The first step is to verify the output of the power supply end of the IPAD when it is working;
By directly verifying the voltage of the output port of the IPAD to ensure that the power supply at the source end is normal; through testing, we found that the voltage value measured at the source end is around 3.4V (500MHZ bandwidth measurement), with a peak-to-peak value of 29mV, which is a very stable power supply ;
Therefore, the problem of source power supply can be excluded. Next, we directly measure the voltage at the power supply pin SDVCC of the MicroSD card after passing through the entire module;
When we tested the points on the picture, we found that there was considerable noise on the high-frequency switching power supply, which caused the voltage to exceed the range required by the specification, with a maximum value of 3.814V and a peak-to-peak value of 854mV;
But when we set the oscilloscope to 20MHZ bandwidth, the high frequency switching power supply becomes very good, completely within the range of power supply requirements;
In this high-frequency switching power supply test process, it is not the ripple measurement of the high-frequency switching power supply, but the noise. Similar to this kind of high frequency switching power supply voltage measurement, if the test is performed according to the limited 20MHZ bandwidth, it will bring misjudgment to the measurement analysis (because there is indeed a relatively large noise/voltage fluctuation), and the front-end filtering of the oscilloscope will cause the product itself to exist. The noise is filtered out; therefore, we use a full bandwidth of 500MHZ for testing;
However, does the above test method truly reflect the noise level of the product? Also, how much skewed the measurement results will be when testing with standard passive probes? Is it within the acceptable range? Further verification is required;
We measured the same test points with different ground loops. The test loop with spring ground reduces the return path of the signal, and the test result will be better than that of the original standard 6 inches, but the difference between the two is small, and the measured maximum value of 3.8V seems to be inaccurate (Judgment from experience); I also learned during the oscilloscope operation training that the standard 10:1 passive probe of the oscilloscope will bring a large deviation to the signal measurement, and the 10:1 attenuation will increase the noise floor of the oscilloscope by 10 times. ; Therefore, we will use a 1:1 attenuation, 50 ohm coaxial cable to measure the product again to ensure that the real condition of the product is accurately reflected, so as to analyze the test results, as shown in the following figure:
The use of 1:1 coaxial cable can reduce the signal transmission path. In addition, the oscilloscope is directly set to 1:1 attenuation, which avoids the amplification of the noise floor of the oscilloscope by the software algorithm, thus bringing the most accurate measurement results;
Using the coaxial cable test results, the maximum value is 3.645V, which is 0.169V different than the measured value using the passive probe 3.814V. It can be seen that when a very accurate measurement is required, a coaxial cable should be selected for measurement to minimize the measurement error.
