Customer Case

Spectrum analyzer using skills


     

Today, I 'd like to introduce some skills in the use of spectrum analyzer-setting the three parameters of SPAN, RBW and VBW reasonably. I hope it can help you in the process of using the spectrum analyzer.

The Frequency (center frequency, SPAN (sweep width) and Amplifier (reference level) of the spectrum analyzer are the three setting parameters that are most frequently operated in actual work. Most digital spectrum analyzer panels have specially increased the buttons for these three settings to facilitate operation. SPAN (sweep width), RBW (resolution bandwidth), Amplifier (reference level) is the three most important parameters of the spectrometer. Many novices like to set large SPAN parameters as soon as they come up, which will cause the spectrometer to automatically adjust and use larger RBW values, which is not conducive to observing signal characteristics. The use of too large RBW will make the signal display distortion, and when two signals with very similar frequencies are easily confused together under large RBW resolution, they cannot be distinguished. The SPAN, RBW and SWEEP TIME (sweep time) of the general sweep digital spectrum analyzer are related and linked in the default automatic setting state. In order to maintain the real-time display of the spectrum chart, the spectrum analyzer will automatically increase RBW when SPAN increases to ensure that the sweep time (SWEEP TIME) is not too long. If the user manually sets a smaller RBW when the SPAN is large, the spectrometer will be forced to have a longer sweep time. Inappropriate settings in actual operation can make the SWEEP TIME of the spectrum as long as tens of seconds or even hundreds of seconds. In an ideal state, the spectrum diagram that is refreshed dozens of times per second is changed into tens of seconds before slowly scanning out a spectrum diagram. At this time, the practicability has been basically lost. In addition, the greater the RBW of the spectrum analyzer, the higher the bottom noise baseline, which will affect the display of small signals. Don't expect to take into account the fine resolution bandwidth with a large scanning frequency bandwidth, unless your spectrum analyzer is ultra-advanced and ultra-expensive. General spectrometers, especially entry-level products and old products, have limited performance, and need to properly set up a narrow SPAN to ensure the real-time performance of the spectrogram under a small RBW.

The common experience is to use wide SPAN, high RBW to find the signal, and then use narrow SPAN, low RBW to accurately display and measure the signal. General spectrum analyzer set SPAN, for narrowband signals, can be 5~10 times the bandwidth of the signal itself, RBW is set to 1/3~1/20 of the bandwidth of the signal itself. For a broadband signal, the sweep width may be set to 2 to 5 times of the signal bandwidth, and the RBW is set to 1/10 to 1/20 of the signal bandwidth. For the need to search for small signals in a wide frequency range, if the performance of the spectrum analyzer is not enough, a segmented scanning method can be used. For small signals that need special attention, a smaller RBW can be set, and the bottom noise baseline of the spectrum analyzer will also be reduced. At the same time, the number of VBW (video bandwidth) and AVG (average) can be set appropriately to smooth the bottom noise of the spectrum map, reduce disturbance, and facilitate the display of small signal spikes.

Many modern entry-level digital spectrometers provide RBW as small as 100Hz, but under such fine RBW, the scanning speed will be very slow. Many products are only designed to obtain better product indexes and have limited practicability. RBW set above 1kHz is usually more practical. In fact, for conventional signals, 1kHz, 3kHz, 5kHz, 10kHz, 30kHz and 100kHz RBW are most commonly used. VBW is less significant in measurement than RBW, but properly set VBW can smooth the spectral noise floor baseline and reduce disturbance. Usually set RBW: VBW = 10:1, set too high RBW/VBW will affect the spectrum analyzer scan time. In order to reduce the spectral noise line disturbance, in addition to optimizing the VBW setting, you can also set the average number of spectral AVG, the greater the average number, the smaller the noise line disturbance, but this setting is not suitable for grasping instantaneous signals.

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