Sharing experience on harmonic testing with spectrum analyzer

　　Recently, I encountered a situation where a customer failed to pass the harmonic test of our module. Combined with some questions I encountered in previous tests, I recently studied some principles of spectrum analyzer harmonic testing and have some experience to share with you. The following is an example. The figure below shows the harmonic test of the same module, but the difference is 13dB. Some friends have asked this question before, but I didn’t understand it at the time. This time I can answer it.

Figure 1 Harmonic test (-30dBm)

Figure 2 Harmonic test (-43dBm)

　　The above figure shows the harmonic test of the same product, but the test results are different; you can see that the difference between the two tests is that the Ref Level settings are different, but this is not the main factor. The real reason is that in the process of adjusting the Ref Level, attenuation will also change. The significance of this attenuation is to ensure that the input signal is not too large. It is this value change that leads to the inconsistency of our test results.

　　Under normal circumstances, the attenuated signal will be mixed and then passed through the intermediate frequency amplifier to the back end. To ensure the correct measurement value, the intermediate frequency amplifier is linked with the attenuator value, that is, the amount of attenuation will be amplified after mixing.

Figure 3 Simple diagram of spectrum structure

　　Therefore, under normal circumstances, changing the attenuator value will not lead to inconsistent final test results. The inconsistency is caused by distortion in the mixing process. The distorted signal is superimposed on the module harmonic signal, causing the harmonic test to be too large. The amplitude of the distorted signal is related to the mixer operating level. The specific relationship is shown in the following figure:

Figure 4 Relationship between the distortion signal amplitude and the mixer operating level

　　The lower the mixer operating level, the smaller the distortion signal generated, the smaller the distortion signal superimposed on the second harmonic and third harmonic, and the more realistic the displayed harmonic value. The way to reduce the mixer operating level is to increase the attenuator value.

　　This is why attenuation affects the harmonic power. However, the attenuator value cannot be set too large, because part of the background noise displayed in the spectrum is generated during the mixing process. The greater the attenuation, the greater the amplification factor of the intermediate frequency amplifier, and the noise will also be amplified, and eventually the background noise will be too large. Therefore, during normal testing, it is recommended to set it to automatic mode. If there is an inconsistency, you can increase the attenuator value from small to large until the harmonic values ​​before and after no longer change. At this time, the harmonic value is more accurate.

　　summary:

　　The larger the attenuation, the more accurate the harmonic value, but the background noise will increase, so a compromise is required;

　　When testing the module power, the background noise generated by the spectrum analyzer is not only the background noise of the module itself, but also includes the noise of the instrument superimposed after being amplified by the intermediate frequency amplifier (when the amplification factor is relatively large, the background noise we see may not represent the background noise of the module). The display of the background noise is also related to RBW and VBW.

　　I hope the above will be helpful to you in future testing.