Uncovering the secrets of testing and measurement - Test every Monday [Week 10]

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2. In order to improve the measurement accuracy, the oscilloscope probe usually needs to be calibrated. What does the calibration include? How to calibrate the frequency response characteristics of the probe, cable and fixture?
(For reference, please refer to the technical article
http://www.home.agilent.com/agilent/redirector.jspx?action=ref&cname=AGILENT_EDITORIAL&ckey=2064864&lc=eng&cc=US&nfr=-11143.0.00&pselect=SR.GENERAL )

3. Why do we need to adjust the vertical scale as much as possible so that the signal can be displayed at full scale when measuring the signal?

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1. What are the main types of loading effects of oscilloscope probes? What methods can be used to reduce these loading effects and thus reduce the impact on signal measurement? (For reference, please refer to the technical article
http://www.home.agilent.com/agilent/redirector.jspx?action=ref&cname=AGILENT_EDITORIAL&ckey=1378155&lc=eng&cc=US&nfr=-11143.0.00&pselect=SR.GENERAL )

(The answer is provided by Agilent's Ma Zhuofan) The load effect of the oscilloscope probe mainly includes resistance effect, capacitance effect and inductance effect. According to the measurement requirements, when selecting a probe, try to choose one with a small load effect. For example, for basic low-speed signal measurement or voltage observation, you can choose a high-resistance passive probe; when measuring signals on high-speed transmission lines, choose a low-resistance passive probe or active probe with a small capacitance load. When using the probe, use a short ground wire or ground wire wrapped around the probe, and the probe ground is connected to the ground closest to the signal. Use a damping resistor at the tip of the active probe, and try to use a shorter needle tip lead.

2. In order to improve the measurement accuracy, the oscilloscope probe usually needs to be calibrated. What does the calibration include? How to calibrate the frequency response characteristics of the probe, cable and fixture?
(For reference, please refer to the technical article
http://www.home.agilent.com/agilent/redirector.jspx?action=ref&cname=AGILENT_EDITORIAL&ckey=2064864&lc=eng&cc=US&nfr=-11143.0.00&pselect=SR.GENERAL )

(The answer is provided by Agilent's Zhuofan Ma) Traditional probe calibration includes vertical attenuation and offset calibration, delay calibration between multi-channel probes. Vertical attenuation and offset calibration can be called DC calibration. The oscilloscope scans and outputs a reference DC signal, and the probe is used to measure the reference signal to obtain attenuation and offset parameters. The frequency response characteristics are functions missing from traditional probe calibration methods. Agilent uses indium phosphide semiconductor materials to implement a calibration chip of less than 15ps in the DSAX90000A series of oscilloscopes, and an independent calibration source N2806A with the fastest edge of less than 7ps. Without the need for additional network meters, TDRs and other instruments, the TDT (time domain transmission) of external connections such as probes or cables can be accurately measured, thereby compensating for the high-frequency attenuation of the probe and cable connections, and correcting the amplitude-frequency characteristics in the frequency domain.

3. Why do we need to adjust the vertical scale as much as possible so that the signal can be displayed at full scale when measuring the signal?

(The answer is provided by Agilent's Ma Zhuofan) The vertical sensitivity adjustment of the oscilloscope is to adjust the attenuation ratio. The larger the measurement range (that is, the higher the voltage value per vertical grid), the greater the attenuation ratio of the front end of the oscilloscope. The digital system after sampling needs to compensate for the attenuation ratio by the number of times the front end is attenuated, and the corresponding multiple is amplified. The background noise of the instrument mainly comes from the analog front end, before ADC. For the inherent noise, the amplification compensation of the digital system behind also amplifies the noise. The greater the attenuation at the front, the greater the amplification compensation at the back, and the greater the noise is amplified.

1. The background noise of the oscilloscope, the greater the vertical sensitivity (the greater the voltage per vertical grid), the greater the background noise.

2. The probe is part of the measurement system. The greater the attenuation ratio of the probe, the more the digital system amplifies, and the worse the signal-to-noise ratio of the measurement result. Therefore, if the amplitude of the measured signal itself is relatively small, especially when measuring signals such as power supply noise, this effect is more obvious. When selecting a probe, try to choose a 1:1 probe, or even better, use a coaxial cable to measure to reduce the noise introduced by the probe.

3. When measuring, try to use a small vertical amplitude scale to reduce the impact of the instrument background noise. From another perspective, a smaller vertical scale means that the signal is closer to the full scale of the screen vertically. The closer to the full scale, the more the signal can utilize the full scale of the ADC and the more precise vertical resolution voltage level of the ADC (full scale votage / 256) (for an ideal 8-bit ADC, the vertical resolution level is 2 to the 8th power), thus the vertical measurement accuracy is higher.