The shape factor of a filter is the ratio of the response at two attenuation values. The 3 dB Gaussian filter is acceptable for many measurements, but for Electromagnetic Compliance (EMC) related measurements, a filter defined at 6 dB may be required. This is also referred to as the filter’s Full-Width -Half-Max (FWHM) value. Recall that 3 dB is equal to 50% of the maximum. The RBW value is the bandpass frequency of the filter 3 dB below the peak response of the filter. Many analyzers have an RBW filter that has a Gaussian shape and a shape factor determined at the 3 dB point. The shape and shape factor of the RBW filter can also be an important selection. Observe that while the frequency of these two similar signal measurement power is completely unchanged, the signal separation is only clear when the RBW is less than the frequency difference between the signals.įigure 1: Spectrum analyzer display showing two signals at three different resolution bandwidth (RBW) settings. The traces were collected using RBWs of 30 kHz (Blue), 10 kHz (Yellow), and 3 kHz (Pink). Simply adjust the RBW to provide the proper balance between speed and resolution for your application.įigure 1 is the measurement of two signals separated by 20 kHz. You may not use 10 Hz RBW often, but it is very useful when you do. Select a spectrum analyzer that has a large number of RBW settings, especially on the lower frequency end. Smaller RBWs also lower the noise floor, but they extended the sweep time for a given span of frequencies. This means that if the RBW is reduced by 100 times the scanning time will be increased by 10000 times in the same SPAN That is to say, the scanning time is proportional to SPAN and is inversely proportional to the square of RBW. Usually, when we do not focus on noise, the VBW can be set to a value greater than or equal to RBW. The second factor means that each selection The time required depends on the smaller value between RBW and video bandwidth (VBW). In this formula, the meaning of the first factor is the number of frequency selections under SPAN, each step is 1 / k of RBW, to ensure the accuracy of amplitude measurement. Here is the equation governing the sweep speed: Sweep time is the length of time it takes to sweep the detector from the start to the stop frequency. Why not use the smallest RBW setting for all measurements? Smaller RBWs provide finer frequency resolution and the ability to differentiate signals that have frequencies that are closer together. In spectrum analysis, the resolution bandwidth (RBW) is defined as the frequency span of the final filter that is applied to the input signal. The bandwidth of a measurement defines the range of frequencies that were used for the measurement. For example, the bandwidth of the transmission signal is the span of frequencies that the transmission occupies. Resolution Bandwidth (RBW)īandwidth is defined as the span of frequencies that are the focus of a particular event. In this application note, we will introduce resolution bandwidth (RBW) and video bandwidth (VBW) and how they affect measurements. There are a number of common adjustments available with many modern analyzers that can optimize performance for a particular application. Spectrum analyzers are useful tools for broadcast monitoring, RF component testing, and EMI troubleshooting.
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