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We see in Fig. 6-4 that the first element of the vertical deflection subsystem is the input coupling selector. Its purpose is to allow the oscilloscope more flexibility in the display of certain types of signals. For example, an input signal may be either a dc signal, an ac signal, or a signal consisting of an ac component superimposed on a dc component (ac + dc signal). Sometimes only the ac components of the (ac + dc) type signal are of interest; at other times both the ac and dc components need to be displayed. The input coupling selector allows us to choose which of the signal components will be coupled to the amplifier circuitry for subsequent display.The input coupling selector is shown in Fig. 6-7 as a three-position switch, with the switch positions being ac, gnd, and dc. When the dc position is selected, the input terminal couples (connects) the entire signal to the subsequent elements of the vertical deflection system. On the other hand, if the ac position is chosen, we see that a capacitor is placed in series with the input branch to the amplifier. This capacitor appears as an open circuit to dc components and hence blocks them from entering the subsequent circuitry. In the ac position very low frequency ac signals are also blocked (below 2-10 Hz, depending on the oscilloscope design).
Ac coupling is used when a high-frequency ac signal is to be displayed without its dc component. This can be a useful option if the magnitude of the ac component of a signal is much smaller than its dc level. (An example is the ac ripple on the dc output of a power supply.) If the signal with both dc and ac components is displayed, any changes in the larger dc component might drive the entire signal off the screen. By eliminating the dc component, the ac part of such a signal can be more conveniently observed. The dc input coupling is generally appropriate for displaying most other types of signals.
The gnd position of the input coupling switch of Fig. 6-7 grounds the internal circuitry of the amplifier. It does not convert the input terminal to a ground point for an input signal. The action of selecting the gnd position causes any charge that is stored in the input attenuator to be removed, and to recenter the electron beam. This capability finds application when it is desired to recenter the beam without removing the input leads to the scope.
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