Classification and function of signal sources
There are many ways of classifying signal sources, one of which can be divided into mixed signal sources and logical signal sources.The mixed signal source mainly outputs analog waveform; Logical signal source output digital code shape. Mixed signal sources can be divided into function signal generator and arbitrary waveform/function generator, wherein the function signal generator outputs standard waveform, such as sine wave, square wave, etc., and arbitrary wave/function generator outputs any waveform customized by users;The logic signal generator can be divided into pulse signal generator and code type generator, in which the pulse signal generator drives the square wave or pulse wave output of a small number, and the code type generator generates the digital code type of many channels. For example, AFG3000 series produced by tektronix includes function signal generator, arbitrary waveform/function signal generator and pulse signal generator.
In addition, the signal source can also be classified according to the type of output signal, such as rf signal generator, scan signal generator, frequency synthesizer, noise signal generator, pulse signal generator and so on.The signal sources can also be classified according to the frequency band used. The signal sources of different frequency bands correspond to different application fields.
Function generator is the most widely used general signal source, providing sine wave, sawtooth wave, square wave, pulse wave and other waveforms, some also have modulation and scanning functions.
Function waveform generator can be divided into analog type and digital synthesis type.
As is known to all, digital synthesis type function signal generator (DDS) regardless of the frequency, amplitude and signal the signal to noise ratio (S/N) is superior to analog, the design of phase lock loop (PLL) frequency in the output signal is not only a precise, and phase Jitter (phase Jitter) and frequency drift can achieve quite a stable state, but in the digital signal source, the interference between the digital circuit and analog circuit is always difficult to overcome effectively, also on the small signal output than analog function signal generator, now on the market most of function signal generator are the DDS signal source.
Arbitrary waveform generator is a special signal source, which can not only generate general signal source waveform, but also simulate any waveform needed in actual circuit test. In the operation of our actual circuit, due to the existence of various interference and response, the actual circuit often has a variety of defective signals and transient signals, if these conditions are not considered at the beginning of the design, some will have disastrous consequences.Arbitrary wave generator can help you complete the experiment, simulation of the actual circuit, your design for a comprehensive test.
Because the occurrence of arbitrary waveform often depends on the computer communication output waveform data.In computer transmission, waveform is generated by special waveform editing software, which is helpful to expand the ability of instrument and further simulation experiment. In addition, built-in a certain amount of non-volatile memory, random access edit waveform, is conducive to reference comparison, or through random interface communication transmitted to the computer for further analysis and processing. Some arbitrary waveform generators have waveform download function. When doing some troublesome and costly or risky experiments, the waveform is recorded in real time by digital oscilloscope and other instruments, and then transmitted to the signal source through the computer interface. The waveform is directly downloaded to the design circuit for further experimental verification.
B. Main technical specifications of signal source
There are some differences between the main indicators of the traditional function generator and the newly developed arbitrary waveform generator.
1. Main indicators of general function generator:
Bandwidth (output frequency range)
The bandwidth of the instrument is refers to the analog bandwidth, has nothing to do with the sampling rate, the bandwidth of the signal source refers to the output signal frequency range, and in general signal source output sine wave and square wave frequency range, for example, a function generator to generate sine wave frequency range is 1 MHZ to 240 MHZ, and the output pulse frequency range is 1 MHZ to 120 MHZ.
Frequency (timing) resolution
Frequency resolution, the minimum tunable frequency resolution, is the minimum increment of time that can be used to create the waveform.
The deviation between the frequency value displayed by the signal source and the true value is usually expressed by relative error. The frequency accuracy of the low-grade signal source is only 1%, while that of the internal high-stable crystal oscillator can reach 108-1010.For example, a signal source has a frequency accuracy of 1ppm.
Frequency stability refers to the value of the deviation of the output frequency of the signal generator from the set reading within the specified time under the condition that the external environment remains unchanged.Frequency stability is generally divided into long-term frequency stability (long stability) and short-term frequency stability (short stability).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 in any three hours after the signal source has been preheated.
The output impedance of the signal source is the equivalent impedance of the signal source viewed from the output end.For example, the low frequency signal generator output impedance is usually 600 Ω, high frequency signal generator is usually only 50 Ω, television signal generator is usually 75 Ω.
Output level range
Output amplitude is generally expressed by voltage or decibel, refers to the effective range of output signal amplitude.In addition, the output amplitude reading of the signal generator is defined as the condition of matching the output impedance, so the problem of matching the output impedance must be paid attention to.
2. Main indicators of arbitrary wave generator
The sampling rate is usually expressed in megabits per second or gigabits per second, indicating the maximum clock or sampling rate at which the instrument can operate.The sampling rate affects the frequency and fidelity of the main output signals. Nyquist's sampling theorem states that the sampling frequency or clock rate must be at least twice the maximum spectral component of the generated signal to ensure accurate replication.
Storage depth (record length)
Storage depth refers to the number of data points used to record the waveform, which determines the maximum number of sample points (equivalent to time) of waveform data.Each waveform sample point occupies a memory location, and each location is equal to the sampling interval time under the current clock frequency.The bandwidth of any waveform generator is determined by the sampling rate and storage depth of any waveform generator.
Vertical (amplitude) resolution
The vertical resolution of the signal source is 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 mixed sources, the vertical resolution is related to the binary word length of the instrument DAC, and the more bits, the higher the resolution.
C. Main function of signal source
A signal source with strong functions, signal modulation, frequency scanning, TTL synchronous output, reference clock output, Burst and frequency meter, etc.
Signal modulation function
Signal modulation refers to the modulation of the signal, amplitude, phase or frequency change of the low frequency information embedded in the high frequency carrier signal, the signal can be transmitted from voice, to data, to video any signal.Signal modulation can be divided into analog modulation and digital modulation, of which 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 signals to represent binary data.
Frequency sweep function
Measuring the frequency characteristics of electronic devices requires "scanning" sine waves, which change frequency over time.Generally divided into linear (Lin) sweep frequency and logarithm (Log) sweep frequency;Advanced signal generators support sweep function, and can choose the start frequency, hold frequency, stop frequency and related time, some signal generators also provide synchronization with sweep trigger signal.
TTL Synchronous output function
The TTL synchronous signal output by the general signal source is the square wave converted by the triode circuit, and the level is 0 (Low) and 3.6-5v (High).Mainly used to synchronize other signal sources, or other types of instruments, to ensure that the trigger synchronization.
Reference clock output function
TTL synchronous output can only ensure the triggering of synchronization. In order to fully synchronize the signal sources, the clock should be synchronized. The reference clock output is designed to synchronize the clocks of two signal sources.
Similar to the One Shot function, input a TTL signal, the signal source can produce a period of signal output, the design is no signal input, the output can be grounded.
In addition to the simple dial display on the market, whether it is LED digital tube or LCD LCD frequency display, and frequency meter circuit is overlapped.