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Spectrum analyzer and vector signal analyzer

the most commonly used signal testing instrument in laboratories and workshops is electronic oscilloscope. People's thinking is sensitive to the concept of time, and they are connected with time-domain events all the time, but the signals often appear in the form of frequency. Observing the simplest AM carrier signal with an oscilloscope also increases the application cost of the fatigue life experimental machine. It is convenient to hold activities such as the selection and award ceremony of "China's good" material owners ", the annual upstream and downstream innovative product exhibition, the video collection of investment environment, and the exchange of the Council of China Singapore Material Association at different times, It is often difficult to see the modulator clearly when displaying the carrier wave. Three spectral lines are obtained on the screen, namely, the carrier frequency and the modulation frequency around the carrier frequency. The more complex the modulation mode is, the more difficult it is for the electronic oscilloscope to display. The spectrum analyzer production line is generally equipped with a man-machine interface operating system with strong expression ability. The spectrum analyzer is a real representative of the frequency domain instrument. The digital expression method of communicating time and frequency is Fourier transform, which decomposes the time signal into the superposition of sine and cosine curves to complete the process of signal conversion from time domain to frequency domain. The early spectrum analyzer was essentially a sweep receiver. After the frequency conversion of the input signal and the local oscillation signal in the mixer, it passed through a group of parallel band-pass filters with different center frequencies, so that the input signal was displayed on the frequency axis defined by a group of band-pass filters. Obviously, because the band-pass filter is composed of passive components, the spectrum analyzer as a whole is very cumbersome, and the frequency resolution is not high. Since Fourier transform can decompose the input signal into discrete frequency components, it can also play a similar role as a filter. With the help of fast Fourier transform circuit instead of low-pass filter, the composition of the spectrum analyzer is simplified, the resolution is increased, the measurement time is shortened, and the sweep frequency range is expanded. This is the advantage of modern spectrum analyzer. Vector signal analyzer is an instrument that automatically measures the circuit gain and the corresponding signal within the predetermined frequency range. It has an internal swept frequency source or a controllable external signal source. Its function is to measure the gain and phase of the circuit under test that inputs the swept signal, so its circuit structure is similar to that of a spectrum analyzer. The spectrum analyzer needs to measure unknown and arbitrary input frequencies, while the vector signal analyzer only measures its own or controlled known frequencies; The spectrum analyzer only measures the amplitude of the input signal (scalar instrument), while the vector signal analyzer measures the amplitude and phase of the input signal (vector instrument). It can be seen that the circuit structure of vector signal analyzer is more complex than that of spectrum analyzer, and the price is also higher. Modern vector signal analyzers also use fast Fourier transform. The following describes the similarities and differences of words. Spectrum analyzer and FFT spectrum analyzer the circuit of traditional spectrum analyzer is a receiver that can be tuned within a certain bandwidth. The input signal is output by the low-pass filter after down conversion. The filtered output is taken as the vertical component and the frequency is taken as the horizontal component. The plot diagram drawn on the oscilloscope screen is the spectrum diagram of the input signal. Since the frequency converter can reach a wide frequency that can meet the basic requirements of current enterprises in the park, such as 30hz-30ghz, it can be expanded to more than 100GHz with the cooperation of external mixers. The spectrum analyzer is one of the measuring instruments with the widest frequency coverage. Whether measuring continuous signals or modulated signals, spectrum analyzer is an ideal measurement tool. However, the traditional spectrum analyzer also has obvious shortcomings. First, it is only suitable for measuring steady-state signals, not for measuring transient events; Second, it can only measure the amplitude of frequency and lacks phase information, so it belongs to scalar instrument rather than vector instrument; Third, it needs a variety of low-frequency band-pass filters, and it takes a long time to obtain the measurement results, so it is regarded as a non real-time instrument. Since the measured signal can be decomposed into discrete frequency components through Fourier operation to achieve the same results as the traditional spectrum analyzer, a spectrum analyzer based on fast Fourier transform (FFT) appears. This new spectrum analyzer uses digital method to sample the input signal directly by analog-to-digital converter (ADC), and then obtains the spectrum distribution map after FFT processing. Therefore, this spectrum analyzer is also called real-time spectrum analyzer, and its frequency range is limited by ADC acquisition rate and FFT operation speed. In order to obtain good instrument linearity and high resolution, the ADC for signal data acquisition needs a resolution of 12-16 bits. According to the sampling principle, the sampling rate of ADC is at least twice the highest frequency of the input signal, that is, the real-time spectrum analyzer with the upper frequency limit of 100MHz needs ADC to have a sampling rate of 200ms/s. At present, the semiconductor technology level can be made into an ADC with a resolution of 8 bits and a sampling rate of 4gs/s or an ADC with a resolution of 12 bits and a sampling rate of 800ms/s, that is, in principle, the instrument can reach a bandwidth of 2GHz. At this time, the vertical resolution is only 8 bits (256 levels). Obviously, the resolution of 8 bits is too low. Therefore, the real-time spectrum analyzer is applicable to the frequency band below 400MHz bandwidth, and at this time, it has a resolution of more than 12 bits (4096 levels). In order to expand the upper frequency limit, a down converter can be added at the front end of the ADC. The local oscillator adopts a direct digital oscillator. This hybrid spectrum analyzer is suitable for the frequency band below a few GHz. The performance of FFT is characterized by sampling points and sampling rate. For example, if 1024 points of input signal are sampled with a sampling rate of 100ks/s, the maximum input frequency is 50KHz and the resolution is 50Hz. If the number of sampling points is 2048, the resolution is increased to 25Hz. Therefore, the maximum input frequency depends on the full sampling rate, and the resolution depends on the number of sampling points. FFT operation time is logarithmic with the number of sampling points. When the spectrum analyzer needs high-frequency, high-resolution and high-speed operation, high-speed FFT hardware or the corresponding digital signal processor (DSP) chip should be selected. For example, the operation time of 1024 points of 10MHz input frequency is 80 μ s. The operation time of 1024 points at 10kHz becomes 64ms, and the operation time of 1024 points at 1kHz increases to 640ms. When the operation time exceeds 200ms, the response of the screen becomes slow, which is not suitable for eye observation. The remedy is to reduce the number of sampling points to reduce the operation time to less than 200ms. Vector network analyzer for spectrum analysis and electromagnetic interference measurement, spectrum analyzer is a commonly used equipment in communication measurement instruments. Due to its dynamic range of more than 100dB, noise lower than -110dbc/hz, bandwidth of 1hz-100hz, and frequency range of more than 50GHz, it can receive extremely weak signals and distinguish two signals with large amplitude differences. The disadvantage of spectrum analyzer is that it can only display the amplitude of frequency component, not

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