The principle of analog scope operation is simple. Take a signal, which for these purposes is a variation in voltage over time. Let’s say it’s a sine wave. The backbone of an analog scope is a very high performance cathode ray tube (CRT). Water analogies seem to be common in electronics so picture the inside of the CRT as a squirt gun. The squirt gun has a little pump – if you hold the trigger it will keep shooting. Now mount the squirt gun on a gimbal. The gimbal allows it to rotate left to right and up and down, but prevents any other movement. Get three people. Person #1 controls the left to right movement of the squirt gun. Person #2 completely ignores the first and can only move the gun up and down. Give this person a voltmeter. Person #3 has the awesome responsibility of pulling the trigger. Got that? Let’s start a display cycle.
First, Person #2 points the squirt gun to your left. Simultaneously, Person #3 pulls the trigger and a beam of water starting heading towards you. Fortunately, someone kindly positions a big piece of plastic between you and them, so all you see is the point where the water splashes onto the plastic. Next, Person #2 begins moving the beam at a constant rate from left to right. Person #1 reads his voltmeter and moves the gun up and down proportionate to the voltage of the signal. When the beam reaches the far side of the plastic shield, Person #3 lets go of the trigger, the beam stops, and Person #2 turns the squirt gun back towards the left side in what is known as a “blanking period”. If the water glows, the voltmeter could measure infinitely fast, and you in fact recruited superhumans who could do all these operations about 60 times per second, you would have the largest functioning oscilloscope known to man!