In the daily work of FAE, we often receive user feedback: the system is already running normally, but the results obtained when testing the quantized waveform data are not satisfactory. Why is there such a discrepancy? The problem may lie in the test method and the use of test equipment!



Before comprehensively investigating whether it is a system product problem (huge workload, time-consuming and labor-intensive), we must first check the environment, means and methods of our own testing. Is it "tested correctly"? Among them, how to correctly select the test method of the oscilloscope is particularly important!

This article mainly explains the three key indicators of the oscilloscope and the test specifications and related precautions to be followed when using the weho test.


1. How to choose a suitable oscilloscope according to the key indicators of the oscilloscope

As a commonly used high-precision testing instrument, an oscilloscope can convert electrical signals invisible to the naked eye into visible images, which is convenient for people to study the changing process of various electrical phenomena. The correct use of the oscilloscope is very important. Often, the "measured data" is very different from the actual working state of the system due to incorrect parameter setting by testers, which brings unnecessary troubles.

Three key indicators of the oscilloscope: bandwidth, sampling rate, and memory depth.



1.Bandwidth: refers to the frequency range when the response causes the output amplitude to drop to 70.7% (-3dB).



With the continuous development of high-frequency power switch and rectifier technology, the operating frequency of power supply continues to increase. At present, the emerging power switches such as GaN MOSFET, SiC MOSFET and SiC Schottky rectifier on the market have a turn-on and turn-off time of less than 5ns (on The off frequency exceeds 200MHz). In the process of engineering measurement, in order to observe such fast-changing signals, a measurement system with sufficient bandwidth is required. The bandwidth is not only the bandwidth of the oscilloscope, but also the bandwidth of the probe.

Weho commonly used differential probes and oscilloscopes have a bandwidth of 100MHz, which can meet the needs of daily testing. The higher the bandwidth, the wider the range of high-order harmonics of the measured signal that can be collected, and the less distortion of the measured signal, but the wider the bandwidth of the probe, the better. The more, the ripple noise test oscilloscope needs to open the 20MHz bandwidth limit for measurement as an example, the bandwidth is limited. Similarly, when the low-frequency signal noise interference of the test is too much, you can also open the bandwidth on the differential probe (5MHz) or oscilloscope limit.



2. Sampling rate: refers to the number of data points that can be collected per second. Generally speaking, the sampling rate index of the oscilloscope indicates the highest sampling rate that the oscilloscope can achieve when it is working.

Storage depth = sampling rate × sampling time. It refers to the number of waveform data when the oscilloscope displays a waveform on the screen. The waveform displayed on the oscilloscope screen we see is composed of many sampling points. The number of all sampling points is the storage depth.

How does memory depth affect measurements? We add a square wave with a frequency of 1KHz and an amplitude of 2V to the oscilloscope, and use an oscilloscope with a memory depth of 28M to intercept the 14S signal. At this time, the sampling rate is 2Msa/S and the signal is amplified by 2000 times, and it is still a square wave.



When using an oscilloscope with a memory depth of 28K to intercept a 14S signal, the sampling rate is 2Ksa/S, and the waveform is amplified by 2000 times, and the obtained waveform is distorted.



It can be concluded from this example that: the same sampling time, the greater the sampling rate, the greater the storage depth of the oscilloscope, and more details can be seen in the saved waveform. When testing, you should ensure that your sampling rate is sufficient to avoid waveform distortion caused by too long sampling time. Generally, the acquisition rate of the oscilloscope can reach up to 4MSa/s in the rolling state, and it can reach higher in the trigger mode.

Take the weho high-power chassis LMF1000-20Bxx product stress debugging waveform as an example:








2. Precautions for using the oscilloscope

1. The oscilloscope must be calibrated when it is connected to a new passive probe or plugged in and out of the probe, otherwise the test result will be inaccurate (the error of the ripple test result is more than 10mV), and the ground wire of the probe should be kept as short as possible during measurement. The probe compensation steps are as follows:

①Connect the probe to a vertical channel, then connect the probe tip to the oscilloscope square wave reference signal;




② Observe the square wave reference signal and adjust the compensation capacitor. The adjustment method can be seen in the figure below




2.The oscilloscope and probe need impedance matching. Generally, general-purpose oscilloscopes have switchable matching resistors of 1MΩ (general circuit) and 50Ω (high-speed circuit) at the input end, which are correctly matched with the probe to reduce the load effect of the circuit under test.




3.When the power line of the oscilloscope is grounded, it is necessary to avoid using ordinary probes directly connected to products powered by the power system. Please use differential probes to test or use isolation transformers to supply power to the oscilloscope, or use floating measurements (connect the oscilloscope without a ground wire power line) ), to avoid ground noise from interfering with real data (the negative end ground of the passive probe is connected to the power supply PE of the oscilloscope). For specific comparison, please refer to the following figure:




4. Do not use passive probes for EMC testing, and all differential measurements must be used to prevent the PE surge of the test product from being introduced into the oscilloscope when the oscilloscope is grounded, causing damage to the oscilloscope or power-down of the product under test (abnormal test results), surge test Both the power supply line of the instrument and the power supply of the oscilloscope are connected to the mains.



Three. Summary

weho has been deeply involved in the power supply field for more than 16 years. The engineers adhere to the rigorous and serious working attitude, strictly abide by the standardized test rules, provide accurate and real product performance data, simplify the customer's product development and test cycle, and comprehensively improve the stability of the system. , safety and reliability, and is committed to providing "worry-free" power supplies for various industries.