The Oscilloscope

Volts/Div Knob Time/Div Knob Screen Function Generator Vertical Control Horizontal Control Trigger Level Control

Pass the mouse over the picture and then click to link to

a description of the object.  You will be able to click on seven

objects:  The function generator, the screen, the trigger control,

the vertical control, the horizontal control, the volts/div

control, and the time/div control.



The oscilloscope is really just a fancy voltmeter.  The main advantage of the oscilloscope over a voltmeter, is that it can be used to follow the rapid change of voltage in an AC circuit, where the current changes directions many times per second.  A normal voltmeter cannot react fast enough to follow rapid voltage changes.


The heart of an oscilloscope is a cathode ray tube.  In this vacuum tube, free electrons are created at the back and accelerated toward the front, where they strike a screen coated with phosphorous, which then gives off light.  If this were all there is to an oscilloscope, the only thing that you would ever see would be a single dot on the screen.  However, beam of electrons coming from the back of the tube  is swept across the screen from left to right by a changing electric field.


Vertical motion of the electron beam is determined by the voltage applied to the leads of the oscilloscope.  So in a sense, the oscilloscope is like a voltmeter; if a constant (D.C.) voltage is applied to the leads, a spot would shift up by an amount proportional to the applied voltage.  If an A.C. sine wave voltage is applied to the leads, the spot will sweep up and down as the sine wave voltage alternates between the maximum positive and maximum negative values.  Since the spot is sweeping left to right at the same time, a sine wave shaped curve will appear on the screen.


Click on  the volts/div knob in the picture above and note that it is set to 50 millivolts. 





Then come back and click on the screen.



The vertical axis is the voltage axis.  Each big block is 3 cm in length.  Each centimeter is a div.  Since the volts/div knob is set at 50 millivolts, then each centimeter or div on the vertical axis is 50 millivolts.  It follows that the maximum and minimum voltage are 150 mV and -150 mV, respectively.  It also follows that the voltage knob on the function generator, must be set at 150 mV, even though there are know settings on this knob.


Next, note that the time/div knob is set at 50 microseconds. 



The horizontal axis is the time axis.  Each block is 3 cm in length.  Each centimeter is a div.  Since the time/div knob is set at 50 micro seconds, then each centimeter or div on the horizontal axis is 50 microseconds.  From the picture of the screen above, we can see that it takes about 4 times 50 microseconds, or 200 microseconds, for one oscillation to occur.  This corresponds to a frequency of 1/200 microseconds or about 5000 Hz.


This result is consistent with the settings on the function generator.  Note that the knob on the right of the generator, the frequency selector, is set close to 5; and, the multiplier, the knob in the middle, is set to 1000.  So we have a frequency of 1000 times 5 equal 5000 Hz.



It should be pointed out that the knob on the far left of the generator sets the voltage.  If you turn the knob to the left, then the amplitude of the sine wave on the screen will increase; and, if you turn the knob to the right, the amplitude will increase.


Sometimes, when you take a voltage reading, the sine curve is moving rapidly from left to right.  This makes it difficult to take a reading.  You can make the sine wave appear to stand still if you turn the trigger control all the way to the right.



Sometimes you will need to make vertical and/or horizontal adjustments of the sine curve.  This can be accomplished with the following two controls.  The arrows indicate if the control is vertical or horizontal.



                                                              vertical control          horizontal control