A Brief Analysis of Power Spectral Density Test

    The energy (power) carried by each unit frequency point in the working frequency band of wireless communication products is called power spectral density (PSD). Generally speaking, this energy is not fixed and varies with the different bandwidths of the products. In the standards for wireless products in various countries, there are certain limits on power spectral density. Therefore, power spectral density testing is a basic test item in RF testing.

    Power spectrum density testing is generally limited to products in non-frequency hopping mode, and the most typical product is WIFI products. There are corresponding requirements in the US FCC standard Part 15C (e) and the new FCC standard 558074 released in 2012. The basic test methods are the same, and the difference lies mainly in the difference in parameter settings. In addition, the European standard EN300 328 also has test requirements for power spectrum density. The difference between the two standards is that the maximum limit of power spectrum density in the European standard is no more than 10dBm, while in the US standard Part 15C (e), the maximum limit of power spectrum density cannot exceed 8dBm.
    Figure 1 below is a result of testing WIFI in standard Part 15C (e). From the figure, we can clearly see the size of the power spectrum density of the tested product.
    Figure 1 is the PSD test result of WIFI products . The specific test process of                                   Figure 1     is as follows:     1. First, we need to build a test system in a shielded room. The test equipment used includes a spectrum analyzer and an attenuator (connect the end with large power attenuation to the spectrum analyzer to prevent excessive power from burning out the spectrum analyzer) as shown in Figure 2.                                    Figure 2     2. The EUT under test is put into test mode through the software, so that the EUT can control the automatic long-send mode, thereby controlling the power and channel of WIFI. If the communication is not smooth, it is recommended to check whether the test software setting parameters are correct.     3. Enter the frequency center option of the spectrum analyzer to set the center frequency to be tested, and then enter the BW option to set the RBW and VBW. Here, set the RBW and VBW to 100KHz (the standard stipulates that RBW≥VBW).     4. Set the width of the Span to 1.5MHz. It should be noted that the set Span is 5-30% of the occupied bandwidth, so we must set the PSD Span after testing the 6dBm occupied bandwidth (the occupied high bandwidth of WIFI is 6dBm occupied bandwidth).     5. Enter the offset option of the spectrum analyzer to set the compensation value (path loss of the cable line), and finally enter the max hold option to peak a maximum point, which is the power spectrum density value to be taken.     For the FCC standard 558074 newly introduced in 2012, we can see the difference in the results in Figure 3 below.                                     Figure 3     The test process is as follows:     1. When we test, we also need to build a test environment in the shielded room (as shown in Figure 2). The only difference is the parameter settings of the spectrum analyzer.     2. Through software control, the EUT enters the test mode, and then controls the maximum power transmission and channel control of the EUT.     3. Also enter the frequency center option of the spectrum analyzer to set the center frequency to be tested, and then enter the BW option to set RBW and VBW. See Figure 3 for the settings of RBW=100KHz and VBW=300KHz. Here, the new FCC standard 558074 issued in 2012 specifically points out RBW=100KHz and VBW≥300KHz.     4. Set the width of the span. The width of the span should be between 5-30% of the occupied bandwidth.     5. Enter the offset option of the spectrum analyzer to set the compensation value (path loss of the cable line), then enter the Trace option, and finally mark the maximum point with max hold.
             



                    






             

    From the above two tests, we can clearly see that the two standard test methods are exactly the same, the difference is the setting of RBW and VBW. But from the results, we can see that the measured graphs are completely different, and the PSD values ​​are also completely different, with a difference of several dB, so we cannot replace these two methods. 
    Although the test method of Part 15c (e) is the same as the test method of the new FCC standard 558074 released in 2012, the results will be completely different and cannot be confused, otherwise the measured results will be very different.