Working principle of signal generator
Classification of signal generators
1. Sine wave signal generator
Sinusoidal signals are mainly used to measure the frequency characteristics, nonlinear distortion, gain and sensitivity of circuits and systems. It is divided into low-frequency signal generator, high-frequency signal generator and microwave signal generator according to frequency coverage. According to the adjustable range and stability of the output level, it can be divided into simple signal generator (signal source), standard signal generator (output power can be attenuated to less than -100 db milliwatt accurately) and power signal generator (output power reaches more than dozens of milliwatts). According to the mode of frequency change, it is divided into tuned signal generator, sweep-frequency signal generator, program-controlled signal generator and frequency synthesis signal generator.
2. Low-frequency signal generator
Sinusoidal wave generators including audio (200 ~ 20000 hz) and video (1 hz ~ 10 MHZ) range. Main vibration level generally used RC oscillator, can also be used as a difference frequency oscillator.In order to test the frequency characteristics of the system, the output amplitude-frequency characteristics and waveform distortion are required to be small.
3. High-frequency signal generator
Frequency of 100 KHZ to 30 MHZ frequency, 30 ~ 300 MHZ VHF signal generator, generally USES the LC tuned oscillator, the frequency can be read by tuning capacitor dial scale, the main purpose is to measure various technical indicators of receiver, the output signal can be used inside or plus low frequency sinusoidal signal amplitude modulation or frequency modulation, the output frequency voltage to below 1 microvolt, high frequency signal generator output signal level can be accurate readings, adjustable amplitude or frequency can also be used for electricity meter reading. In addition, the instrument has good shielding against signal leakage.
4. Micro-wave signal generator
The signal generator from the split-meter wave to millimeter-wave band is usually generated by uhf triode with distributed parameter resonator cavity and reflected klystron, but it is gradually replaced by microwave transistor, field effect tube, genn diode and other solid devices. nstruments generally rely on mechanical tuning cavity to change frequency, each covering about an octave, signal power by coupled cavity generally can reach more than 10 milliwatts, requires only simple signal source can add 1000 HeFang wave amplitude modulation, while standard signal generator will output the benchmark level adjustment to one milliwatt, again from the back with the attenuator db milliwatts of the readout signal value; It is also necessary to have an internal or external rectangular pulse amplitude modulation to test receivers such as radar.
5. Frequency sweep and program control signal generator
Sweep frequency signal generator can generate signals with constant amplitude and linear variation of frequency within a limited range. In high frequency and VHF segments, low-frequency sweep voltage or current control oscillator circuit elements (such as capacitive tube or magnetic core coil) can realize sweep frequency oscillation. At the early stage of the microwave section, the voltage tuned sweep frequency was used to change the oscillation frequency by changing the dc voltage of the helical electrode of the backwave tube. Later, the magnetically tuned sweep frequency was widely used. YIG ferrite ball was used as the tuning loop of the microwave solid oscillator, and the scanning current was used to control the dc magnetic field to change the resonant frequency of the ball.Frequency sweep signal generator has automatic frequency sweep, manual control, program control and remote control.
6. Frequency synthesis signal generator
The signal of this generator is not directly generated by the oscillator, but takes the quartz oscillator with high stability as the standard frequency source, and USES the frequency synthesis technology to form the signal of any frequency needed, which has the same frequency accuracy and stability as the standard frequency source. The output signal frequency can usually be selected by decimal digits, up to extremely high resolution of 11 digits. Besides manual frequency selection, it can also be controlled by program and remote control, and it can also be stepper frequency sweeping, which is suitable for automatic test system.The direct frequency synthesizer is composed of crystal oscillation, addition, multiplication, filtering and amplifying circuits. The conversion frequency is fast but the circuit is complex. The highest output frequency can only reach about 1000 MHZ. The more commonly used indirect frequency synthesizer USES a standard frequency source to control an electrically tuned oscillator through a phase-locked loop (in the loop at the same time to achieve frequency multiplication, frequency division and mixing) to generate and output signals of various required frequencies.The highest frequency of the synthesizer can reach 26.5 GHZ. High stability and high resolution of the frequency synthesizer, with a variety of modulation functions (amplitude modulation, frequency modulation and phase modulation), plus amplification, amplitude stabilization and attenuation circuits, constitute a new high performance, programmable synthetic signal generator, can also be used as a phase-locked sweep generator.
Also known as waveform generator. It can produce some specific periodic time function waveform (mainly sine wave, square wave, triangular wave, sawtooth wave and pulse wave) signal. The frequency range can range from ultra-low frequencies of a few millihertz or even a few microhertz to tens of megahertz.In addition to communication, instrument and automatic control system test, also widely used in other non-electrical measurement fields. Schmitt circuit can make the triangular wave rise to a certain threshold or fall to another threshold jump and form a square wave, the frequency can change with the change of RC value in the integrator, but also can control the change of two thresholds by applying voltage. The sine wave can be formed by adding triangle wave to the shaping network composed of many different bias diodes and forming many polyline segments with different inclination. Another form of formation is to use frequency synthesizer to generate sine wave, and then to it many times magnify, clipping and form a square wave, and then integrate the square wave into triangular wave and positive, negative slope of the sawtooth wave. The frequency of these function generators can be controlled by electric control, program control, locking and frequency sweeping. In addition to working in the continuous wave state, the instrument can also work by keystroke control, gate control or trigger.
8. Pulse signal generator
A generator that generates rectangular pulses with adjustable width, amplitude, and repetition rate that can be used to test transient responses of linear systems or to test the performance of radar, multiplex, and other pulse digital systems with analog signals. Pulse generator is mainly composed of main control oscillator, delay stage, pulse forming stage, output stage and attenuator. Usually, after the external trigger signal, a pre-trigger pulse is output first, so as to trigger the oscilloscope and other observation instruments in advance, and then the main signal pulse is output after an adjustable delay time, whose width can be adjusted. Some can output a pair of main pulses, while others can output two main pulses with different delays.
9. Random signal generator
Random signal generator is divided into noise signal generator and pseudo-random signal generator.
10. Noise signal generator
Completely random signal is white noise with uniform spectrum in the working frequency band.
The commonly used white noise generators mainly include: the saturated diode white noise generator which works on the coaxial line system below 1000 MHZ;White noise generator with discharge tube for microwave waveguide system;Solid state noise sources that utilize the reverse current of the crystal diode (which can work in the whole frequency band below 18 GHZ), etc. The output intensity of a noise generator must be known, usually in terms of the decibels of output noise power over the thermal noise of the resistance (known as the ultra-noise ratio) or in terms of its noise temperature. The main purposes of the noise signal generator are: (1) to introduce a random signal in the system to be tested, to simulate the noise in the actual working conditions and determine the performance of the system; (2) to measure the noise coefficient by comparing a given noise signal with the internal noise of the system; (3) Use random signal instead of sine or pulse signal, to test the dynamic characteristics of the system.For example, the impulse response function of the network can be obtained by measuring the cross-correlation between the output signal and the input signal with white noise as the input signal.
11. Pseudo-random signal generator
When measuring correlation function with white noise signal, if the average measuring time is not long enough, statistical error will appear, which can be solved by pseudo-random signal. When the pulse width of binary coded signal T is small enough, and contains a code cycle T number N is large, is lower than fb = 1 / T uniform amplitude spectrum of the signal in frequency band, known as the pseudo random signal. Statistical errors are not introduced as long as the measured time is equal to an integral multiple of the coded signal period. Binary code signals can also provide the time delay required for relevant measurements.The pseudo-random coded signal generator is composed of n-level shift registers with feedback loop, and the generated code length is n =2-1.