Evaluating Oscilloscope Bandwidths for Your Application
The main oscilloscope specification – and one which usually features in the product part number – is bandwidth. Simply, this can be thought of as the highest signal frequency that the instrument can display. But here’s a little bit more depth on what that really means and some reasons why you might want to get a higher bandwidth oscilloscope.
Oscilloscope Bandwidth is defined at the point at which a simple sinusoidal signal can be captured and displayed with less than -3dB attenuation. The ‘curve’ of this response over the frequency range is either Gaussian (figure 1) or “Maximally flat” (also known as “Brickwall”, figure 2).
Oscilloscopes with a maximally-flat response attenuate in-band signals less than scopes with Gaussian response, meaning that scopes with maximally-flat responses are able to make more accurate measurements on in-band signals. To achieve the flat frequency response DSP is used, as this impacts waveform update-rate performance and is expensive, it’s normally only found in higher-end instruments and most general-purpose oscilloscopes provide a Gaussian response.
Advantages of higher frequency oscilloscope
Having a higher frequency oscilloscope allows you to see higher frequency signals. But more importantly, it allows you to see higher frequency content in the in-band signal. Consider a square wave: this is constructed from a sine wave of the fundamental signal frequency, and then ideally an infinite number of odd harmonics of 1/n * fn (where n = 3, 5, 7…∞). If you’re looking at a clock signal with a fundamental frequency of 100MHz using a 100MHz scope, you’ll see the following waveform:
If you use a 500MHz scope on the same signal, you’ll see a very different waveform:
So with just the fundamental (100MHz) then the 300MHz and 500MHz harmonics, we get a much better representation of the waveform. An even higher bandwidth scope will display more harmonic content and other signal artefacts such as noise, as well as a faster rise time.
What else does bandwidth affect?
As seen from the above images of a 100MHz and 500MHz scope, a low bandwidth will mean any rise-time measurements you make on your scope will be impacted. As the bandwidth is effectively a filter, a lower bandwidth oscilloscope is cutting out the higher-frequency or “faster” signals. (The bandwidth of your scope can also be defined by the signal rise-time.)
What else affects bandwidth?
Your probe! Standard oscilloscope passive probes are usually rated at 500MHz or below. However, the actual effective bandwidth can be much less: the load a passive probe has on your circuit can be very high as the small compensation capacitor becomes a very low impedance at high frequency. A typical passive probe with a compensation circuit of 1MΩ//10pF is just 32Ω at 500MHz!
To get around this, use high-impedance active probes with very low capacitance.
How much bandwidth do you need?
The simple rule of thumb is that you should use a scope which has a bandwidth which is 5x higher than the signals you want to measure. For digital applications, if you check the datasheet of the signals you are looking at, you can determine the fastest edge speeds, calculate the “knee” frequency and then decide on the measurement accuracy you can accept – simply, the higher the bandwidth the more accurate your timing measurements will be. A full explanation is in the Keysight Application Note.
Is too much bandwidth a bad thing?
Not really: while it will mean you could capture and display noise you don’t want to see, it’s usually better that you are aware of it so you can make a judgement on whether or not it is affecting your circuit. Most scopes now allow you to limit the signal you are viewing using either hardware filters (typically 20MHz) or software filters (configurable as either low-pass, high-pass or band-pass).
How can I get more bandwidth?
Some newer oscilloscopes allow you to upgrade bandwidth via a license key or by replacing a board – and you usually only have to pay for the difference in cost.
From August 1st, 2019 until March 31st, 2020, Keysight are providing customers with a free bandwidth upgrade on the 4-channel MSOX3000T range of oscilloscopes. Buy a 100MHz scope and get a 200MHz model; buy a 200MHz model and get 350MHz, buy 350MHz and get 500MHz, buy 500MHz and get 1GHz!