Electronic devices that generate electronic signals

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"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] These categories cover a wide range of applications and functionalities, from simple periodic functions to complex arbitrary waveforms.

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 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

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] In April 1929, the first commercial frequency standard was marketed by General Radio with a frequency of 50 KHz.[3]

General-purpose signal generators

Function generator

A function generator is a device that produces simple repetitive waveforms. Such devices contain an electronic oscillator, a circuit capable of creating a repetitive waveform. Modern devices may use digital signal processing to synthesize waveforms, followed by a digital-to-analog converter (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

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

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

Analog 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 the purpose and design of radio-frequency and audio-frequency signal generators.

RF

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 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 sideband phase noise at various carrier frequencies, spurs and harmonics, frequency and amplitude switching speeds, and modulation capabilities.

AF

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 the 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 analyzer 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

With 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, 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

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 a stimulus source for digital integrated circuits and embedded systems, for functional validation and testing.

Special purpose signal generators

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There are several classes of signal generators designed for specific applications.

Pitch generators and audio generators

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 that varies the output frequency over a range, 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, and 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

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 as freeware. Simple systems use a standard computer sound card as the output device, limiting the accuracy of the output waveform and limiting frequency to lie within the audio-frequency band.

Video signal generator

A video signal generator is a device that 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

• AN/URM-25D signal generator, 1950s hardware still in use today.
• Digital pattern generator, for generating digital (logic) types of signals
• Inductive amplifier, used to find an individual telephone cable pairs

References

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RF Radio Frequency Signal Generator

Radio Frequency, RF or microwave signal generators are used to provide a stimulus for many RF circuits.

Signal Generators includes: RF signal generator basics RF signal generator specs

Signal generator types: Signal Generator Basics Arbitrary waveform generator Function generator Pulse generator

Radio frequency, RF signal generators are an essential item of test instrumentation for any area where RF or microwave test and development is undertaken.

A microwave or RF signal generator provides a signal source that can be used to test the operation of the circuit being tested or developed. Unlike many other items of test equipment, the signal generator does not make any measurements, but provides the right test conditions for other items of test instrumentation to measure the output signals from the unit under test.

A typical RF signal generator

The purpose of the signal generator is to generate a signal with known characteristics: frequency, amplitude, modulation and the like. It is therefore possible to look at the response of the circuit, knowing exactly how it has been exercised.

Often an RF signal generator is used alongside other test instruments like oscilloscopes, spectrum analyzers, power meters, and frequency counters.

Types of RF signal generator

It is possible to design radio frequency signal generators in a variety of ways. Also, with developments that have been made in electronics circuitry over the years, different techniques have evolved.

RF signal generators are an essential tool for RF design engineers. With the growing need for wireless products and general wireless communication of all forms, the need for RF signal generators has grown.

Along with this, the complexity of modern signal generators has increased to enable them to accommodate the ever-increasing complexity of the signals being transmitted for applications from 5G to Wi-Fi, Bluetooth, and many more applications from commercial and private applications to professional and defense.

The capabilities of the test instruments available have developed considerably in recent years, but the basic concepts remain the same.

It can be said that there are two forms of signal generator that can be used:

• Free running RF signal generators: These RF generators are rarely used these days as their frequency tends to drift. Sometimes low-end simple signal generators utilized one or two transistors and had a very basic level of performance and their cost was affordable for many experimenters. These very basic RF signal generators are now rarely seen these days.
  
  However, high-end free-running RF generators have been made and they have the advantage that the signal produced is very clean and does not have the level of phase noise either side of the main signal that is present on some other radio frequency signal generators.
  
  Some signal generators used a form of frequency-locked loop to provide a means of adding some frequency stability while still retaining the very low levels of phase noise. Again, these are not common these days because the performance of RF signal generators using frequency synthesizer technology has considerably improved.
• Synthesized radio frequency signal generators: Virtually all radio frequency signal generators used today employ frequency synthesizer technology. Using this technique enables frequencies to be entered directly from a keypad, or via remote control and it also enables the output signal to be determined very accurately. The accuracy being dependent upon either an internal reference oscillator that can have a very high degree of accuracy, or the signal can be locked to an external frequency reference which can be exceedingly accurate.
  
  There are two main techniques that are used within synthesized RF signal generators:
  
  • Phase locked loop synthesizer: Phase-locked loop synthesizers are used within most RF signal generators as they enable signals to be generated over a wide range of frequencies with a relatively low level of spurious signals. Phase-locked loop synthesizer technology is well developed and enables high-performance RF signal generators to be produced using them.
  • Direct Digital Synthesizer, DDS: Direct digital synthesis techniques may be used in RF signal generators. They enable very fine frequency increments to be achieved relatively easily. However, the maximum limit of a DDS is normally much lower than the top frequencies required for the signal generator, so they are used in conjunction with phase locked loops to give the required frequency range.

Whatever the type of oscillator used, stability, control, accuracy, and also phase noise are key issues. For many of today’s test instrumentation requirements, the use of frequency synthesizers means that the stability, control, and accuracy are very good. However, phase noise can be an issue in some applications.

RF signal generator operation

In order to understand the operation of a generic microwave or RF signal generator it is useful to understand what is included in terms of a basic block diagram.

Within a modern RF signal generator there are a number of major circuit blocks or sections:

• Oscillator: The most important block within the RF signal generator is the oscillator itself. This can be any form of oscillator, but today it would almost certainly be formed from a frequency synthesizer. This oscillator would take commands from the controller and be set to the required frequency.
• Amplifier: The output from the oscillator will need amplifying. This will be achieved using a special amplifier module. This will amplify the signal, typically to a fixed level. It would have a loop around it to maintain the output level accurately at all frequencies and temperatures. This loop is closely controlled because the accuracy of the final output is dependent upon it.
• Attenuator: An attenuator is placed on the output of the signal generator. This serves to ensure an accurate source impedance is maintained as well as allowing the generator level to be adjusted very accurately. In particular, the relative power levels, i.e., when changing from one level to another are very accurate and represent the accuracy of the attenuator. It is worth noting that the output impedance is less accurately defined for the highest signal levels where the attenuation is less. Levels may often be adjusted in increments of 0.1dB over the range.
• Control: Advanced processors are used to ensure that the RF and microwave signal generator is easy to control and is also able to take remote control commands. The processor will control all aspects of the operation of the test equipment. Also, a large screen and controls are present on many modern signal generators.

RF signal generator functions

Microwave and RF signal generators are able to offer a large variety of functions and facilities these days. These include some that are detailed below:

• Frequency range: Naturally, the frequency range of the RF signal generator is of paramount importance. It