Common Mistakes in Using Spectrum Analyzers

 

    Assume that a system has excessive emissions during testing, making it unable to meet the electromagnetic radiation limits in the electromagnetic compatibility standard. After preliminary investigation, there may be four reasons, which are:

 

    • Radiated emissions from common-mode currents on the interconnect cable (cable 1) between the host and keyboard

 

    • Radiated emissions from common mode currents on the interconnect cable (cable 2) between the host and printer

 

    • Leakage caused by the gap between the chassis panel and the chassis base (opening 1)

 

    • A display window (opening 2) leaks

 

    During the diagnosis, a ferrite ring was first placed on cable 1 to reduce common mode radiation. It was found that the signal displayed on the spectrum analyzer screen did not decrease significantly. Therefore, the tester believed that cable 1 was not a major leakage source, so the ferrite ring was removed and placed on cable 2. It was found that the signal displayed on the spectrum analyzer screen did not decrease significantly. As a result, the tester concluded that the cable was not the leakage source.

 

    Then the leakage on the chassis was checked again. Opening 1 was blocked with shielding tape, and it was found that the signal displayed on the spectrum analyzer screen did not decrease significantly. The tester believed that opening 1 was not the main source of leakage, so the shielding tape was removed and blocked on opening 2. As a result, the displayed signal on the spectrum analyzer did not decrease. The tester was at a loss. The reason why this problem occurred was that the tester ignored that the signal amplitude displayed on the spectrum analyzer was displayed in dB. Let's take a look at why this phenomenon occurs.

 

    Assuming that the four leakage sources each account for 1/4 of the total radiation, and the measures taken on each radiation source can completely suppress the radiation source, when we take one of the above four measures, the amplitude ΔA of the signal reduction displayed on the spectrum analyzer is:

 

    ΔA=20lg(4/3)=2.5dB

 

    A reduction this small is obviously insignificant, but it has reduced leakage by 25%.

 

    The correct method is that after taking suppression measures for a possible leakage source, even if there is no obvious improvement, do not remove this measure, and continue to take measures for possible leakage sources. When a certain measure is taken, if the interference amplitude is greatly reduced, it does not necessarily mean that this leakage source is the main one, but only that this interference source is the last one. The results of treating the four leakage sources one by one according to this step are shown in Figure 1.

 

    In the previous description, we assumed that after taking measures for a certain leakage source, the leakage source is 100% eliminated. If so, when the last leakage source is removed, the reduction of electromagnetic interference should be infinite. In fact, this is impossible. When we take any measure, it is impossible to eliminate the interference source 100%. The degree of elimination of the leakage source can be 99%, or 99.9%, or even more than 99.99, but it can never be 100%! So when the last leakage source is removed, although the improvement is great, it is still a finite value.

 

    When the equipment fully complies with the relevant regulations, if in order to reduce product costs and unnecessary components, the measures taken can be removed one by one. The first thing to consider is to remove the high-cost components/materials, or measures that are difficult to implement on the official product. If the electromagnetic emission of the product does not exceed the standard after removal, the measure can be removed. Through testing, the product cost can be minimized.