How to adjust the parameters to make better use of the spectrum analyzer

Mar. 17, 2021

The simplest and most intuitive purpose of using a spectrum analyzer is to find and observe the spectrum signal within the range, and even to save it for further digital processing or analysis. Therefore, except of basic settings such as center frequency and sweep width, only reasonable setting of VBW, RBW and other parameter can obtain more real and accurate signals.

1. RBW

The FFT bin in the frequency domain, or the Resolution Bandwidth (RBW), determines the frequency resolution of the signal that can be resolved and analyzed by the spectrum analyzer. For example, if a larger FFTbin or smaller RBW range is set, then spectrum analyzer can observe and analyze more details of the signal. If a higher sensitivity is desired, user needs to turn down RBW, so as to lower the noise level to ensure that the signal is not drowned by noise. On the other hand, lowering the RBW also reduces the speed of each sweep, so there is a tradeoff between sweep resolution and test speed.


In addition, power is found less than some dB when use spectrum analyzer to test the maximum power. In this situation, some users maybe think that spectrum analyzer is not calibrated or something wrong with it. But most likely the setting for RBW is not reasonable.

RBW determines the number of collection points in the sampling process. When the sampling points are few (RBW is large), it is likely that the peak value of the signal falls between the two frequency points, resulting in that the captured peak power is not the actual value.


2. VBW

Except of RBW, Video Bandwidth (VBW) filters digitally filtering and process signal output by FFT engine and carry out smooth process to signal amplitude.

The video bandwidth reflects the bandwidth of the video filter behind the envelope detector in the spectrum analyzer receiver. This filter also affects the display noise, but unlike the way bandwidth is resolved, in a video filter the average level of noise remains constant but the variation of noise is reduced. Therefore, the role of the video filter is to "smooth out" the noise of the signal, so changing the VBW does not improve the sensitivity, but when measuring low power signals, the VBW improves the recognition and reproducibility, with a smoother test curve but a corresponding increase in scan time.

If spikes from phase-locked loop (PLL) or intermodulation distortion products may overwhelm the signal of interest, and lowering the RBW will lower the noise baseline, and lowering the VBW will further smooth the remaining noise, then the spectrum analyzer can reveal precise measurements of previously hidden signals.

3. AMP

Low Noise Amplifiers (LNAs), or preamplifiers (AMPs), are generally divided into RF front-end Amplifiers and mid-frequency Amplifiers, all are used to amplify the signal appropriately for a subsequent processing. If the signal power is very low, the ADC may not be able to adequately process it after the preselected or mid-frequency filter, increasing the gain of the preamplifier can make the signal reach enough level to be collected and processed. There are various types of preamplifiers available. 

4. ATT

Attenuator (ATT) plays the opposite function compared with the amplifier. When measure a signal with higher level, to protect spectrum analyzer not be damaged, this signal must be attenuated, as well as get a larger dynamic range through adjusting the attenuator into the mixer level.

In addition, turn down the noise level value if adjust the attenuator to lower so as ensure the signal is not drowned by noise to improve the sensitivity of spectrum analyzer.

When use spectrum analyzer to observe and capture required signal, it is necessary for user to adjust according to the signal characteristics and try every parameter according to needs. So reasonable use of spectrum analyzer can ensure a better observe effect.