Signal Generator Basics and Selection Guide

This article mainly introduces the basic knowledge of signal generators. First, it introduces the classification of common signal generators and briefly explains the characteristics and functions of various signal sources. In addition, it focuses on the main indicators of signal generators and introduces some special functions of existing signal generators.
Keywords: arbitrary waveform generator, function signal generator, frequency resolution, memory depth

1. Introduction A signal source is an electronic instrument that generates signals according to the user's command for its waveform. The signal source mainly provides the required known signals (various waveforms) to the circuit under test, and then uses other instruments to measure the parameters of interest. It can be seen that the signal source does not measure any parameters in electronic experiments and test processing, but simulates various test signals according to the user's requirements and provides them to the circuit under test to meet the test needs. 2. Classification and function of signal sources There are many ways to classify signal sources, one of which can be divided into mixed signal sources and logic signal sources. Among them, the mixed signal source mainly outputs analog waveforms; the logic signal source outputs digital code patterns. Mixed signal sources can be divided into function signal generators and arbitrary waveform/function generators. Among them, function signal generators output standard waveforms, such as sine waves, square waves, etc., and arbitrary waveform/function generators output user-defined arbitrary waveforms; logic signal generators can be divided into pulse signal generators and pattern generators. Among them, pulse signal generators drive a small number of square waves or pulse waves to output, and pattern generators generate digital patterns for many channels. For example, the AFG3000 series produced by Tektronix includes the functions of function signal generator, arbitrary waveform/function signal generator, and pulse signal generator. In addition, signal sources can also be classified according to the type of output signal, such as RF signal generator, sweep signal generator, frequency synthesizer, noise signal generator, pulse signal generator, etc. Signal sources can also be classified according to the frequency band used, and signal sources in different frequency bands correspond to different application fields. Below we will briefly introduce function signal generators and arbitrary waveform/function generators: 1. Function signal generator Function generator is the most widely used general signal source, providing waveforms such as sine wave, sawtooth wave, square wave, pulse wave, and some also have modulation and sweep functions. Function waveform generators are divided into analog and digital synthesis types in design. As we all know, the digital synthesized function signal source (DDS) is superior to the analog type in terms of frequency, amplitude and even signal-to-noise ratio (S/N). The design of its phase-locked loop (PLL) allows the output signal to be not only accurate in frequency, but also in a fairly stable state in terms of phase jitter and frequency drift. However, in digital signal sources, the interference between digital circuits and analog circuits is always difficult to overcome effectively, which also causes the output of small signals to be inferior to analog function signal generators. Most function signal generators on the market today are DDS signal sources. 2. Arbitrary waveform generator An arbitrary waveform generator is a special signal source that not only has the waveform generation capability of a general signal source, but also can simulate arbitrary waveforms required in actual circuit testing. In the operation of our actual circuits, due to the existence of various interferences and responses, the actual circuits often have various defective signals and transient signals. If these situations are not considered at the beginning of the design, some will have disastrous consequences. An arbitrary waveform generator can help you complete experiments, simulate actual circuits, and conduct comprehensive tests on your design. Since arbitrary waveform generation often relies on computer communication to output waveform data. In computer transmission, waveforms are generated through dedicated waveform editing software, which is conducive to expanding the capabilities of the instrument and further simulating experiments. In addition, a certain amount of non-volatile memory is built in, and waveforms can be randomly accessed and edited, which is conducive to reference comparison, or transmitted to the computer through random interface communication for further analysis and processing. Some arbitrary waveform generators have waveform download functions. When doing some experiments that are troublesome, expensive or risky, the waveforms are recorded in real time through instruments such as digital oscilloscopes, and then transmitted to the signal source through the computer interface, and directly downloaded to the designed circuit for further experimental verification. The AFG3000 series three-in-one signal source launched by Tektronix can complete the above-mentioned functions, and has greatly improved the accuracy and stability of waveform output. It is a new generation of arbitrary waveform generators at the forefront of the industry. 3. Main technical indicators of signal sources The main indicators of traditional function generators and the main indicators of newly developed arbitrary waveform generators are somewhat different. We introduce them separately here. (I) Main indicators of ordinary function generators: Bandwidth (output frequency range) The bandwidth of the instrument refers to the analog bandwidth, which has nothing to do with the sampling rate, etc. The bandwidth of the signal source refers to the range of the output frequency of the signal, and generally speaking, the frequency ranges of the sine wave and square wave output by the signal source are inconsistent. For example, the frequency range of a sine wave generated by a function generator is 1mHz to 240MHz, while the frequency range of the output square wave is 1mHz to 120MHz. Frequency (timing) resolution Frequency resolution, that is, the minimum adjustable frequency resolution, is the minimum time increment that can be used when creating a waveform. Frequency accuracy The deviation between the frequency value displayed by the signal source and the true value is usually expressed as a relative error. The frequency accuracy of a low-end signal source is only 1%, while the frequency accuracy of an internal high-stability crystal oscillator can reach 108 to 1010. For example, the frequency accuracy of a certain signal source is 1ppm. Frequency stability Frequency stability refers to the deviation of the output frequency of the signal generator relative to the set reading within a specified time when the external environment remains unchanged. Frequency stability is generally divided into long-term frequency stability (long stability) and short-term frequency stability (short stability). Among them, short-term frequency stability refers to the maximum change of signal frequency within 15 minutes after preheating; long-term frequency stability refers to the maximum change of signal frequency within any three hours after the signal source has been preheated. Output impedance The output impedance of the signal source refers to the equivalent impedance of the signal source as seen from the output end. For example, the output impedance of a low-frequency signal generator is usually 600Ω, the output impedance of a high-frequency signal generator is usually only 50Ω, and the output level range of a TV signal generator is usually 75Ω. Output level range The output amplitude is generally expressed in voltage or decibels, which refers to the effective range of the output signal amplitude. In addition, the output amplitude reading of the signal generator is defined under the condition of output impedance matching, so the problem of output impedance matching must be paid attention to. (II) Main indicators of arbitrary wave generators: sampling (or sampling) rate The sampling rate is usually expressed in megasamples or gigasamples per second, indicating the maximum clock or sampling rate that the instrument can run. The sampling rate affects the frequency and fidelity of the main output signal. Nyquist sampling theorem states that the sampling frequency or clock rate must be at least twice the highest spectral component in the generated signal to ensure accurate reproduction. Memory Depth (Record Length) Memory depth refers to the number of data points used to record a waveform, which determines the maximum number of samples of waveform data (equivalent to time). Each waveform sample occupies a memory location, and each location is equal to the sampling interval time at the current clock frequency. The bandwidth of an arbitrary waveform generator is determined by the sampling rate and memory depth of the arbitrary waveform generator. Vertical (amplitude) resolution

































