Analog vs Digital Oscilloscopes – The Key Differences

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!

This is actually a pretty good analogy of CRT operation in an oscilloscope. The water represents a beam of electrons accelerated out of the CRT onto a phosphor coated screen. When an electron hits a phosphor dot, energy is transferred, and the dot glows briefly. Instead of people, the electron beam is “deflected” by electromagnets in the form of coils wrapped around the cathode ray tube. Person #1 is the vertical deflection coil which is driven by amplifiers connected to the input signal. Person #2 represents a ramp generator. The higher the voltage from this circuit, the farther the beam is deflected to the right. The ramp waveform begins at its lowest point and over a period of time (determined by the “timebase”) increases the voltage to the horizontal deflection coil causing the beam to move across the screen. Person #3 is a blanking circuit for which the sole function is to turn the beam off as it returns to the other side. Now you ask yourself: what am I seeing as the beam returns? Isn’t it off? Shouldn’t the screen be blinking or something? Good point my attentive reader! Thanks to persistence of vision, the pattern of the waveform is imprinted in our eyes for a short period of time, long enough for the horizontal deflection circuit to reset. If you want to see the flickering on the scope, find a video of analog scope. This one will work. Because the camera captures a complete frame at a different rate than the scope completes a trace, you see jumps between where you think the trace should be and where the next complete video frame places it.

Above I mentioned something called a “timebase”. Timebase is one of those words that makes sense once you think about it. Take for example a 1 Hz 2V peak-to-peak sine wave. If the voltages of this wave were graphed over a period of 1 second, you would see one complete cycle of the wave. Now try to cram 1 second of a similar wave at 100 Hz into the same space. The vertical deflection would be identical (still +- 1V), but each full wave cycle would now take up a hundredth of the space it did previously, making it impossible to figure out what it looks like. The oscilloscope therefore offers to show you only short lengths of a waveform by increasing the speed at which the ramp generator deflects the signal across the screen, effectively stretching the waveform.

At this point I think it prudent to mention a key part of the scope – the “trigger”. This means slightly different things for analog vs digital scopes – this paragraph deals with it solely in the context of analog scopes. [“Triggers” to be added later]

Analog oscilloscope have been around in some form since the late 1930s. By the 1950s, bandwidth hover between 10-20 MHz. I would say that the late 80s and 90s were the golden age for analog scope technology. At that point, a major manufacturer called Tektronix managed to produce scopes capable of displaying signals at 400 MHz. To my knowledge, no other company surpassed this.

Unfortunately, as industrial infrastructure became more dependent on complex digital systems, a need was created for not only high frequency bandwidth, but a way to store signals so that they could be analyzed later. Initially, companies tried to adapt analog oscilloscopes. Remember how an electron hitting a phosphor dot made the dot glow? Well, if a whole lot of electrons hit the same dot, and the dot was specially made to store the electrons and then slowly release them to glow, then the phosphor screen could be made to store the image of a waveform for periods from several seconds up to several minutes.

Analog vs Digital Oscilloscope

$ - $$$$$ - $$$$$$$ (not kidding)
live view of the signaldisplays a record of the signal
decent bandwidth for the moneybandwidth gets expensive quickly
good for learning how a scope worksstill a good tool, but can occasionally be misleading to beginners (aliasing)
best Tektronix scope maxed out at 400 MHzcurrent bandwidth cap is around 100 GHz

[Comparison / Digital scopes to be added later]

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