1. Spectrum analyzer
A spectrum analyzer, also known as a spectrum analyzer, is a test and measurement device that is mainly used for frequency domain analysis of radio frequency and microwave signals, including measuring signal power, frequency, and distortion products.
According to the working principle, there are two basic types of spectrum: real-time spectrum analyzer and swept-frequency tuning spectrum analyzer. Real-time spectrum analyzers include multi-channel filter (parallel type) spectrum analyzers and FFT spectrum analyzers. Sweep-frequency tuned spectrum analyzers include swept-frequency tuned spectrum analyzers and superheterodyne spectrum analyzers.
Principle of spectrum analyzer
The real-time frequency analyzer has corresponding filters and detectors for different frequency signals, and then a synchronized multi-task scanner transmits the signals to the CRT screen.
Scanning tuned spectrum analyzer is a local oscillator whose input signal is directly applied to the mixer through the attenuator and then modulated. The scan generator synchronized with the CRT generates an oscillation frequency that changes linearly with time, and is mixed with the input signal through the mixer wave.
2. Oscilloscope
An oscilloscope is a very widely used electronic measuring instrument. It can transform electrical signals that are invisible to the naked eye into visible images, which is convenient for people to study the changing process of various electrical phenomena. The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit a screen coated with a fluorescent substance to produce a small light spot (this is the working principle of a traditional analog oscilloscope). Under the effect of the measured signal, the electron beam is like the tip of a pen, and the curve of the instantaneous value of the measured signal can be drawn on the screen. Using an oscilloscope, you can observe the waveform curves of various signal amplitudes changing with time, and you can also use it to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude adjustment, and so on.
working principle
The oscilloscope uses a narrow electron beam composed of high-speed electrons to hit a screen coated with a fluorescent substance to produce a small light spot. Under the effect of the measured signal, the electron beam is like the tip of a pen, and the curve of the instantaneous value of the measured signal can be drawn on the screen. Using an oscilloscope, you can observe the waveform curves of various signal amplitudes changing with time, and you can also use it to test various electrical quantities, such as voltage, current, frequency, phase difference, amplitude adjustment, and so on.
3. What is the difference between a spectrum analyzer and an oscilloscope
From the four aspects of real-time bandwidth, dynamic range, sensitivity and power measurement accuracy, the difference between the analysis performance indicators of the oscilloscope and the spectrum analyzer is compared.
1. Real-time bandwidth
For an oscilloscope, the bandwidth is usually its measurement frequency range. The spectrum analyzer has bandwidth definitions such as intermediate frequency bandwidth and resolution bandwidth. Here, we take the real-time bandwidth that can analyze the signal in real time as the object of discussion.
For the spectrum analyzer, the bandwidth of the final analog intermediate frequency can usually be used as the real-time bandwidth of its signal analysis. The real-time bandwidth of most spectrum analysis is only a few megahertz, and the wider real-time bandwidth is usually tens of megahertz. The FSW spectrum analyzer with the widest bandwidth can reach 500 MHz. The real-time bandwidth of the oscilloscope is the effective analog bandwidth for real-time sampling, which is generally hundreds of megahertz, and the high can reach thousands of megahertz.
It should be noted here that most oscilloscopes may not have the same real-time bandwidth when the vertical scale is set differently. When the vertical scale is set to the most sensitive, the real-time bandwidth usually decreases.
In terms of real-time bandwidth, oscilloscopes are generally superior to spectrum analyzers, which is especially beneficial for some ultra-wideband signal analysis, especially in modulation analysis has an unparalleled advantage.
2. Dynamic range
The dynamic range index varies depending on its definition. In many cases, the dynamic range is described as the level difference between the maximum and minimum signals measured by the instrument. When changing the measurement settings, the instrument's ability to measure large and small signals is different. For example, when the spectrum analyzer has different attenuation settings, the distortion caused by the measurement of large signals is different. Here, we discuss the ability of the instrument to measure large and small signals at the same time, that is, the best dynamic range of the oscilloscope and spectrum analyzer under the appropriate settings without changing any measurement settings.
For the spectrum analyzer, without considering the near-end noise and spurs such as phase noise, the average noise level, second-order distortion, and third-order distortion are the most important factors that restrict the dynamic range. , Its ideal dynamic range is about 90dB (limited by second-order distortion).
