Signal generators, also known variously as function generators, RF and microwave signal generators, pitch generators, arbitrary waveform generators, digital pattern generators or frequency generators are electronic devices that generate repeating or non-repeating electronic signals (in either the analog or digital domains). They are generally used in designing, testing, troubleshooting, and repairing electronic or electroacoustic devices; though they often have artistic uses as well.
There are many different types of signal generators, with different purposes and applications (and at varying levels of expense); in general, no device is suitable for all possible applications.
Traditionally, signal generators have been embedded hardware units, but since the age of multimedia-PCs, flexible, programmable software tone generators have also been available.
Contents |
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 audio frequency 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.
Function generators are typically used in simple electronics repair and design; where they are used to stimulate a circuit under test. A device such as an oscilloscope is then used to measure the circuit's output. Function generators vary in the number of outputs they feature, frequency range, frequency accuracy and stability, and several other parameters.
Arbitrary waveform generators, or AWGs, are sophisticated signal generators which allow the user to generate arbitrary waveforms, within published limits of frequency range, accuracy, and output level. Unlike function generators, which are limited to a simple set of waveforms; an AWG allows the user to specify a source waveform in a variety of different ways. AWGs are generally more expensive than function generators, and are often more highly limited in available bandwidth; as a result, they are generally limited to higher-end design and test applications.
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 source modules. RF and microwave signal generators can be classified further as analog or vector signal generators.
Analog Signal Generators are capable of producing CW (continuous wave) tones. The output frequency can be tuned anywhere over their entire frequency range. In addition, 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. Another common feature is a built-in attenuator which makes it possible 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. Analog signal generators have been available for decades from various manufacturers. One of the earliest implementations was the HP200A Audio Oscillator, the first product sold by the Hewlett-Packard Company in 1939.
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 vector signal generators, also known as digital signal generators. 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.
Also known as 'data pattern generator' or more often 'digital pattern generator', this type of signal generators produces logic types of signals - that is logic 1's and 0's in the form of conventional voltage levels. The usual voltage standards are: LVTTL, LVCMOS. As such, they must be distinguished from 'pulse/pattern generators', which refers to signal generators able to generate logic pulses with different analog characteristics (such as pulse rise/fall time, high level length, ...).
Logic signal generators (digital pattern generators) are used as stimulus source for digital integrated circuits and embedded systems - for functional validation and testing.
In addition to the above general-purpose devices, there are several classes of signal generators designed for specific applications.
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 audio frequency range (20 Hz–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 AES-3, 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.
Whilst professional signal generators can be expensive, casual hobbyists can make use of computer programs which generate signals and use the sound card to output the signal as audio. These programs can be fun for experimentation but are often limited by the hardware capabilities of the sound card to generate signals only within the aural band.
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).
New high-speed DACs provide up to 16-bit resolution at sample rates in excess of 1 GS/s. These devices provide the foundation for an AWG with the bandwidth and dynamic range to address modern radio and communication applications. In combination with a quadrature modulator and advanced digital signal processing, high-speed DACs can be applied to create a full-featured vector signal generator with very high modulation bandwidth. Example applications include commercial wireless standards such as Wi-Fi (IEEE 802.11), WiMAX (IEEE 802.16) and LTE, in addition to military standards such as those specified in the Joint Tactical Radio System (JTRS) initiative. Also, broad modulation bandwidth allows multi-carrier signal generation, necessary for testing receiver adjacent channel rejection.
|
|||||