EMI testing: Using MDO to test EMI problems caused by switching power supplies

The electronic device circuit board is a circle with a diameter of 15 cm and is placed in a metal shielding shell. There is no problem with EMC certification, but the device itself does not work properly. It is suspected that the switching power supply ripple is the cause. Since the frequency of the switching power supply ripple is usually very low, the traditional test method is to use an oscilloscope to monitor the power supply waveform and perform FFT calculations on the waveform to display the spectrum of the power supply ripple. Since the span of the spectrum calculated by the oscilloscope FFT is limited by the oscilloscope's time base, and the dynamic range is much lower than that of the spectrum analyzer, the test results are not ideal and the power supply ripple problem is not confirmed. Since MDO is a combination of an oscilloscope and a spectrum analyzer, the customer decided to use MDO for another test.

The actual measurement process:

First, test the +5V power supply ripple of the circuit board. MDO oscilloscope channel 1 is directly connected to +5V, and the ripple is tested using AC coupling. For comparison, one end of a BNC cable is connected to the MDO 50 ohm RF input terminal, and the other end is stripped and directly connected to +5V. The center frequency is set to 2.5MHz and the span is 5MHz. The test results are as follows:

From the test results, we can see that the power supply ripple is about 20mV, and its spectrum is about -50dBm in the low frequency band. In order to see the impact of the power supply ripple, we do FFT on the oscilloscope and set the span of the spectrum analyzer to 50MHz. The test results are as follows:

From the test results, we can see that the spectrum displayed by the FFT function of the oscilloscope is similar to the 5MHz span spectrum of the spectrum analyzer, but we cannot see any problems. However, the spectrum in this figure shows that the occupied bandwidth of the +5V power supply ripple extends to 16MHz, which has a serious impact on the device. Thus, we can find the real reason why the device is not working properly.

Summary:

In this case, an oscilloscope and a spectrum analyzer were used to test the power ripple. The spectrum analyzer's span can be flexibly set to easily test the bandwidth occupied by the power ripple in the frequency domain. MDO combines the functions of an oscilloscope and a spectrum analyzer, and can display the time domain and frequency domain together, making it very suitable for such applications.