Most oscilloscopes are limited by their AD effective sampling digits and noise floor. The ideal dynamic range of traditional oscilloscopes usually does not exceed 50dB. (For R & S RTO oscilloscope, the dynamic range can be as high as 86dB at 100KHz RBW)
From the perspective of dynamic range, spectrum analyzer is better than oscilloscope. But it should be pointed out here that this is true for the spectrum analysis of the usual signal. However, the spectrum of the oscilloscope is the same frame of data, and the spectrum of the spectrum analyzer is not the same frame of data in most cases. , The spectrum analyzer may not be able to measure. The oscilloscope has a much higher probability of finding transient signals (when the signal meets the dynamic range).
3. Sensitivity
The sensitivity discussed here refers to the minimum signal level that the oscilloscope and spectrum analyzer can test. This index is closely related to the instrument settings.
For the oscilloscope, when the oscilloscope is set to the most sensitive file on the Y axis, usually the minimum signal can be measured by the oscilloscope at 1mV / div. Aside from factors such as port mismatch, the noise and trace generated by the oscilloscope's signal channel are not The noise brought by stability is the most important factor restricting the sensitivity of the oscilloscope.
From Figure 1, we can see that because of the increase in the number of sampling points, the spectrum noise floor can be reduced to an ideal level. However, when the signal cannot be reproduced clearly and accurately in the time domain, a lot of clutter is generated in the frequency domain, which limits our ability to observe small signals.
Most oscilloscopes, as shown in Figure 1, can stably measure 0.2mV signals, corresponding to the frequency domain, which is equivalent to the level of -60dBm. In fact, whether the oscilloscope can accurately measure small signals is not only related to the sensitivity of the vertical system, but also related to the performance of the X-axis jitter and trigger sensitivity.
In order to compare the technical indicators analyzed in the article, I specially went to the R & S company's Chengdu Open Laboratory (thanks to the assistance provided by the Chengdu branch) to compare the indicators. Surprisingly, the RTO oscilloscope is very good in sensitivity indicators.
It can be seen that RTO can accurately measure the signal of -60dBm, and its noise floor is around -80dBm. The most pleasant thing is that in the entire frequency band (DC-4GHz), no large clutter that can affect the sensitivity is found, thereby greatly improving the measurement sensitivity.
In the absence of clutter, lower noise can be obtained by increasing the number of sampling points. For example, as shown in Figure 3, when Span and RBW are set smaller, the noise floor of the RTO oscilloscope can be reduced to below -100dBm.
From this point of view, RTO can definitely allow the surveyor to change the feeling that "the oscilloscope is a tasteless analysis in the frequency domain".
For the spectrum analyzer, the factors such as port mismatch are also discussed. The average noise level of the spectrum analyzer with the maximum gain and the minimum attenuator setting can be regarded as the limit of the spectrum analyzer to measure small signals. Without involving a preamplifier, most good spectrum analyzers can achieve -150dBm.
4. Power measurement accuracy
For frequency domain analysis, power measurement accuracy is a very important technical indicator. Whether it is an oscilloscope or a spectrum analyzer, the amount of influence on the accuracy of power measurement is very large, and the main influences are listed below:
For oscilloscopes, the influence of power measurement accuracy includes: reflection caused by port mismatch, vertical system error, frequency response, AD quantization error, calibration signal error, etc.
For the spectrum analyzer, the influence of power measurement accuracy includes: reflection caused by port mismatch, reference level error, attenuator error, bandwidth conversion error, frequency response, calibration signal error, etc.
Here we do not analyze and compare the influence amount one by one. We compare by measuring the power of the 1GHz frequency signal. The comparison between the RTO oscilloscope and the FSW spectrum analyzer can be seen. At 1GHz, the power measurement value of the oscilloscope and the spectrum analyzer The difference is only about 0.2dB, which is a very good measurement accuracy index. Because the measurement accuracy of the spectrum analyzer at 1 GHz is very good.
In addition, in the frequency range, the frequency response index of the oscilloscope is also very good, not exceeding 0.5dB in the 4GHz range. From this point of view, the oscilloscope is even better than the performance of the spectrum analyzer.
In general, oscilloscopes and spectrum analyzers have their own advantages in frequency domain analysis, spectrum analyzers are superior in technical indicators such as sensitivity, and oscilloscopes are better than spectrum analyzers in real-time bandwidth. When measuring different types of signals, you can choose according to test requirements and different technical characteristics of the instrument.
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