Bandwidth is one of the key banner specifications of oscilloscope probes.
Understanding just a few concepts can keep you from making common mistakes
that can cause inaccurate measurements and errors.
Hi everyone i'm Erin and welcome to the new series Probing Pitfalls. In this
series you'll learn all about common probing pitfalls and how to avoid them.
This will give you the confidence that the signal you see on the scope screen
is the best representation of what's coming out of your device under test or
your DUT. In this episode you'll learn about critical bandwidth pitfalls like
not knowing about the three DB point, forgetting to consider bandwidth with
respect to rise time, not understanding harmonics, or not calculating the entire
system bandwidth. Many engineers believe using a 100 megahertz probe to analyze a
100 megahertz signal is the perfect setup but there are a few things they
don't realize. The first is the three DB point. The easiest way to understand
probe bandwidth is by learning this theory. The bandwidth is defined by the
point at which the frequency components passing through the probe are attenuated
or decreased by 3 DB. In other words the amplitude of your signal is reduced by
70.7% of what it should be. As an example let's say we're working
with a 20 megahertz probe. To start out I'm measuring a 1 megahertz signal with
one volt peak-to-peak that's coming from my waveform generator. The peak to peak
measurement you see on screen is accurate at 1 volt peak-to-peak because
we're using a probe that has plenty of bandwidth for the signal however if I
increase the frequency of the signal to 20 megahertz you can see we hit the 3 DB
point of the probe which means we'll see a measurement error of -3 DB or
-0.3 volts peak-to-peak. You can see in our peak to peak measurement here
our 1 volt peak-to-peak is now reading off at 0.7 volts peak-to-peak. You can
fix this pitfall by calculating the correct bandwidth for your desired
frequency. A common rule of thumb is your probe bandwidth should be at least three
times the size of the sine wave you wish to measure. As a result to measure this
20 megahertz sine wave we would want to probe with at least 60 megahertz.
Now remember this only works for perfect sine waves but most analog measurements
aren't perfect sine waves, if this is the case the rise time theory
is critical in determining the necessary probe bandwidth. To truly calculate the
best probe bandwidth needed you need to know the rise time of your signal.
The rise time is the time it takes for your signal to get from the 10% level to the
90% level of a rising edge this formula states that the bandwidth of a signal
equals 0.34 divided by the rise time for a signal with gaussian
response and again this is when evaluating rising edges from 10% to 90%.
Many times the rise time specification is listed along with the other banner
specs of your design. As an example think about a clock signal with a frequency of
500 megahertz. Well you may think alright I just need a one and a half gigahertz
probe to measure that signal according to what Erin said a minute ago but your
rise time could change everything. If your rise time for the 500 megahertz
clock signal is something like 350 Pico seconds the calculated bandwidth of that
signal based on the formula is actually equal to one gigahertz. This means that
the real bandwidth of your signal is one gigahertz not 500 megahertz 500
megahertz is your clock frequency for an entire cycle but your clocks rising edge
is much faster at one gigahertz. This would mean that you would actually need
a three gigahertz probe to measure that signal accurately. Avoid the pitfall of
not having enough bandwidth by remembering to check your rise time
specification. Another way to visualize the effects of bandwidth is by looking
at the different harmonics of a signal the yellow trace here shows the original
signal and is made from the fundamental harmonics. The first harmonic can be seen
as this green trace you can see it has the same period in duty cycle as the
original signal but the rising edges are slower and the corners are more rounded.
The first and third harmonic are combined in the purple signal and have a
faster rising edge and the corners are even more crisp but the first third and
fifth harmonic are combined in the pink trace and have faster edges, crisper
corners, and even more detail on the top in the bottom of the waveform. The higher
the probe bandwidth the more detailed your waveform is going to become because
the more harmonics can be captured. Now let's take a look at this harmonic
knowledge a little bit further by looking at a 20 megahertz clock wave.
Here you can see the result of measuring a 20 megahertz clock with a 20 megahertz
probe. Basically we can't capture enough of the harmonics of the 20 megahertz signal
because we don't have enough probe bandwidth and the result is going to
look like a sine wave. Any measurements made on that signal will be inaccurate.
If we measure the same signal using a 200 megahertz probe the result is a
clean square wave that you can see here because the critical harmonics of the 20
megahertz signal are able to be captured in the measurement. The last common
pitfall when it comes to bandwidth is when engineers forget about the system
bandwidth. Your probe combined with your scope creates the system and the
bandwidth of this whole system is lower than you may think.
Let's say both your oscilloscope and your probe bandwidth are 500 megahertz
using this formula the system bandwidth would be 353 megahertz so you can see
that the system bandwidth is degraded greatly from the two individual
bandwidth specifications of the probe and the scope and this will just keep
getting worse if you start using accessories but I'll save that for a
later episode. Obviously system bandwidth is another important aspect to consider
when you're selecting the correct probe bandwidth and with now that you understand these
common pitfalls you can understand why choosing a probe with adequate bandwidth
is crucial for making accurate measurements.
Having too little bandwidth means that you won't be able to see a true
representation of your signal making it difficult for you to make good
engineering decisions while designing. To learn more about probing click on the
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And for anyone wondering about my cast, this is what happens when you steal probes.