What is the difference between an oscilloscope and a spectrum analyzer?

Oscilloscopes and spectrum analyzers are commonly used test instruments in engineers' daily work. Antai engineers compared the differences in the analysis performance indicators of oscilloscopes and spectrum analyzers from four aspects: real-time bandwidth, dynamic range, sensitivity and power measurement accuracy:

1. Real-time bandwidth

For an oscilloscope, bandwidth is usually its measurement frequency range. For a spectrum analyzer, bandwidth is defined as intermediate frequency bandwidth, resolution bandwidth, etc. Here, we will discuss the real-time bandwidth that can perform real-time analysis on the signal.

For spectrum analyzers, 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 analyzers is only a few megahertz, and the wider real-time bandwidth is usually tens of megahertz. Of course, the FSW spectrum analyzer with the widest bandwidth can reach 500 megahertz. The real-time bandwidth of an oscilloscope is the effective analog bandwidth of its real-time sampling, which is generally hundreds of megahertz and can reach several thousand megahertz.

It should be pointed out here that the real-time bandwidth of most oscilloscopes may not be consistent 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 particularly beneficial for certain ultra-wideband signal analysis, especially in modulation analysis where they have incomparable advantages.

2. Dynamic Range

Dynamic range varies depending on its definition. In many cases, dynamic range is described as the level difference between the maximum and minimum signals measured by the instrument. When the measurement settings are changed, the instrument's ability to measure large and small signals is different. For example, the distortion caused by measuring large signals by a spectrum analyzer is different when the attenuation settings are different. Here, we discuss the instrument's ability to measure both large and small signals at the same time, that is, the optimal dynamic range of an oscilloscope and spectrum analyzer under appropriate settings without changing any measurement settings.

For spectrum analyzers, without considering proximal noise and spurious signals such as phase noise, the average noise level, second-order distortion, and third-order distortion are the main factors restricting the dynamic range. Calculated based on the technical indicators of mainstream spectrum analyzers, the ideal dynamic range is about 90dB (limited by second-order distortion).

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

In terms of dynamic range, spectrum analyzers are better than oscilloscopes. But it should be pointed out that this is true for spectrum analysis of constant signals. However, the spectrum of an oscilloscope is the same frame of data, while the spectrum of a spectrum analyzer is not the same frame of data in most cases. Therefore, for transient signals, a spectrum analyzer may not be able to measure them. The probability of an oscilloscope finding transient signals (when the signal meets the dynamic range) is much greater.

3. Sensitivity

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

For an oscilloscope, when the Y-axis is set to the most sensitive level, usually 1mV/div, the minimum signal that the oscilloscope can test. Leaving aside factors such as port mismatch, the noise generated by the oscilloscope's signal channel and the noise caused by unstable track are the most important factors limiting the oscilloscope's sensitivity.

As we can see from Figure 1, the spectrum noise floor can be reduced to a relatively ideal level due to the increase in the number of sampling points. However, when the signal can no longer 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 can stably measure 0.2mV signals, as shown in Figure 1. This corresponds to a -60dBm level in the frequency domain. In fact, whether an oscilloscope can accurately measure small signals is not only related to the sensitivity of the vertical system, but also to the jitter of the X-axis, trigger sensitivity and other performance.

In order to compare the technical indicators analyzed in this article, the author went to the open laboratory of R&S Chengdu (thanks to the help provided by the Chengdu branch) to compare the indicators. Surprisingly, the RTO oscilloscope is very good in terms of sensitivity indicators, as shown in the following figure:

From this point of view, RTO can definitely help measurement personnel change their perception that "oscilloscopes are useless for frequency domain analysis."

For spectrum analyzers, leaving aside factors such as port mismatch, the average noise level can be regarded as the limit of the spectrum analyzer in measuring small signals when the gain is maximum and the attenuator is set to minimum. Without involving preamplifiers, most spectrum analyzers with good performance can reach -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, there are many factors that affect power measurement accuracy. The main factors are listed below:

For an oscilloscope, the factors that affect power measurement accuracy include: reflection caused by port mismatch, vertical system error, frequency response, AD quantization error, calibration signal error, etc.

For spectrum analyzers, the influencing factors of power measurement accuracy include 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 influencing quantities one by one. We compare them by measuring the power of the 1GHz frequency signal. From the measurement comparison of the RTO oscilloscope and the FSW spectrum analyzer, we can see that at 1GHz, the power measurement values ​​of the oscilloscope and the spectrum analyzer differ by only about 0.2dB, which is a very good measurement accuracy indicator. This is because the measurement accuracy of the spectrum analyzer at 1GHz is very good.

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

In general, oscilloscopes and spectrum analyzers have their own strengths in frequency domain analysis performance. 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 the test requirements and the different technical characteristics of the instrument.