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In electronic circuits, the process of sending the output signal from a preceding circuit (or signal source) to a subsequent circuit (or load) is called coupling. The function of coupling is to transfer (or convert) the energy of one circuit to other circuits. In an oscilloscope, there are two types of coupling settings: one relates to the coupling of channels, and the other relates to the coupling of triggers. Today we will go into detail about the functions and differences of these two coupling settings. Let’s start with the coupling methods for the oscilloscope channels. Generally, by opening the oscilloscope’s channel menu, one can see that there are three settings for channel coupling: DC coupling, AC coupling, and ground. We feed the oscilloscope a sine wave signal with a frequency of 1KHz, an amplitude of 100V, and a bias of 50V (i.e., this signal contains a 50V DC component). DC coupling, also known as direct current coupling, means that when this option is selected, the signal travels directly to the front-end amplifier through wires. Both the DC component and the AC component present in the signal can pass through, allowing for the observation of waveforms as low as 0Hz with no significant DC offset. At this point, the signal is displayed as shown in the figure: AC coupling is also known as AC coupling. When this option is selected, the signal is coupled to the front-end amplifier through a capacitor; the DC component of the signal being measured is blocked, allowing only the AC components to pass through. This approach can be used to view waveforms with significant DC offsets. At this point, the signal is shown as follows: it can be seen that the signal has moved downward from the zero point (the zero point is indicated by the number 1 inside the yellow pentagon on the left). In the previous diagram, the zero point was located below the waveform, while now it is in the middle of the waveform, as the DC component of the signal has been filtered out. The vertical scale of the oscilloscope is 20 V/div; the signal was shifted down by 2.5 divisions, which is roughly equal to 50 V. When the coupling mode is to ground, it means the internal input is grounded while the external input is disconnected. At this point, the signal is displayed as shown in the figure: The function of ground coupling is to help us locate the zero point accurately when it is not convenient to make an external connection, or when there is significant external interference. Channel coupling is used to control the delivery of signal energy to the front-end amplifier of the oscilloscope. Trigger coupling is used to control the manner in which energy is delivered from the signal to the oscilloscope’s trigger circuit. Common types of trigger coupling include DC, AC, high-frequency suppression, low-frequency suppression, and noise suppression. Similar to channel coupling, when DC coupling is selected, both the DC component and the AC component can pass through the trigger. When AC coupling is selected, the oscilloscope filters out the DC component in the trigger signal. High-frequency suppression suppresses signals in the trigger signal that are above 50KHz, while low-frequency suppression suppresses signals below 50KHz. Noise suppression, on the other hand, uses low-sensitivity DC coupling to eliminate high-frequency noise in the trigger signal. Let’s take a look at the following signal: This signal uses AC coupling, and even when the trigger level exceeds the waveform, the signal can still be synchronized with the scanning. Because this signal is a 2V square wave with a 1V DC component. Therefore, when the triggering coupling mode is AC, the signal should actually drop by 1V; as a result, it can still be triggered at a trigger level of -500mV. Let’s take a look at the signal below: This signal makes use of low-frequency suppression. Although the trigger level falls within the acceptable range for the signal, the signals with frequencies below 50KHz in the trigger signal are suppressed, which prevents the signal from being synchronized during scanning and results in unstable signal behavior.