Super practical 100 questions and answers on oscilloscope basics

1. How to use an oscilloscope to test and analyze the reliability of a designed product?

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

Answer: There are many types of oscilloscope probes with different performances, such as high voltage, differential, active high-speed probes, etc., and the prices range from a few hundred RMB to nearly 10,000 US dollars. The main determinants of price are of course bandwidth and features. The probe is the part of the oscilloscope that contacts the circuit, and a good probe can provide the fidelity required for testing. In order to do this, even passive probes must have a lot of passive component compensation circuits (RC networks) inside.

3. What is the service life of a typical Keysight oscilloscope probe? Does the probe need to be calibrated regularly?

Answer: It’s hard to say how long the probe life of an oscilloscope will last, 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 the probe is replaced or the probe changes channels, probe compensation adjustment must be made. All active probes should be warmed up for at least 20 minutes before use. Some active probes and current probes require zero drift adjustment.

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

Answer: The real-time sampling rate is the reciprocal of the sampling interval of one acquisition (one trigger) of the oscilloscope. It is understood that the current highest level in the industry is the simultaneous use of four channels.

5. What is the equivalent time sampling of an oscilloscope?

Answer: Equivalent time sampling refers to the oscilloscope piecing together the waveforms collected by multiple acquisitions (multiple triggers) into one waveform. Each sampling rate may be very slow, and the trigger points of the two acquisitions have a certain offset, and the final waveform is formed. The reciprocal of the minimum sampling interval between two points is called the equivalent sampling rate. Its index can reach very high, such as 1ps.

6. What is power factor? How to measure?

Answer: Power factor: In a DC circuit, voltage multiplied by current is active power. But in an AC circuit, the voltage multiplied by the current is the apparent power, and the part of the power that can do work (that is, the active power) will be less than the apparent power. The ratio of active power to apparent power is called power factor, represented by COSΦ. In fact, the simplest measurement method 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. Generally, W/in3 is 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 obtain the core loss.

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

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

10. How can a LeCroy oscilloscope test the radiation from the starting power supply?

Answer: There is radiation interference in switching power supplies. The general approach is to try to detect the source of interference and then shield it. An oscilloscope can use the Fourier transform function to analyze its frequency component composition and determine the type of interference 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 transformers?

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

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

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

13. Can an oscilloscope perform Fourier decomposition?

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

14. Can an oscilloscope be filtered? How to perform low-pass filtering on PWM wave?

Answer: TDS5000 can perform 20MHz, 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 8bits to 12bits. The above system It can output a signal such as PWM that has a similar sine wave waveform according to the trend of pulse width change.

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

Answer: Tektronix's oscilloscope supports the A and B trigger function. Simply put, it can be triggered by dual event sequences. When AB seq is selected, the A event serves as the main trigger and cooperates with the B event to capture complex waveforms. The trigger method is the A event arm trigger system, which is triggered at the B event when the defined B event occurs. For detailed triggering instructions, please refer to the oscilloscope manual.

16. How to use TDS3052B to measure the maximum value of the modulated wave when the carrier frequency is tens of K and the modulation wave frequency is the power frequency?

Answer: The power frequency input may be low frequency 50Hz/60Hz, while the carrier wave is tens of K, and a power frequency cycle is about 20ms. If the oscilloscope needs to observe a 20ms signal, that is, the duration acquisition window of the oscilloscope is at least 2ms/div × 10 divisions. 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.

17． Use a DSO oscilloscope with a nominal 100MHz to measure a high-frequency switching amplitude of 400V, f=50M. How does the oscilloscope depict its waveform and rise time?

answer:

① The bandwidth of the oscilloscope is defined based on the -3dB point where the sine wave amplitude attenuates.

② The depiction of waveforms and rise times in digital oscilloscopes is obtained by obtaining waveform data through real-time sampling circuits and high-speed A/D converters, and then through interpolation operations.

③ In Tektronix oscilloscopes, there is a real-time processing circuit to complete the so-called sine interpolation function, which is completed in the signal acquisition circuit part. Of course, many oscilloscopes also perform mathematical operations through the oscilloscope's main processor, which takes a lot of time.

④ I am afraid that the signal you are measuring cannot be measured using a 100MHz oscilloscope. For a 50MHz square wave, in theory, an oscilloscope of 450MHz or above should be used to accurately reconstruct the most important harmonics below the 9th order in the signal, thereby ensuring that the waveform is not distorted. What's more, you may also have to consider the signal rise time. In theory, the rise time of the oscilloscope should be more than 5 times faster than the signal.

⑤ The same goes for probes. Since ordinary probes will produce high-frequency distortion effects when measuring high voltage, you should use special differential probes or high-voltage probes such as Tektronix's P5205 and P5100 for measurement.

18． How to make good use of digital oscilloscope in analog circuits, such as measuring small signals of audio amplifiers, clutter of power supplies, etc.?

Answer: Issues to pay attention to include:

① The grounding problem of the oscilloscope. The oscilloscope’s chassis and the reference ground wire of the probe are both connected to the ground wire. Therefore, good grounding is the primary condition for measuring interference.

② The interference problem caused by the reference ground wire of the oscilloscope. Since ordinary probes usually have a ground wire, they will form an interference path similar to a loop antenna with the point to be measured, introducing relatively large interference. Therefore, to minimize this interference, you can use The best method is to remove the probe cap, do not use the ground wire drawn from the probe, and directly use the probe tip and the location inside the probe to contact the point to be measured for measurement.

③ Use differential measurement method to eliminate common mode noise. Tektronix provides a series of differential probes, such as the ADA400A, which is specifically designed for small signals and can measure hundreds of microvolts, and the P7350, which is used for high-speed signal measurement, providing bandwidths up to 5GHz.

④ Many Tektronix oscilloscopes provide high-resolution acquisition (Hi-Res) signal capture mode, which can filter random noise superimposed on the signal.