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Spectrum analyzer selection

2024-03-08 14:47

1. Resolution bandwidth

Refers to the small spectral line spacing between two adjacent components in the resolution spectrum, measured in Hz. It represents the ability of a spectrograph to distinguish two equiamplitude signals that are very close to each other at a specified low point. The spectral line of the measured signal seen on the spectrum analyzer screen is actually a dynamic amplitude frequency characteristic graph of a narrowband filter (similar to a bell shaped curve), so the resolution depends on the bandwidth generated by this amplitude frequency. The 3dB bandwidth of the amplitude frequency characteristics of this narrowband filter is defined as the resolution bandwidth of the spectrograph.

2. Input frequency range

The frequency range in which the spectrograph can operate normally, represented by HZ, is determined by the frequency range of the scanning local oscillator. The frequency range of modern spectrographs usually ranges from low frequency to radio frequency, and even microwave, such as 1KHz to 4GHz. The frequency here refers to the center frequency, which is the frequency located at the center of the displayed spectrum width.

3. Sensitivity

The ability of a spectrograph to display small signal levels, expressed in units such as dBm, dBu, dBv, V, etc., given a given resolution bandwidth, display mode, and other influencing factors. The sensitivity of a superheterodyne spectrograph depends on the internal noise of the instrument. When measuring small signals, the signal spectral lines are displayed above the noise spectrum. In order to easily see the signal spectral lines from the noise spectrum, the signal level should generally be 10dB higher than the internal noise level. In addition, sensitivity is also related to the scanning speed. If the scanning speed is fast, the dynamic amplitude frequency characteristic peak will be lower, resulting in lower sensitivity and amplitude difference.

4. Dynamic range

It refers to the ability to measure the difference between two signals that appear simultaneously at the input end with specified accuracy. The upper limit of dynamic range is constrained by nonlinear distortion. There are two ways to display the amplitude of a spectrograph: linear logarithm. The advantage of logarithmic display is that it can achieve a larger dynamic range within a limited effective height range of the screen. The dynamic range of a spectrograph is generally above 60dB, and sometimes even above 100dB.

5. Frequency scan width (Span)

There are different terms for analyzing spectrum width, sweep width, frequency range, spectrum span, etc. Usually refers to the frequency range (spectrum width) of the response signal that can be displayed within the vertical scale lines on the left and right of the spectrograph display screen. Automatically adjust or manually set according to testing needs. The scanning width represents the frequency range displayed by the spectrometer during a measurement (i.e. a frequency scan), which can be less than or equal to the input frequency range. The spectrum width is usually divided into three modes.

1) The full sweep frequency spectrometer scans its effective frequency range at once.

2) Each frequency spectrum analyzer only scans one specified frequency range at a time. The spectral width represented by each grid can be changed.

3) The zero sweep frequency width is zero, and the spectrograph does not sweep frequency and becomes a tuned receiver.

6. Sweep Time (abbreviated as ST)

The time required to perform a full frequency range scan and complete the measurement, also known as analysis time. The shorter the scanning time, the better.

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