3 Coupling of Input Signals
Thursday, 12 July 2018
8:11 AM
The DSO has the ability to insert a capacitor between the external input and its internal analog acquisition circuitry. This can be represented by the circuit below, known as the input ‘coupling’ circuit:

We will investigate the effect and use of the input coupling circuit.

Turn Channel 2 on by pressing the 2 key. Set 1 V/div on both channels.

Set the FG frequency to around 3 Hz and measure the FG output on DSO Channels 1 and 2 simultaneously. Adjust the Time/Div knob to display approximately 2 cycles of the sinusoid.

On the Channel 2 menu, set the Coupling to AC. You should see a “shifted” sinusoid on Channel 2. Sketch the observed waveforms in the correct time relationship below. Show the voltages and time on your plot.

Sketch of phase-shifted sinusoids


Change the FG to a square wave. Note the significant change in wave shape. Sketch the observed waveforms in the correct time relationship below. Show the voltages and time on your plot.

Sketch of square wave and AC-coupled square wave


Change the FG to a triangle wave. Note the significant change in wave shape. Sketch the observed waveforms in the correct time relationship below. Show the voltages and time on your plot.

Sketch of triangle wave and AC-coupled triangle wave


Note: AC coupling should be used with caution, because at low frequencies it can radically alter the observed waveform!


Change the FG to a 20 kHz sinusoid, and adjust the Time/Div knob to display 2 cycles of the sinusoid.

Set the trigger on the DSO to use Channel 2 with HF Reject selected.

On the FG, turn on the “DC offset” and apply approximately 3 V of DC to the sinusoid.

Now reduce the amplitude of the sinusoid to a minimum. Turn the Volts/Div knob on Channel 2 to 100 mV/div to display a fairly large sinusoid. We can now get “AC detail” from a waveform that has a large DC component:


AC coupling will remove the DC component of a waveform

Note: The AC input coupling capacitor has blocked the DC component of the waveform, allowing us to observe only the AC component.


Sketch the observed waveforms in the correct time relationship below. Show the voltages and time on your plot.

Sketch of shifted sinusoid and AC component


Set the trigger on the DSO to use Channel 1 with HF Reject off.

Set Channel 2 to 1.00 V/div and then turn it off.

On the FG, turn the DC offset to ‘off’.