Oscilloscope FAQs

　　A: Oscilloscopes have long been one of the most effective tools for testing electronic circuits. By observing the voltage and current waveforms at key nodes of the circuit, one can visually check whether the circuit is working properly and verify whether the design is appropriate. This is extremely helpful in improving reliability. Of course, the correct analysis and judgment of the waveform depends on the engineer's own experience.

　　2. What are the main factors that determine the price of an oscilloscope probe?

　　A: There are many types of oscilloscope probes with different performances, such as high voltage, differential, active high-speed probes, etc. The price ranges from a few hundred RMB to nearly $10,000. The main determinants of price are of course bandwidth and function. The probe is the part of the oscilloscope that contacts the circuit. A good probe can provide the fidelity required for testing. To achieve this, even a passive probe must have a lot of passive device compensation circuits (RC networks) inside.

　　3. How long is the service life of a general oscilloscope probe? Does the probe need to be calibrated regularly?

　　Answer: It is difficult to say the life of an oscilloscope probe, as it depends on the environment and method of use.

　　The standard does not have clear measurement regulations for probes, but for passive probes, at least when replacing probes or switching channels, probe compensation adjustment must be performed. All active probes should be preheated for at least 20 minutes before use, and some active probes and current probes require zero drift adjustment.

　　4. What is the real-time sampling rate of an oscilloscope?

　　A: The real-time sampling rate refers to the reciprocal of the sampling interval of an oscilloscope's one acquisition (one trigger). It is understood that the highest level in the industry is the simultaneous use of four channels.

　　5. What is equivalent time sampling of an oscilloscope?

　　A: Equivalent time sampling means that the oscilloscope pieces together the waveforms collected by multiple acquisitions (multiple triggers) into one waveform. The sampling rate may be very slow each time, and there is a certain offset between the two acquisition trigger points. The inverse of the minimum sampling interval between the two points is called the equivalent sampling rate. Its index can be very high, such as 1ps.

　　6. What is power factor? How to measure it?

　　Answer: Power factor: In a DC circuit, voltage multiplied by current is active power. But in an AC circuit, voltage multiplied by current is apparent power, and the part of power that can do work (i.e. active power) will be less than the apparent power. The ratio of active power to apparent power is called power factor, which is represented by COSΦ. In fact, the simplest way to measure it is to measure the phase difference between voltage and current, and the result is the power factor.

　　7. How to express and test power density?

　　Answer: Power density is the power per unit volume, and W/in3 is generally used in power supplies.

　　8. Is there any way to use an oscilloscope to measure the working condition of a high-frequency transformer or inductor core?

　　Answer: The power test solution launched by TEK has a function - BH curve analysis, which can reflect the working status of the magnetic core, measure the dynamic inductance value, and derive the core loss.

　　9. There are many types of noise in switching power supplies, such as cross interference caused by unreasonable wiring, inductor leakage, diode reverse spikes, etc. How to use an oscilloscope to identify noise? [page]

　　A: TEK's TDS5000 oscilloscope has frequency domain analysis. By analyzing the frequency band of the noise, the type of noise can be analyzed, and the corresponding processing method can be used. The oscilloscope can only provide data analysis and band shape display.

    10. How can I use an oscilloscope to test the radiation of the power supply?

　　A: Switching power supplies have radiated interference. The general approach is to find out the source of interference and then shield it. An oscilloscope can be used to analyze its frequency component composition through Fourier transform function, and the type of interference can be determined based on the frequency range.

　　11. In the design process of flyback power supply, the conversion efficiency of the transformer is often reduced due to the large leakage inductance of the transformer. The method of winding the secondary with the primary in the middle is still not ideal. Are there any techniques for winding the transformer?

　　Answer: Wind the high-power output winding inside, as close to the primary side as possible to enhance coupling.

　　12. Is there an oscilloscope that can analyze switching losses?

　　A: Tektronix's power supply test system, namely the TDS5000 series digital phosphor oscilloscope plus TDSPWR2 power analysis software, can easily analyze switching losses and power losses per cycle, even including RDS ON.

　　13. Can an oscilloscope perform Fourier decomposition?

　　Answer: Most modern digital oscilloscopes have FFT function, and the above system can even pre-test the current harmonics according to EN61000-3-2 standard.

　　14. Can an oscilloscope perform filtering? For example, low-pass filtering of PWM waves?

　　Answer: TDS5000 can perform 20MHz and 150MHz low-pass filtering, and can also perform a digital low-pass filtering called high-resolution acquisition. In this mode, the vertical resolution of the sampling point can be increased from 8 bits to 12 bits. The above system can output a sine wave waveform similar to a signal such as PWM according to the trend of pulse width change.

　　15. When using a digital oscilloscope, what are the principles for setting the B trigger and trigger level and the measured signal?

　　A: Tektronix oscilloscopes support A and B trigger functions. Simply put, they can trigger dual event sequences. When AB seq is selected, the A event is used as the main trigger to capture complex waveforms in conjunction with the B event. The trigger method is the A event arm trigger system. When the defined B event occurs, it triggers at the B event. For detailed trigger instructions, please refer to the oscilloscope manual.

　　16. How to use TDS3052B to measure the maximum value of a modulated wave with a carrier frequency of tens of K and a modulating wave frequency equal to the power supply frequency?

　　A: The power frequency input may be a low frequency of 50Hz/60Hz, and the carrier is tens of K. A power frequency cycle is about 20ms. If the oscilloscope needs to observe a 20ms signal, the duration acquisition window of the oscilloscope must be at least 2ms/div ×10 grids. At the same time, the sampling rate of the oscilloscope is determined based on the carrier signal of tens of K. Finally, the required acquisition memory length can be estimated to determine whether it can meet the test requirements.