Development, classification and operation of spectrum analyzers

A spectrum analyzer is an instrument that studies the spectral structure of an electrical signal. It is used to measure signal distortion, modulation, spectral purity, frequency stability, and intermodulation distortion. It can be used to measure certain circuits such as amplifiers and filters. The parameter is a versatile electronic measuring instrument. It can also be called a frequency domain oscilloscope, a tracking oscilloscope, an analytical oscilloscope, a harmonic analyzer, a frequency characteristic analyzer or a Fourier analyzer. Modern spectrum analyzers can display analysis results in an analog or digital manner, and can analyze electrical signals in all radio frequency bands from very low frequency to sub-millimeter band below 1 Hz. If the digital circuit and the microprocessor are used inside the instrument, it has the function of storage and calculation; when the standard interface is configured, it is easy to form an automatic test system.

Traditional product

The front-end circuit of the traditional spectrum analyzer is a tunable receiver within a certain bandwidth. The input signal is output by the low-pass filter after being frequency-converted by the inverter, the filtered output is used as the vertical component, and the frequency is used as the horizontal component, and the coordinate map is drawn on the oscilloscope screen. Is the spectrum of the input signal. Since the inverter can

To achieve a wide frequency, such as 30Hz-30GHz, with the external mixer, can be extended to more than 100GHz, spectrum analyzer is one of the widest frequency measurement instruments. Whether measuring continuous or modulated signals, spectrum analyzers are ideal measurement tools. However, the traditional spectrum analyzer also has obvious shortcomings. It can only measure the amplitude of the frequency and lack the phase information, so it belongs to the scalar instrument instead of the vector instrument.

Modern product

A modern spectrum analyzer based on Fast Fourier Transform (FFT) decomposes the measured signal into discrete frequency components by Fourier operation to achieve the same results as traditional spectrum analyzers. The new spectrum analyzer uses a digital method to directly sample the input signal from an analog/digital converter (ADC) and then obtain a spectral distribution map after FFT processing.

In this spectrum analyzer, in order to obtain good instrument linearity and high resolution, the sampling rate of the ADC when data is collected is at least twice the highest frequency of the input signal, that is, the real-time spectrum analysis with the upper frequency limit of 100 MHz. The instrument requires an ADC with a sampling rate of 200 MS/S.

The semiconductor process 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 2 GHz, in order to expand the upper frequency limit, The downconverter is added to the front end of the ADC, and the local oscillator uses a digitally tuned oscillator. This hybrid spectrum analyzer can be extended to frequencies below a few GHz.

The performance of the FFT is characterized by the number of sampling points and the sampling rate. For example, if the input signal is sampled at 1024 points with a sampling rate of 100 KS/S, the highest input frequency is 50 KHz and the resolution is 50 Hz. If the number of sampling points is 2048 points, the resolution is increased to 25 Hz. It can be seen that the highest input frequency depends on the sampling rate, and the resolution depends on the number of sampling points. The FFT operation time is in logarithm with the sampling and the number of points. When the spectrum analyzer needs high frequency, high resolution and high speed operation, high speed FFT hardware or corresponding digital signal processor (DSP) chip is used. For example, the operation time of 1024 points of the 10 MHz input frequency is 80 μs, and the operation time of 1024 points of 10 KHz becomes 64 ms, and the operation time of 1024 points of 1 KHz is increased to 640 ms. When the operation time exceeds 200ms, the response of the screen becomes slower and is not suitable for eye observation. The remedy is to reduce the number of sampling points and reduce the operation time to less than 200ms.

The spectrum analyzer is divided into real-time analysis and sweeping. The former can obtain all the required spectrum information in the actual time when the measured signal occurs and analyze and display the analysis result; the latter needs to complete the repeated information analysis through multiple sampling processes. Real-time spectrum analyzers are primarily used for signal analysis that is non-repetitive and has a short duration. Non-real-time spectrum analyzers are mainly used for the analysis of a certain continuous RF signal and periodic signal from audio to sub-millimeter.

Swept spectrum analyzer

It is a swept superheterodyne receiver with display device, mainly used for spectrum analysis of continuous signals and periodic signals. It works in audio until Asia

The millimeter wave band only shows the amplitude of the signal and does not show the phase of the signal. Its working principle is: the local oscillator adopts a swept oscillator, and its output signal and each frequency component in the measured signal are sequentially subjected to difference frequency conversion in the mixer, and the generated intermediate frequency signal passes through the narrowband filter. After amplification and detection, the video amplifier is applied to the vertical deflection signal of the oscillating tube so that the vertical display on the screen is proportional to the amplitude of each frequency component. The sweep of the local oscillator is controlled by the sawtooth voltage generated by the sawtooth sweep generator. The sawtooth voltage is also used as a horizontal sweep of the oscilloscope so that the horizontal display on the screen is proportional to the frequency.

The working principle is shown in Figure 4(a). Using the swept oscillator as the local oscillator of the superheterodyne receiver, when the selection switch S is set to 1, the sawtooth sweep voltage sweeps the local oscillator I, and the respective frequency components in the input signal are in the mixer. The difference frequency is matched with the local frequency sweeping signal, and they fall into the passband of the first intermediate narrowband filter in turn, are selected by the filter, and are added to the vertical deflection of the oscilloscope after being subjected to secondary frequency conversion, detection, and amplification. The system causes the vertical display on the screen to be proportional to the amplitude of each frequency component. The scanning voltage is simultaneously applied to the horizontal deflection system of the oscilloscope so that the X coordinate of the frequency screen becomes the frequency coordinate and the spectrum of the input signal being analyzed is displayed on the screen. The above mode of operation is performed on the local oscillator I, which is called a "pre-sweep" mode of operation, and has a wide analysis frequency band. When S is set to 2, the frequency sweep can also be performed on the local oscillator II, which is called the "sweep intermediate frequency" mode of operation. At this time, narrowband spectrum analysis can be performed.

Real-time spectrum analyzer

The instrument that extracts the entire spectrum information of the signal for a limited time in the presence of the measured signal and analyzes the results is mainly used to analyze non-repetitive stationary stochastic processes and transient processes with short durations, and can also analyze below 40 MHz. Low frequency and very low frequency continuous signals that show amplitude and phase. The Fourier analyzer is a real-time spectrum analyzer. Its basic working principle is to transform the analyzed analog signal into a digital signal through an analog-to-digital conversion circuit, and then add it to the digital filter for Fourier analysis; it is controlled by the central processor. The orthogonal digital local oscillator generates a digital local oscillator signal that varies in sine and cosine, and is also applied to the digital filter and the measured signal for Fourier analysis. The orthogonal digital local oscillator is a swept oscillator. When its frequency is the same as the frequency in the signal under test, it has an output. After the integral processing, the analysis result is obtained for the waveguide to display the spectrum pattern. The analysis results obtained by the sine and cosine signals of the orthogonal local oscillator are complex numbers and can be converted into amplitude and phase. The results of the analysis can also be sent to a print plotter or connected to a computer via a standard interface.