Any instrument that produces a test signal is referred to as a signal source.

Also known as a Signal Generator, it is used to generate electrical test signals with specific parameters required by the circuit under test. When testing, studying or adjusting electronic circuits and equipment, in order to measure some electrical parameters of the circuit, such as measuring frequency response, noise coefficient, voltmeter calibration, etc., it is required to provide electrical signals that meet the specified technical conditions, in order to simulate the excitation signal of the equipment to be tested in actual work. A sinusoidal signal source with known amplitude and frequency is used when measuring the steady-state characteristics of the system. 

When testing the transient characteristics of the system, a rectangular pulse source with known leading edge time, pulse width and repetition period is required. And requires the signal source output signal parameters, such as frequency, waveform, output voltage or power, etc., can be accurately adjusted in a certain range, with good stability, there are output instructions. According to different output waveforms, signal sources can be divided into four categories: sinusoidal signal generator, rectangular pulse signal generator, Function Generator and random signal generator. Sinusoidal signals are the most widely used test signals. This is because the method of generating sinusoidal signal is relatively simple and it is convenient to measure with sinusoidal signal. Sinusoidal signal sources can be divided into several types according to the different working frequency range.