July 11, 2013
Among the equipment at Hackforge, we have a 100MHz dual trace oscilloscope. This extremely versatile piece of test equipment is one of the most useful made. What is it, and how can you put it to use in your next project? Check below the fold to find out!
The oscilloscope creates a graph of voltage vs. time. Voltage is the amount of push that electricity has, much like water in a garden hose. Voltage is used to encode information so that we can do useful things with it. One of the most common uses of changing voltage is to represent sound. When we have a problem, we must often see exactly how the voltage is changing to know exactly what the nature of the problem is.
The oscilloscope incorporates three fundamental components:
- A display
- A vertical amplifier, and
- A time base
The display is used for viewing the waveform. The oscilloscope at Hackforge is an analog machine, so it uses a CRT(cathode ray tube) for display. The glass front is coated with phosphor. A device called an electron gun fires a focused beam at the rear of the glass, striking the phosphor coating. This makes it glow. The coating used by an oscilloscope differs from that used by a TV set, whereas the TV uses individual dots, a scope uses a continuous coating. The electron beam in a TV is constantly scanning over the entire surface of the screen, whereas in a scope it only moves side to side and up and down as needed. A TV set(and most displays) are termed raster displays, and the motion of the electron beam is called rastering. An oscilloscope, however, uses a vector display. Instead of sweeping all over, the beam is moved to specific places, drawing whatever shape is needed along the way.
We have already seen here a fundamental concept of oscilloscopes: wiggling. We want to make the electron beam wiggle. Surrounding a part of the CRT called the neck are four large electromagnets, these are called the yoke. Left, right, up, and down. To make the beam move, we apply power to one of the magnets. The magnetic field thus created pulls the electron beam toward the magnet. The first thing we need to do is to make the electron beam wiggle up and down. So we have to apply power to the up and down magnets in the yoke. This has to be done in a way that mimics what’s happening with the voltage we are monitoring. The problem is that these are pretty big magnets, and it takes a lot of power to drive them. Most of the signals we will be dealing with aren’t nearly strong enough to drive the magnets directly. We need to make the signal louder. How do we make signals louder? With an amplifier. This is a device that can take very small signals and make them much bigger. The vertical amplifier makes the electron beam wiggle up and down(vertically) by making the signal bigger, and using it to power the up and down magnets in the yoke. The amplifier used in a scope is special, though. It is calibrated so that we can use the distance the beam moves up and down to make measurements. This way not only can we see what the wave looks like, we can also measure the voltage at different points. This lets us see if the wave is too loud or not loud enough for our circuit.
Wonderful! We’ve got the beam wiggling! But wait- there’s a problem. The signal is changing too fast. It just looks like a vertical line. What we need now is to make it wiggle from side to side. This is where the timebase comes in. This makes the beam move from side to side at a very precise speed. This means that we can see the whole wave properly, and we can measure things like frequency. This is how fast the wave is changing.
Now, lets put the scope to work!
My amp is on, but there’s no sound!
You’ll remember that we use a changing voltage to represent sound. How can we troubleshoot this problem? We follow some basic steps:
- Identify path that the signal takes through the circuit. This is not always easy, so persevere and when you don’t understand what the circuit is doing, ask for help and check with google. Fortunately amplifiers are usually very simple circuits. Getting a handle on what is happening may take a little while the first time, but be assured that you can understand what’s happening.
- Setup a signal generator. This can be anything from a complex piece of bench test equipment, to something as simple as an MP3 player. A sine wave tends to be preferable, it’s easier to identify and allows you to see in what way the signal is being changed by the circuit.
- Follow the signal and see where it stops. The fault probably lay at that point.
Clip the ground from the oscilloscope probe to an appropriate ground. The closer to where you are testing the better. Start looking at the point where the signal enters the system. Follow the path, and it will lead you to the problem.
Why is my microcontroller not running?
When troubleshooting, it’s important to understand that every subsystem has certain things it needs to work. A microcontroller needs power, ground, a clock signal, and a program. You’ve loaded it with the program, and checked that the power supply and ground are good. But you need to check the clock. Most modern microcontrollers have clocks that run around 20MHz, a multimeter is useless at that speed. This is where the scope can come to the rescue!
You’ll probably want to set the vertical amplifier at its lowest setting, around 10mV/division. That means that each square on the grid is equal to ten millivolts. The tick marks inside the squares will be 2,4,6, and 8 millivolts. Connect the probe to one of the leads of the crystal, or in the case of a ceramic resonator, one of the outside leads. The middle is usually ground. Adjust the timebase. It has a knob graduated in seconds per division. You’ll notice that this is period, not frequency. So we need to figure out what the period of a 20MHz signal is. Dividing 1/20,000,000 we get 50 nanoseconds. We set the knob to about 50ns/div. On the timebase there is a little light that says “triggered”. That should be lit. If it isn’t, that means the scope isn’t seeing a signal.
Tweak the settings and you will be able to see whether the circuit is oscillating.
The oscilloscope is the most versatile of test equipment, and can save hours of head scratching. Take some time, and come down to Hackforge and we’ll show you how to run it in person!