Spectrum analyzer, do you really know how to use it?

The following is an introduction to the precautions in the use of the spectrum analyzer, and I hope it will be helpful to you.
To use a spectrum analyzer to measure system indicators, generally only need to directly connect the spectrum analyzer to the system, and then operate according to the measurement method of the indicators. During the measurement process, special attention should be paid to the following issues.
(1) Adjustment of signal input size

If the input of the spectrum analyzer is too high, the analyzer will cause it to produce nonlinear distortion, and the measured results will be inaccurate due to distortion; if the signal level is too low, the signal may be covered by the noise floor of the analyzer and cannot be measured correctly. signal, both of which reduce the dynamic range of the measurement. Therefore, it is necessary to clearly understand the input range of the signal before use, and select the input attenuation correctly.

When the RF signal is input, it should also be noted that the characteristic impedance of the cable matches the input impedance of the instrument, otherwise the signal mismatch will cause attenuation and cause measurement errors. In the cable TV system, the characteristic impedance of the cable is generally 75Ω, and the input impedance of the analyzer can generally be selected between 50Ω and 75Ω. Therefore, the input impedance of the analyzer should be correctly selected during measurement to reduce the measurement error.

(2) Selection of resolution bandwidth

In a spectrum analyzer, frequency resolution is a very important concept. It is determined by the bandwidth of the intermediate frequency filter, which determines the resolution bandwidth BWRES of the instrument. If the filter bandwidth is 100Hz, then the spectral line frequency is There is 100Hz uncertainty. If two spectral lines appear in the bandwidth frequency range of a filter, it is impossible to detect that the two spectral lines are different frequency components, but its energy in the frequency range will be detected regardless of how many spectral lines This energy is generated and, therefore, for two closely related spectral lines, the resolution depends on the width of the filter.

In the actual measurement process, the size of the frequency resolution bandwidth should be correctly selected, and the unwanted signal cannot be mixed into the measurement signal, nor can the required signal be excluded.

(3) Selection of signal detector

The signal detectors in the spectrum analyzer include peak detection and sampling detection. Peak detection is a commonly used type. The output of the IF filter is connected to a detector, which generates a DC level proportional to the AC signal level output by the IF stage. We can choose different detection methods according to different signal measurement indicators, such as peak detection when measuring signal level, and sampling detection when measuring noise.

(4) Selection of vertical scale

In the spectrum analyzer, since the signal level changes greatly, the logarithmic scale is generally used, and there is a logarithmic amplifier before the detector. The logarithmic amplifier compresses the signal level according to the logarithmic function, that is, for the input level amplitude V, the output voltage amplitude is lgV, which greatly reduces the signal level variation detected by the detector, and at the same time provides the user with a logarithmic vertical scale calibrated in decibels. In addition, the linear vertical scale can also be selected according to the different signals, and the signal range it represents is small.

(5) Selection of video filter bandwidth

The video filter is a low-pass filter that reduces the noise variation at the detector output, revealing some signals that have been masked and are close to the noise floor, and also helps stabilize the measurement if noise is to be measured .

When a wideband video filter is used, the noise fluctuation is larger; when a narrowband video filter is used, the fluctuation is significantly reduced. The average value of the noise is the same, but the noise fluctuation is different. Therefore, we can choose the bandwidth of the video filter according to the type of the test signal, for example, when the test signal is only noise signal, the narrow-band video filter can smooth the fluctuation of these noise signals, if the wide-band video filter is selected, due to the influence of noise , the measurement will vary.