Oscilloscope

OSCILLOSCOPE

The oscilloscope is an indispensable tool that allows the technician to see how an electronic signal looks and behaves. Unlike the VOM (volt-ohm-meter), which is generally limited to measurements of voltage and continuity, the scope has the ability to sample a signal and display its characteristics on a screen (when applied to a time constant). In the most general terms the scope operates similar to a television. It takes an incoming signal and reproduces it as an electron beam or dot moving across a screen. Making that dot move quickly enough over the screen creates a solid image. In the case of the scope we are not wanting a picture like a TV produces but rather a video representation of electronic input displayed under a special set of conditions. Looking at the blue-green screen on the scope above and you see black crossing lines called a grid, or graph. Let us say you assign a starting point and an ending point to the movement of a dot across the screen plus an upper and lower measurement. You can gather information about how it behaves when referenced to the lines on the grid. To better illustrate this let's suppose that we have set up a dot that moves across the screen starting at the left most vertical line moving across to the line on the far right. If it travels over and over rapidly enough it would appear as a solid line. We can set up a time interval so we know how many trips it is making per second. Now we sample some kind of a signal and make it act on the traveling dot, or line. For this discussion let's use common household alternating current. The sampling of the signal causes the dot on its travels from left to right to go above the line when the voltage is positive and below the line when is is negative. It acts this way on the screen because the scope electronics is able to take the signal sample and convert it into a charge that is applied to vertical and horizontal deflection plates mounted inside display tubes neck. (The tube is formed similar to a TV picture tube in that it has a long slender neck on the starting end and flares out to the bell shape on the screen end.) The strength and location of the plates is what causes the beam of electrons coming from an emitter or gun to bend or deviate inside the display tube while it is on its way to hitting the screen. Now if we have set our scope time interval to 100 events per second and spread it across the screen from 0 on the left to 100 on the right (dot going from left to right 100 times per second) and the divisions (lines that divide the screen into sections running left to right going above and below the center line) to each line equaling 50 volts, we should see a sine wave that represents 110 volts AC on the screen. Its starting point would be on the line somewhere starting on the left at a location we called zero and would arc up and peak at a positive 110 volts (we said each line division equaled 50 so it would peak at just over two lines) then arc back down and pass through the centerline repeating the arc below the line to a negative110 units and end up back at the line sixty events down from its starting point. The process would repeat over and over again until we shut off the power or terminated the signal. What this would tell us by looking at the picture on the screen is that the reference signal going into the scope based upon our settings is fluctuating from positive to negative 110 volts sixty time per second, which is exactly what AC line voltage does. Suppose for some reason the cycles were to go to 50 times per second. We could see it on the scope because our sine wave would travel a shorter distance. A VOM wouldn't show this difference, it would still read the voltage as 110 AC volts. Now take this same principle and apply it to other kinds of signals and you can see the value of seeing the signal as opposed to just measuring it. Sampling and evaluating signals is what a scope is all about and this is why it is a valuable tool. Say you want to compare two signals at the same time. You can do that with the dual trace scope show above, you just have two incoming samples. The scope can also be set up so that the incoming signal triggers the start of the dot travel. This is a commonly used function and you'll see that used numerous times throughout this refurbishing program. The scope is a fairly simple device, it only becomes complicated when you factor in the different settings and functions, most of which are beyond this discussion. I've purposely kept it simple here because this isn't intended to be class in using the scope. I know some of you technicians out there may think it's too general but this description was only intended to cover the basics. If you wish to learn more about the principles, operation and theory of using an oscilloscope I recommend researching at the library. Tektronix publishes a nice training manual: XYZs of Oscilloscopes, publication number 070869001 available online, click here www.tektronix.com. Another good one is by Design Electronics, to access it click here http://www.doctronics.co.uk/scope.htm. To go back to the original scope page click here.