A Quick Guide to Using an Oscilloscope

WisdomAugust

An oscilloscope is a test instrument that allows you to “see” how the voltage at a point in a
circuit varies with time. A ‘scope’ samples the voltage and displays it on a screen that is
marked with a grid in division. From the vertical gain in Volts/div we can measure the
amplitude of the signal in Volts, and from the horizontal scale in sec/div (the ‘timebase’), we
can measure the timing of that signal. Scopes always have two or more channels so that the
relative timing or amplitude of two signals can be compared.


Scopes are often a complete mystery to students when they begin a course. Since we expect
you to be proficient in using a scope, we have written this short note to help you get started.
Scopes are actually quite straightforward to use. Regardless of how fancy a scope may appear
to be, any scope really has only three controls – vertical, horizontal, and triggering. These are
described below.  

WisdomAugust

5.1 Whatever Else You Do it is ALWAYS Good Practice to? Check the input gain and probe attenuation settings by measuring a known voltage, such
as a +5V or +12V supply rail.



5.2 and Please, NEVER, EVER
? Use a scope probe for anything other than probing circuits;
? Use anything other than a scope probe for probing circuits;
? Poke the pointed probe tip into anything, including a solderless breadboard. It will get
snapped off, ruining the probe and costing you money;
? Twist or kink the scope probe lead – it is a small coaxial cable, and easily damaged.
Even slight damage can cause internal reflections of high frequency signals;
? Connect the probe ground clip to a mains voltage (active or neutral) when measuring
mains AC circuits. This will short the mains voltage to protective earth through the
scope, destroying the scope.
? Use a multimeter in place of a scope unless you are absolutely certain that you are
measuring a DC voltage. Check the circuit node first with a scope, and then you might
as well not bother to get the multimeter out…  

WisdomAugust

5 Useful HintsThis section provides some hints and suggestions for good practice.

? If nothing shows on the screen, follow these steps:
o Make sure that the scope is powered on.
o Make sure that at least one input channel (say Ch1) is turned on.
o Set the VERTICAL gain to 5V/div. Set the HORIZONTAL time base to 1ms/div. Set the
TRIGGERING to Ch1, Auto, and DC Coupling. Turn the screen brightness up.
Ground the probe, and wiggle the VERTICAL POSITION until you see the trace – it
will be a horizontal line. You can then slowly adjust the controls, hoping that the
beam will not vanish again. If it does, perhaps you should go back to the start of
these notes.
o On an analog scope there may be a button marked Beam Finder. On a digital
scope, you can use the Auto Set button. This is about all it is good for!

? The VERTICAL control will always include an INVERT setting. In conjunction with the
INPUT MODE control this allows one input signal to be subtracted from another.  

WisdomAugust

4 Grounds
A scope measures the voltage of the input signal relative to “ground”, the voltage on the outer
conductor in the BNC input connector. This ground is usually tied to the oscilloscope chassis
for shielding reasons. The protective earth conductor of the AC power supply (the third pin of
the power plug) is also connected to the instrument chassis. In other words, one is usually
forced to measure the voltage of the input signal relative to the arbitrary ground of the
protective earth conductor. If a circuit under test is floating above ground, you cannot connect
the probe ground clip to the circuit because this will cause a direct short circuit to protective
earth through the scope. In this situation, you must make a differential measurement of the
voltage between two circuit nodes by using two probes. Invert the input of one probe, then use
the oscilloscope’s ADD function.  

WisdomAugust

3 Digital Scopes

In an analogue scope, the input signal is amplified and used to deflect an electron beam that
excites a phosphor to produce the screen trace – like a TV. A digital scope is quite different: it
rapidly samples the input signal with an analog to digital converter, then stores the digitised
samples in memory. The samples are then retrieved for display. It is clear that a digital scope
is intrinsically also a storage scope, capable of capturing and displaying single, non-repetitive
events. Digital scopes are therefore very useful for looking at digital signal.