The vertical resolution of a signal source refers to the minimum voltage increment that can be programmed in the signal source, that is, the binary word width of the instrument's digital-to-analog converter, in bits, which specifies the amplitude accuracy of the waveform. In a mixed signal source, the vertical resolution is related to the binary word length of the instrument's DAC. The more bits, the higher the resolution.
4. The main functions of a signal source A more powerful signal source also has functions such as signal modulation, frequency scanning, TTL synchronization output, reference clock output, Burst and frequency meter: Signal modulation function Signal modulation refers to the amplitude, phase or frequency changes in the modulated signal to embed low-frequency information into a high-frequency carrier signal. The resulting signal can transmit any signal from voice to data to video. Signal modulation can be divided into two types: analog modulation and digital modulation. Analog modulation, such as amplitude modulation (AM) and frequency modulation (FM), are most commonly used in broadcast communications, while digital modulation is based on two states, allowing the signal to represent binary data. Frequency scanning function Measuring the frequency characteristics of electronic equipment requires "scanning" a sine wave, which changes frequency over a period of time. Generally divided into linear (Lin) sweep and logarithmic (Log) sweep; advanced signal generators support sweep function, and can select the start frequency, hold frequency, stop frequency and related time. Some signal generators also provide trigger signals synchronized with the sweep. TTL synchronization output function The TTL synchronization signal output by the general signal source is a square wave converted by a triode circuit, with a level of 0 (Low) and 3.6~5V (High). It is mainly used to synchronize other signal sources or other types of instruments to ensure trigger synchronization. Reference clock output function TTL synchronization output can only guarantee trigger synchronization. If you want to synchronize the signal source completely, you must synchronize the clock. The reference clock output is designed to synchronize the clocks of two signal sources. Generally, the reference clock output frequency is a square wave signal with a relatively stable frequency. Burst function Similar to the One Shot function, inputting a TTL signal can make the signal source generate a cycle of signal output. The design method is that when there is no signal input, the output can be grounded. Frequency meter In addition to the simple dial display on the market, whether it is an LED digital tube or an LCD liquid crystal display frequency, it overlaps with the frequency meter circuit. Summary Signal generator is one of the basic general instruments and the best tool for electronic engineers to conduct signal simulation experiments. It has a wide range of applications in many fields. This article introduces the basic knowledge of function signal generators to facilitate your selection and understanding of concepts. When purchasing, in addition to the basic indicators of the signal source, we also need to pay attention to its advanced functions, such as waveform editing and downloading functions, communication capabilities with computers, synchronization capabilities of several signal generators, etc. According to the engineer's own application needs, choose a suitable signal generator to conduct comprehensive and real tests for the engineer's design, so that you can have more confidence in the products you develop. References [1] Chen Yongming and Wang Hongchao: "Waveform Generator Based on LabVIEW". Electronic Measurement Technology. October 2006. [2] Bob Buxton, Tektronix: "Adding Value to Arbitrary Waveform Generators". Radio Technology Issue 402. [3] Marine Instruments: "Electronic Test Maintenance Solutions". [4] Tektronix: "Signal Source XYZ". [5] Combs: "Electronic Instrument Handbook". Science Press. First edition in May 2006.