtop signal generators and provides measurement consistency and compatibility from product development to manufacturing and support.
Signal generators are also classified based on their capabilities.
Analog signal generators supply sinusoidal continuous wave (CW) signals with optional capability to add amplitude modulation, frequency modulation, phase modulation, and pulse modulation. The maximum frequency range for analog signal generators spans from RF to microwave. Most generators feature step/list sweep modes for passive device characterization and calibration.
The newer vector signal generators or digital signal generators have a built-in quadrature, also called IQ modulator, to generate complex modulation formats such as Quadrature Phase-Shift Keying (QPSK) and 1024 Quadrature Amplitude Modulation (QAM). When combined with an IQ baseband generator, virtually any signal can be emulated and transmitted within the information bandwidth supported by the system.
Agile signal generators are optimized for speed to quickly change the frequency, amplitude, and phase of the signal. They also have the unique capability to be phase-coherent at all frequencies, all the time. This attribute, along with extensive pulse modulation and wideband chirp capabilities, is ideal for electronic warfare (EW) and radar applications.
An AWG is used to output any arbitrarily defined waveform. These arbitrary waveforms include cardiac, pulse, sawtooth and other real-life signals. An AWG can also generate sinewaves. 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.
If that is the case, what good is a signal generator if an AWG can generate all the types of waveforms that a signal generator can? A signal generator is used when you need low phase noise or accurate power level in your signal. Low phase noise is critical especially when you have dense constellations in your modulation scheme or if you need to work with low-level signals. Accurate power is critical when testing RF power amplifiers and receiver.
As digital technologies improve, the spurious-free dynamic range (SFDR) and phase noise of modern AWGs is coming close to that of signal generators. This has allowed AWGs to replace signal generators in certain RF applications.
Another key advantage a signal generator has over an AWG is the ease of changing modulation carrier frequency with the turn of a knob. An AWG requires a re-calculation of the waveform if there are changes to the carrier frequency.
We will discuss, in-depth, the differences between AWGs, signal generators, pulse generators, and function generators in an upcoming post. Stay tuned.
Understanding signal generator specifications is critical when determining the right type of source for an application. Specifications are generally divided into three broad categories – frequency, amplitude, and spectral purity.
Range, resolution, accuracy, and switching speed are the main frequency specifications. Range specifies the range of output frequencies that the source can produce. Resolution is the smallest frequency increment of the source. Accuracy represents how close the source’s output frequency is to the set frequency. Switching speed is how fast the output settles down to the new frequency. Switching speed is an important specification for manufacturing because time is cost.
The frequency accuracy of a source is affected by two factors; they are the aging rate of the time base reference oscillator, and the amount of time since the source was last calibrated. The aging rate indicates how fast the reference will drift from its specified value. Let’s say, for example, a signal generator’s 1 GHz reference oscillator has an aging rate of 0.152 ppm (parts-per-million) per year. If this oscillator has not been calibrated for one year, the signal generator’s output frequency will be within 152 Hz off its set frequency. The calculation is shown below.
Frequency Accuracy (Hz) = Output Frequency (Hz) x Aging Rate (ppm/year) x Time since last calibration
Figure 2: Calculating the accuracy of a typical oscillator
Dynamic range, accuracy, resolution, and switching speed are the main amplitude specifications. The dynamic range of a signal generator is the difference between the maximum output power capability and minimum output power capability. The resolution of a source indicates the smallest possible amplitude increment. Switching speed is a measure of how fast the source can change from one amplitude level to another.
Figure 3: Power output accuracy
The specifications associated with spectral purity are often the most difficult to understand. These specifications include phase noise, harmonics, and spurious. Harmonics occur at integer multiples of the output frequency. Spurious signals, also known as non-harmonics, can occur at any frequency. The ideal output is a sine wave at a single frequency. Unfortunately, there are no ideal sources. All signal generators are made with non-ideal components which introduce phase noise and unwanted distortions. Phase noise is a frequency-domain view of the noise spectrum around the oscillator signal; it describes the frequency stability of an oscillator. Phase noise is expressed in dBc/Hz at a certain frequency offset. The unit dBc/Hz represents the noise power contained a 1 Hz bandwidth relative to the power contained in the fundamental frequency. For example, the phase noise of M5182B at 1GHz is <-145 dBc/Hz at 20 kHz offset as shown in Figure 4.
Figure 4: Measured phase noise performance for N5182B
Harmonic spurs are spurious with frequencies that are integer multiples of the fundamental frequency (fo) output. These harmonics are caused by non-linear characteristics of components used in the signal generator. These non-linear components are needed to provide a broad range of output frequencies and power. Harmonics are expressed in dBc. For example, when the second harmonic is specified at less than -30 dBc, this means the second harmonic is at least 30-dB below the output level of the fundamental frequency.
Non-harmonic spurs come from a variety of sources, such as power supply and are typically are lower than -65 dBc. Multipliers are often used in sources to extend the output frequency. This can result in the presence of sub-harmonics, which appears as spurs with frequencies less than the fundamental frequency. Figure 5 shows how each spectral purity components relate to each other.
Figure 5: Spectral purity specifications
Signal generators are versatile instruments capable of providing complex and accurate signals for your RF test needs. Building a solid foundation in signal generator know-how will help you design more effective and efficient test strategies.
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