How to choose a spectrum analyzer and repair it? Fault analysis of spectrum analyzers

In the following content, the editor will focus on introducing and explaining the relevant content of spectrum analyzers, mainly on how to choose a spectrum analyzer and how to solve the loss of lock failure of the spectrum analyzer. I hope this article can help you increase your understanding of spectrum analyzers

1. What is a spectrum analyzer?

A spectrum analyzer is an instrument for studying the spectrum structure of electrical signals. It is used to measure signal parameters such as signal distortion, modulation, spectrum purity, frequency stability, and intermodulation distortion. It can be used to measure certain parameters of circuit systems such as amplifiers and filters. It is a multi-purpose electronic measuring instrument. It can also be called a frequency domain oscilloscope, tracking oscilloscope, analytical oscilloscope, harmonic analyzer, frequency characteristics analyzer, or Fourier analyzer.

Spectrum analyzers are divided into two categories: real-time analysis and sweep frequency. The former can obtain all the required spectrum information and analyze and display the analysis results within the actual time when the measured signal occurs; the latter requires multiple sampling processes to complete repeated information analysis.

2. How to choose a spectrum analyzer

1. Frequency range

A spectrum analyzer can only work properly within a specific frequency range, so when purchasing a spectrum analyzer, you need to consider the situations in which you may use the spectrum analyzer in the future. It is best to choose a spectrum analyzer with a large frequency measurement range.

2. Input power

The input power of the spectrum can be divided into two categories: pulse input power and average continuous power. So, what is pulse input power? In fact, it is very simple, that is, the value of the pulse input power that the spectrum can measure. The maximum power value of the continuous input signal is the average continuous power we are talking about.

3. Input impedance

The terminal impedance presented by the spectrum analyzer to the signal source is also called the input impedance. According to practical experience, the impedance values ​​of various instruments are usually different. For example, the impedance of a microwave analyzer is usually 50Ω, while for some systems, its impedance value is higher. Please note that if the impedance does not match, it will affect the measurement process, mainly the measurement accuracy.

4. Average noise level (DANL)

The average noise level is equivalent to the size of the spectrum's own noise. There is a certain relationship between the size of the noise and the minimum signal amplitude to be measured. As everyone thinks, the smaller the average noise level, the better, because when the average noise level is large, it will affect the measurement error. However, the equipment with a smaller average noise level usually costs more. Therefore, when purchasing a frequency analyzer, you need to comprehensively consider its cost performance.

5. Preamplifier

If you need to measure tiny signals, you need to consider whether the spectrum analyzer has a tiny signal amplification module when purchasing it. If not, the corresponding spectrum analyzer will be "incapable" of measuring tiny signals.

3. How to troubleshoot the spectrum analyzer lock failure

If the spectrum analyzer loses lock at 2GHz but not at 6GHz, it means that the first local oscillator is normal and the second local oscillator is locked; if both frequency points are locked, it may be that the first local oscillator is locked or both the first and second local oscillators are locked. The principle diagram is as follows:

Spectrum analyzer lock failure troubleshooting steps:

(1) Determine whether the first local oscillator is unlocked. Check according to the "First Local Oscillator Preset". If it is abnormal, follow the inspection steps in the first local oscillator preset debugging. If it is normal, proceed to the next step.

(2) Determine whether the second local oscillator is unlocked. Check according to the "Second Local Oscillator Preset". If it is abnormal, follow the inspection steps in the second local oscillator preset debugging. If it is normal, proceed to the next step.

(3) Determine whether the 300MHz third local oscillator is normal. If this local oscillator is unlocked, it is actually the 100MHz crystal oscillator that is locked abnormally. At this time, measure the external 100MHz. If the signal jitter is abnormal, it may be that the 100MHz crystal oscillator is locked.

(4) Determine whether the 28.9MHz fourth local oscillator is normal. This oscillator is a crystal oscillator. If it loses lock, it may deviate from the century frequency point by about 20kHz. The bandwidth of the spectrum analyzer should be set small enough, about 100kHz, to be able to observe.