The foregoing has avoided much description of the form of the operator controls, since both
analog and digital scopes have essentially the same controls. The only real difference is that
an analog scope will have many knobs and push buttons, whereas only the most important
controls on a digital scope will have dedicated knobs, and the other controls are usually
implemented through screen-based “soft” keys that change in function as the user steps
through menus on the screen.  

WisdomAugust

2.1 Probe Compensation
In practice, adjustment of probe capacitance is very easy. All scopes provide a reference
output, labelled something like PROBE ADJ or CALIBRATE, that is typically a 1kHz square
wave at 5V peak-to-peak. Just clip the probe onto the reference output, set up the triggering,
and adjust the probe capacitance by turning the tiny screw in the probe head. When correctly
adjusted, the screen trace should show a crisp, “square” square wave, free of rounded-off
corners, overshoot or oscillations.

Do NOT attempt to adjust the probe compensation screw on the Lab scopes – it is fastened
with a thread locking compound. The Lab scopes have a feature that allows checking of probe
compensation. To use this feature, set the Probe entry in the VERTICAL menu to match the
probe, clip the probe onto the “PROBE COMP” output (see Figure 3), and press the “PROBE
CHECK” button. The message PASSED should be displayed on the screen.  

WisdomAugust

The input impedance of a scope is typically around 1M? in parallel with about 100pF. Even
this may load the circuit under test too much, with the input capacitance distorting the
measured waveform. The usual solution is to use a “10x” passive scope probe which has a
resistance of 9M? in parallel with an adjustable capacitor of about 5pF. When the capacitor is
adjusted to be 1/9th of the capacitance of the scope input plus probe lead, the probe will have
an impedance of 10 M? at all frequencies. Loading of the circuit under test will have been
reduced by a factor of 10, and waveform distortion eliminated through the process of probe
compensation. You should normally use the scope probe in the “10x” mode.


In Figure 4a, the short black lead with the alligator clip is the ground connection, to be
clipped to the +0V reference point in the circuit. The probe hook can be seen just above the
probe itself. This style of probe hook pulls (gently!) off the probe; some styles screw off.  

WisdomAugust

2 The Importance of Probes

The function of an oscilloscope is to allow you to “see” a time-varying waveform. The
function of a scope probe (see Figure 4) is to transmit the waveform from the circuit node to
the scope input without distortion or added noise. That is, a scope probe is not just any old bit
of wire that happens to be lying around on the lab bench.




Figure 4: a) A scope probe;                                                                                                                           b) Not a scope probe.

WARNING! Scope probes are delicate and expensive. We will not tolerate anyone misusing
scope probes. You have been warned!  

WisdomAugust

Typical triggering controls are shown in Figure 3.

                  


Figure 3: Typical trigger controls of an analog (left) and digital
oscilloscope (right).


Fancy scopes often have more, fancy, triggering options. You may come across scopes that
trigger when a particular series or parallel pattern appears on one or more inputs, or a scope
that triggers when a waveform goes outside of a pre-set voltage-and-time envelope.

WisdomAugust

Now that you understand the basics, here are descriptions of all the usual trigger controls:
? LEVEL: This knob allows the trigger reference voltage to be set;
? SLOPE: Sets the slope (+ or –) of the signal that will cause triggering as it crosses the
level of the trigger reference voltage;
? MODE: Allows selection between
o NORMAL: the trace will be drawn on the screen when the trigger source signal
crosses the trigger reference voltage with the correct slope;
o AUTO: the trace “free runs”. This is sometimes useful for looking at signals that
have very little AC component, and also for signals that sometimes get very small.
Both types of signals can be hard to trigger on;
oSINGLE: Once “armed”, the scope will trigger the next time that the input signal
matches the trigger level and slope condition. This triggering option is essential for
viewing non-repetitive waveforms, and is most useful when the oscilloscope is a
digital storage scope.
? SOURCE: Determines which signal is compared to the trigger reference voltage.
Usually selectable between
oCH1, CH2, etc: The various input signals;
oEXTERNAL: some other signal that is injected through a dedicated input;
o LINE: the AC power supply for the scope, and maybe some other options.
? HOLDOFF: is a time delay before the scope can re-trigger. It is sometimes useful when
an input signal is nearly repetitive.