What is bandwidth, really? Does it matter?

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Transcript:

Today, we’re going to take a look at a different way to think about bandwidth. What really is bandwidth, and what does it mean for your systems?

Hi, I’m Daniel Bogdanoff, and today we’re going to dig into arguably the most fundamental of all test gear specs – bandwidth.
In test gear 101 you hear that bandwidth is simply the highest frequency component that you can measure or produce. For example, a 100 MHz oscilloscope can measure a 100 MHz sine wave.
But, this signal is going to be attenuated by 3 dB or so, because in test gear 201 you hear that the signal will actually roll off a bit near the maximum bandwidth of your system. In fact, you can actually see frequencies that exceed the bandwidth of your scope.
For example, here’s a 40 MHz, 5 Vpp signal piped into my 1.5 GHz oscilloscope. No problem.
But then, I turn on my scope’s 20 MHz bandwidth limit, I can still see my 40 MHz sine wave, but the peak to peak voltage has attenuated down to 2.1 volts. So essentially I’m able to see a signal that’s double the bandwidth of my scope.
The same is true for the output of a function generator, but it’s harder to see that in practice because the software won’t let you set a higher frequency than the generator’s stated max output. On a 100 MHz generator, you simply can’t turn the frequency setting any higher than 100 MHz.
So, this begs the question, why don’t we, the designers of test and measurement gear, simply say the generator can produce a higher frequency and then just limit the amplitude at those extended frequencies? Or, why can’t I say my oscilloscope has a higher bandwidth? Here I am able to measure double the bandwidth on this scope. It would certainly make our specs look better!
To answer that, we need to look more closely at what bandwidth really means. As it turns out, it’s not about the Hertz, the cycles per second, it’s about rise time. It’s about how fast the signal goes up and down. Edge speed.

Let’s look an example. A delivery driver has a car that can travel at 100 km/h max. He has to make a delivery 50 km away, so it’ll take him 1 hour to deliver the package and get back. His frequency is 1 delivery per hour. Next, he has to deliver two packages that are only 25 km away. It’ll take him only 30m for each package, and his frequency will be 2 deliveries per hour. His frequency doubled, but his speed didn’t change.
This is the same with test gear and bandwidth. An oscilloscopes bandwidth or generator’s output is like the delivery rate (deliveries per second), while the rise time is like the driver’s speed.
Sure, he can deliver more packages if the deliveries are closer together, but the travel speed doesn’t change. This is the same with test gear.
Another way to think about this is in terms of frequency components. Bandwidth refers to the frequency components when you look at the frequency domain, but the rise time or edge speed in the time domain. Watch my edge speed change as a toggle the bandwidth limit on and off.
Here’s what’s happening.
When the signal goes positive, the scope trace will start to move positive. But, it has to do so linearly, meaning it can’t just magically jump up to the new voltage value. The speed that the scope adjusts to the change is dictated by its bandwidth. So, you can technically measure a frequency higher than the max bandwidth of the scope, but you can’t measure an edge speed faster than the max bandwidth of the scope.
The same is true for a function generator. The edge speed of the output maxes out at its max bandwidth. That’s why a 100 MHz function generator can create a 100 MHz sine wave, but not a 100 MHz square wave. The Hertz is the same, but the edge speeds are very different.
So, if you’re picking out a function generator, think about what types of signals you want to generate, not just the max bandwidth. A general rule of thumb for square waves is that you need a generator with 3x the bandwidth of your square wave frequency.
You can use this knowledge to push some of the specs of your scope – which is a topic for an upcoming video.


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