The Basics of Oscilloscopes


Oscilloscopes (commonly called "scopes" or "o-scopes"), at first glance appear to be very complex, intimidating devices.  In reality, they are actually rather simple to use.   They are used to view signals in the time domain, where the horizontal axis is time and the vertical axis is amplitude, although some modern digital scopes have FFT (fast Fourier transform) capability to allow viewing of signals in the frequency domain, where the horizontal axis is frequency and the vertical axis is amplitude.  All oscilloscopes, digital or analog, share the same basic four sections: vertical, horizontal, triggering, and display.


The vertical portion of the scope sets the amplitude, or "height", of the waveform on the display.  There is a knob to set the number of volts per division (volts/div) on the vertical axis of the scope display.   There are several settings, usually ranging from as low as 5mV up to 50V or more per division.   Use this knob to set the amplitude of the waveform to the level best for the viewing the signal. There is also a knob usually labeled "position", which allows adjustment of the vertical position of the waveform being displayed.

There are also a few buttons, or sometimes a switch, to set the coupling mode.  Coupling refers to the way the signal gets into the scope.   Usually there are choices for AC, DC, and GND, and sometimes a few other modes. 

If the coupling is set for AC, there is a capacitor in between the scope input amplifier and the probe, so it only passes AC signals, not DC signals.  If the scope is set to DC coupling, it will pass the DC component of the signal.  Don't confuse AC with the AC coming out of the wall mains outlet!  AC simply refers to any signal that has an alternating component, such as an audio sine wave, square wave, or a complex waveform like a guitar signal.

DC mode is used to measure the DC value of a signal, or to view the AC signal and the DC level simultaneously.

Use AC mode to view a signal that is at a high DC level, such as the signal on the plate of a preamp tube.  If the scope is set to DC coupling when viewing a signal at the plate, the amplitude will have to be set to a very high level, such as 50V/div, to get the signal on the screen, which would make the signal riding on that DC level very small and hard to see, unless it is also very large in amplitude.  By switching to AC coupling, the volts/div can be changed to a level where the signal is more easily viewed.

GND coupling disconnects the scope input and sets it to ground, or 0VDC, so a flat line trace will be displayed.  This mode is used to set the scope to a known reference point so DC measurements can be correctly measured.   For example, set  the scope coupling to GND, then adjust the vertical position knob until the flat line is centered in the "railroad tracks" (this is what the center horizontal and vertical axis lines are commonly called, because they look like railroad tracks). Then set the scope back to DC coupled mode, and the DC level of the signal can be measured by counting the number of divisions and multiplying by the volts/div.  For example, if the scope is set to 1V/div, and you count 1.5 divisions from the point set when the scope was GND coupled, the signal is at a 1.5V DC level.

High-bandwidth scopes will have a button to set the bandwidth to a lower level for better viewing of noisy signals.  This is usually a 20MHz BW Limit, and should be used if the signal trace is too "fat" because of high-frequency noise.



The horizontal portion of the scope sets the timebase, or horizontal time, of the waveform on the display.  There is a knob to set the number of seconds per division (sec/div) on the horizontal axis of the scope display.   As with the vertical knob, there are also several settings, usually ranging from as low as 5nS up to 500mS or more per division.   Use this knob to set the horizontal time axis of the waveform to whatever best suits the waveform being viewed.  There will also be a horizontal position adjustment knob, and usually a 10x magnification button for zooming in by a factor of 10.
For example, if viewing a 1kHz waveform, which has a 1mS period (period = 1/frequency, so 1/1000Hz = 1mS), the horizontal timebase can be set to 1mS/div, and there would be one cycle of the waveform shown in each major division of the display.  Since there are usually 10 major divisions in the horizontal, 10 cycles of the 1kHz waveform would be seen. Alternately, to "zoom in" on a smaller number of cycles, the horizontal timebase can be set to a smaller number, such as 200uS/div.  At this setting, only two cycles of the 1kHz waveform would be seen, because 10 divisions at 200uS/div = 2mS, so only two cycles would fit on the display.


The trigger portion of the scope determines how the scope "triggers", or locks onto a waveform.  If the scope is not triggered, the waveform will be rolling through horizontally, and it will be difficult to determine what is going on.  The trigger section allows the waveform to be "stopped", or held stationary on the display, by triggering on a particular portion of the signal each time the scope sweeps in the horizontal direction.   Modern digital scopes take this concept one step further by "capturing", or writing into memory, only a certain number of samples of the waveform and then displaying them on the screen.  
There are usually several trigger modes, including DC, which allows triggering when the signal reaches are certain DC level (adjusted by the level knob), AC, Noise, which rejects noise to allow stable triggering of noisy signals, HF and LF, which reject high or low frequencies.
There are also several trigger source modes, usually one for each channel of the scope, so CH1 would be used to trigger on a signal applied to channel 1, CH2 for channel two, etc., and LINE, which allows triggering on the AC mains signal supplying power to the scope.  This last mode is particularly useful when viewing hum on a power supply, because it is easier to trigger the scope on the AC mains signal than trying to trigger on the hum signal itself.
There is also a polarity button, usually labeled "+" and "-", which allow triggering on either the rising or falling edge of a pulse signal.  The trigger will start at the leftmost side of the display, so if the trigger is set to "+", and a positive-going pulse is applied to the scope probe, the signal on the display will start out high, then go low.  If the trigger polarity is set to "-", the display will start out low, then go high, because it triggers on the falling edge of the pulse.


The display section of an analog scope will usually have controls for intensity, focus, readout intensity, scale illumination, trace rotation, astigmatism correction, and sometimes a beam finder button.   Digital scopes have different controls, such as contrast, viewing angle, and other features, because they have LCD screens instead of analog CRT screens.

Copyright © 2015,  Randall Aiken.  May not be reproduced in any form without written approval from Aiken Amplification.

Revised 04/29/15