Spectrum analyzer signal amplitude varies when changing reference level
时间：04-04
整理：3721RD
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There are several posts about this problem but none have good answers and all are now closed.
The question usually goes like this: "I change the SpecAn reference level up or down and the amplitude of the monitored signal changes a great deal". The answers usually suggest that the amplitude change is just the result of the specan changing the input attenuation (which most do automatically in response to the selected reference level). However, this is almost never the answer the OP is looking for because the change does not correlate with the specific input attenuation (e.g. if the attenuation drops 10dB, the signal changes much more than 10dB).
The likely issue is that there are multiple signals on the input and that one is much stronger than the signal being monitored and overloads the front end of the spectrum analyzer when the input attenuation is reduced. For example, when examining harmonics that are much lower than the fundamental, it is easy for the fundamental to overload the specan front-end. The operator looks at the fundamental with a high reference level and it looks fine; a large attenuator is automatically placed in line by the SpecAn; but when looking at the lower level harmonics, the operator adjusts the reference level down, the spectrum analyzer reduces the input attenuation, and the front end overloads due to the high power fundamental.
Solutions: 1) manually select a higher attenuation value if your dynamic range allows it or 2) use a filter to attenuate just the fundamental frequency or other strong interfering signal so you can use your dynamic range for the signal of interest.
The question usually goes like this: "I change the SpecAn reference level up or down and the amplitude of the monitored signal changes a great deal". The answers usually suggest that the amplitude change is just the result of the specan changing the input attenuation (which most do automatically in response to the selected reference level). However, this is almost never the answer the OP is looking for because the change does not correlate with the specific input attenuation (e.g. if the attenuation drops 10dB, the signal changes much more than 10dB).
The likely issue is that there are multiple signals on the input and that one is much stronger than the signal being monitored and overloads the front end of the spectrum analyzer when the input attenuation is reduced. For example, when examining harmonics that are much lower than the fundamental, it is easy for the fundamental to overload the specan front-end. The operator looks at the fundamental with a high reference level and it looks fine; a large attenuator is automatically placed in line by the SpecAn; but when looking at the lower level harmonics, the operator adjusts the reference level down, the spectrum analyzer reduces the input attenuation, and the front end overloads due to the high power fundamental.
Solutions: 1) manually select a higher attenuation value if your dynamic range allows it or 2) use a filter to attenuate just the fundamental frequency or other strong interfering signal so you can use your dynamic range for the signal of interest.