Oscilloscope Overview and How to Use it

1. Overview of oscilloscope: It is a very important electronic measuring instrument, generally composed of an oscilloscope body and an oscilloscope probe.

Marked 150MHz: The effective bandwidth of the oscilloscope is 150MHz, also known as the -3dB bandwidth of 150MHz.

Marked 1GS/s: The sampling rate of the oscilloscope, which means that the oscilloscope samples 1000_000_000 data points per second.

2. Oscilloscope probe: The three main limitations of the oscilloscope are: insufficient sensitivity, too small input voltage allowable range, and limited bandwidth.

(1) Sensitivity: Ensure that the level of the signal being measured is higher than the lowest signal sensitivity level of the oscilloscope.

(2) Input voltage allowable range: The maximum level is less than 5V, which is the maximum voltage measurement range of most oscilloscopes.

(3) Bandwidth: Ensure that the maximum bandwidth of the signal being measured is less than the nominal rated bandwidth of the oscilloscope.

Note 1: Equivalent circuit of oscilloscope probe:

The ground loop self-inductance of the oscilloscope probe: The ground loop self-inductance of the probe and the parasitic capacitance of the probe will form a resonant loop. When the self-inductance value is large, high-frequency resonance may occur under the excitation of the input signal, causing signal Distortion or ringing. Therefore, the smaller the value, the better. Generally, the ground clamp of the oscilloscope is not used for grounding. Instead, a grounding spring is used for grounding to reduce the self-inductance of the ground loop.

Input impedance of the oscilloscope probe: The larger the input impedance, the better. In order to avoid the load effect of the probe input impedance, the probe input impedance is generally required to be more than 10 times the signal source impedance and load impedance. The input impedance of most probes is between tens of K ohms and tens of M ohms.

Input capacitance of the oscilloscope probe: The input capacitance is the parasitic capacitance of the oscilloscope probe and is the most important factor affecting the bandwidth of the probe. The input capacitance of the probe will attenuate the high-frequency component of the signal and slow down the rising edge of the signal. The input capacitance of most probes is between tens of pF and hundreds of pF.

Note 2: 1× gear and 10× gear of oscilloscope probe:

1× gear: Indicates that the signal enters the oscilloscope without attenuation.

10× gear: indicates that the signal is attenuated 10 times (1/10 of the original signal) entering the oscilloscope. At this time, when the oscilloscope is also set to the 10× gear, the attenuated signal is amplified 10 times inside the oscilloscope, and then read directly; when the oscilloscope is set to the 1× gear, the oscilloscope reading should be amplified 10 times to get the true signal value.

3. Use of oscilloscope:

Oscilloscope measures the ripple of a switching power supply:

(1) In order to reduce the ground loop inductance and reduce high-frequency noise interference. The oscilloscope's probe ground lead should be as short as possible. Usually the probe cap should be removed and the probe's own grounding spring should be used for grounding instead of using a grounding clamp for grounding.

(2) Select the 1× gear of the oscilloscope probe to measure the ripple. Because if you choose the 10× gear to measure the ripple, the signal will first be attenuated by 1/10, and at the same time, the oscilloscope will be amplified 10 times later. This will superimpose and amplify 1/10 of the noise component generated by the oscilloscope probe and the signal component, causing greater noise interference and making the ripple measurement inaccurate.

(3) Pay attention to the bandwidth information of the 1× gear of the oscilloscope probe. Generally, the bandwidth of 1× gear probes is relatively limited, and it is generally required that the bandwidth of the oscilloscope probe should be at least greater than 20MHz.

(4) The oscilloscope must enable the 20MHz bandwidth limit function. Generally, the power output ripple of a switching power supply is in the range of DC~20MHz; while the high-frequency synchronous switching noise and the reflected noise of the signal are in the range of DC~1GHz. Therefore, turning on the bandwidth limit of the oscilloscope can attenuate the high-frequency noise and avoid high-frequency noise and Ripple aliasing.

(5) Select AC coupling as the channel coupling method of the oscilloscope. AC coupling isolates DC voltage and only displays AC signals, which is helpful for observing ripple information. If you choose DC coupling, the DC signal is much larger than the ripple AC signal. At this time, you need to reduce the vertical scale of the oscilloscope and adjust the vertical offset to see the ripple signal, which is inconvenient to watch.