Feifei recently visited a lot of users of electronic instruments, including college teachers, R&D engineers, production test engineers and so on. The two most frequently asked questions are: What is the difference between an oscilloscope and a spectrum analyzer? When should I use a spectrum analyzer? For the most basic and most commonly used instruments in the field of RF, there are so many people who are unfamiliar with its basic concepts and uses. Feifei is indeed a bit strange, so I wrote my own experience and wanted to share with you. Discuss this topic together. Students know that for an electrical signal, we can describe it from a time perspective (time domain) or from a frequency perspective (frequency domain).
The common oscilloscopes on the market have spectral display function drops. This is mainly the use of Fourier transform (FFT) for time domain and frequency domain conversion, but what is the difference between oscilloscope FFT and spectrum analyzer in RF test applications? What? 1. Applicable signal types are different: The main purpose of the oscilloscope is to observe the time domain characteristics of the signal (that is, the voltage conversion characteristics with time), which is mainly applicable to the analysis of baseband signals (sinusoidal, square wave, bit stream, etc. unmodulated signals). ), and the spectrum analyzer is mainly for the analysis of radio frequency signals (especially complex signals with multiple modulation or multi-frequency signals, such signals have almost no regularity on the time axis). Although the oscilloscope can also display signals from the frequency domain by FFT, its performance specifications are generally insufficient to analyze RF, modulated signals. This Fifi will explain further below. 2. The measured bandwidth is different: the design of the oscilloscope is mainly used to observe the baseband signal, so in general the bandwidth is not very wide, the most common is tens to hundreds of MHz. Of course, with the rapid development of digital circuit technology, the rate of baseband signals is also rapidly increasing, so some medium and high-end oscilloscopes can also reach the order of GHz. The spectrum analyzer is mainly used to analyze the carrier and modulated RF signals, so the frequency range of the spectrum analyzer is usually much wider. For example, Agilent's entry-level spectrum analyzer N9322C is 7GHz, and the high-end N9000A CXA is 26.5 GHz. The N9030A PXA can go to 50 GHz.
3. The measurement content is different: the oscilloscope observes the voltage change with time, so usually see sine wave, square wave, bit stream, etc., pay attention to voltage, period, rise, fall, overshoot, glitch, and multipath Features such as timing between signals. The spectrum analyzer looks at the power, frequency, distortion (harmonics and intermodulation products) of the RF signal, the bandwidth after modulation, the size of leakage to adjacent channels, noise testing, and in-depth analysis of complex modulated signals (modulation, IQ constellation, modulation error, etc.)
4. Sensitivity is different: the oscilloscope looks at the baseband signal and is connected by conduction. The signal amplitude is generally strong, in a few volts, a few tenths or a few percent volts (power in milliwatts), and many spectrum analyzers It is necessary to measure the spectrum of the transmitted signal or the RF signal received from the air. The power is often a few times lower than 1 milliwatt or even a dozen or ten times, which is a few microvolts or less. 5. Different dynamic range: The so-called dynamic range refers to the ability to simultaneously observe large signals and small signals. When the oscilloscope observes a signal with a main signal scale at the volt level, it can easily observe the fine signal or fluctuate at a fraction of a fraction or a few tenths of a volt. That is to say the resolution of a tenth or a few percent of the voltage (percent or ten thousandth of the power). The small signal that the spectrum analyzer can observe at the same time can be one millionth of the power of the large signal, one ten thousandth, one hundred thousandth. In the field of radio frequency measurement, such a large dynamic range is often required. Ok, I hope that Fifi has not said that everyone is dizzy. . . . . . Therefore, students can see that although they are all signal analysis, you may need a spectrum analyzer instead of an oscilloscope in the following situations: 1. Focus on RF equipment and RF signal testing (carrier and modulated) Later signals), such as testing RF transceivers 2. Checking unknown signals in a wide frequency band, such as the composition of air signals and various RF interferences, EMC testing of electronic products, etc. 3. At the same time pay attention to large signals and small signals, such as I want to know the distortion, intermodulation, clutter, etc. of the transmitter signal. 4. Pay attention to weak signals below the millivolt level, such as focusing on the coupling between signals on the printed circuit board, noise on the power supply and clock, and so on. As shown below
The first three applications are typical spectrum analyzer applications, and the fourth case is not familiar to engineers, and is often the most problematic for circuit design engineers. Many times, engineers only consider this when they find that the whole machine has problems in troubleshooting and debugging.
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