The difference between spectrum analyzer and oscilloscope
Can not tell the difference between oscilloscope and spectrum analyzer often joke, in order to avoid flaws, this article briefly summarizes the following four points - with real-time bandwidth, dynamic range, sensitivity, power measurement accuracy, compare the oscilloscope and spectrum analyzer analysis performance indicators To distinguish between the two.

1 Real-time bandwidth

For oscilloscopes, the bandwidth is usually its measurement frequency range. The spectrum analyzer has bandwidth definitions such as IF bandwidth and resolution bandwidth. Here, we discuss the real-time bandwidth that can analyze the signal in real time.

For spectrum analyzers, the bandwidth of the final analog IF 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 wide real-time bandwidth is usually tens of megahertz. The widest bandwidth FSW can reach 500 MHz. The oscilloscope's real-time bandwidth is its effective analog bandwidth for real-time sampling, typically hundreds of megahertz, and high up to several gigahertz.

What needs to be pointed out here is that most real-time oscilloscopes may not have the same real-time bandwidth when the vertical scale setting is different. When the vertical scale is set to the most sensitive, the real-time bandwidth usually decreases.

In terms of real-time bandwidth, the oscilloscope is generally better than the spectrum analyzer, which is particularly beneficial for some ultra-wideband signal analysis, especially in the modulation analysis has unparalleled advantages.

2 dynamic range

The dynamic range indicator varies according to its definition. In many cases, the dynamic range is described as the level difference between the maximum and minimum signal measured by the instrument. When changing the measurement settings, the instrument's ability to measure large and small signals is different. For example, if the spectrum analyzer is not the same in attenuation settings, the distortion caused by measuring large signals is not the same. Here, we discuss the ability of the instrument to measure large and small signals at the same time, that is, the optimal dynamic range of the oscilloscope and the spectrum analyzer under suitable settings without changing any measurement settings.

For spectrum analyzers, the average noise level, second-order distortion, and third-order distortion are the most important factors that limit the dynamic range without considering the near-end noise and spurious conditions such as phase noise. The calculation is based on the technical specifications of mainstream spectrum analyzers. Its ideal dynamic range is about 90dB (limited by second-order distortion).

Most oscilloscopes are limited by the number of AD sampling bits and the noise floor. The ideal dynamic range of traditional oscilloscopes usually does not exceed 50dB. (For R&S RTO oscilloscopes, the dynamic range can be as high as 86dB at 100KHz RBW)

In terms of dynamic range, spectrum analyzers are superior to oscilloscopes. However, it should be pointed out here that this is true for the spectrum analysis of the signal. However, the frequency spectrum of the oscilloscope is the same frame data. The spectrum of the spectrum analyzer is not the same frame data in most cases, so for the transient signal, The spectrum analyzer may not be able to measure it. The probability that an oscilloscope finds a transient signal (where the signal satisfies the dynamic range) is much greater.

3 Sensitivity

The sensitivity discussed here refers to the minimum signal level that the oscilloscope and the spectrum analyzer can test. This indicator is closely related to the instrument settings.

For an oscilloscope, when the oscilloscope is set to the most sensitive position on the Y-axis, the minimum signal can be measured by the oscilloscope when it is usually 1mV/div. Aside from port mismatch, the noise and trajectory generated by the oscilloscope's signal channel are not The noise caused by stability is the most important factor that limits the sensitivity of the oscilloscope.

From Fig. 1, we can see that because of the increase of sampling points, the spectrum noise floor can be reduced to a more ideal level. However, when the signal cannot be clearly and accurately reproduced in the time domain, there is a lot of clutter in the frequency domain, which limits our ability to observe small signals.


Figure 1 Sensitivity limits affected by noise

Most oscilloscopes, like those shown in Figure 1, can stably measure a signal of 0.2mV corresponding to the frequency domain, which is equivalent to a level of -60dBm. In fact, whether an 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, the author specifically went to the R&S Chengdu Open Laboratory (thanks to the Chengdu Division for the help) to compare the indicators. Surprisingly, the RTO oscilloscope is very excellent in sensitivity indicators, as shown below Shown:

Figure 2 Full Spectrum Spectra of RTO Oscilloscope

As can be seen from Figure 2, RTO can accurately measure -60dBm signal, and its noise floor is about -80dBm. What is most gratifying is that, at the entire frequency band (DC-4 GHz), no large clutter that can affect the sensitivity was found, which greatly increased 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 the Span and RBW are set smaller, the bottom noise of the RTO oscilloscope can be reduced to less than -100 dBm.

Figure 3 RTO oscilloscope narrow-band spectrogram

From this point of view, RTO definitely allows the measurement personnel to change the "oscilloscope is a tasteless analysis of the frequency domain" feeling.

For spectrum analyzers, the same considerations such as port mismatching are discussed. With the maximum gain of the spectrum analyzer and the minimum attenuator setting, the average noise level can be regarded as the limit of the spectrum analyzer's measurement of small signals. Most well-performing spectrometers can reach -150dBm without the preamplifier involved.

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 power measurement accuracy is very large. The following are the main influence quantities:

For oscilloscopes, the impact of power measurement accuracy is: port mismatch caused by reflection, vertical system error, frequency response, AD quantization error, calibration signal error.

For the spectrum analyzer, the impact of power measurement accuracy is: port mismatch caused by reflection, reference level error, attenuator error, bandwidth conversion error, frequency response, calibration signal error.

Here, we do not analyze and compare the influence quantities one by one. We compare the power measurement of the 1GHz frequency signal. Through measurement comparison between the RTO oscilloscope and the FSW spectrum analyzer, we can see that the power measurement values ​​of the oscilloscope and the spectrum analyzer are at 1GHz. Only about 0.2dB difference, this is a very good measure of measurement accuracy. Because the spectrum analyzer's measurement accuracy at 1GHz is very good.

In addition, in the frequency range, the oscilloscope's frequency response is also very good, not exceeding 0.5dB in the 4GHz range. From this point of view, the oscilloscope is even better than the spectrum analyzer performance.

In general, oscilloscopes and spectrum analyzers have their own advantages in frequency domain analysis performance. Spectrum analyzers are superior in terms of sensitivity and other technical indicators. Oscilloscopes are superior to spectrum analyzers in real-time bandwidth. When measuring different types of signals, you can choose according to the test requirements and the different technical characteristics of the instrument.

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