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Oscilloscopes Blog

2 Posts authored by: schoenecker Employee

Oscilloscope users are constantly reviewing the signals of their design in the “normal” time vs voltage display of the scope.  It is easy to overlook the FFT (Fast Fourier Analysis) view of the same signal. It is a completely different way of reviewing the signal characteristics that often reveals clues to some very difficult to solve problems. FFT can be an invaluable tool for identifying noise, crosstalk, and other common problem in many designs that can stall prototype development. In digital designs, it is often used to highlight and pinpoint the source by the frequency content on power rails.

FFT measurement with an input sine wave

By applying the FFT algorithms to the sampled data, you convert the time domain operation of the oscilloscope into a frequency view of the signal. This results in two primary benefits. One, you can easily identify each of the frequency components. Two, it reveals the magnitude of each contributing signal.

Identifying the frequency

By identifying the frequency components, it reveals if there are any signals on that are not expected. For example, a digital signal should only have frequencies that are harmonics of the base signal. If you have a 10 MHz data, there should be only frequencies at 10 MHz (the primary harmonic), 30 MHz (the third harmonic), 50 MHz (the fifth harmonic), and the continuing odd harmonics up to the bandwidth of the source.

Any other frequency is a result of noise, or crosstalk, or some type of coupling on to the signal.

Frequency components of 10MHz clock

Figure 1: In this capture of a 10 MHz clock, we can easily identify the frequency components related to the fundamental frequency, but we can also see a 20 MHz signal that is -55 dB from the fundamental.


It’s important to understand how the oscilloscope sampling characteristics play into the quality of this FFT measurement. The oscilloscope analog bandwidth, sample rate, memory depth, and related time capture period all can have a profound effect on the measurement result. The math that is utilized for the calculation is using the data that was sampled at 5 GSa/sec, and it makes it possible to calculate a 10 GHz FFT. However, the front end of this scope is 1 GHz, so the FFT is only valid up to the bandwidth of the oscilloscope.

Identifying the Magnitude

The other key component of the signal is the power of each signal component. When looking at a signal in the time domain, it is only possible to see the very large signal power components. In the spectrum view (or FFT display) the horizontal axis is changed from a linear voltage scale to a logarithmic voltage scale (or dB for decibel).

The display on the right side of the display is listing the power level in dBV (decibel volts, or power relative to 1volt) of each frequency in order with the respective power level.  The first, or fundamental frequency of our signal is at just less than 10 MHz, and a power level of -13.9775 dBV, which is about 200mV rms. Looking at the time display of the signal (in green), you can see that it looks about right. We can also see that the next highest power signal is at 30 MHz and a power level of -30dBV, or about 3 mVrms-- something that cannot be seen in the time display that we are used to looking at.

FFT is just a button away

On Keysight oscilloscopes, the FFT operation is often enabled by simply pressing a button on the front panel. The new 1000X low cost oscilloscopes include this feature standard. The FFT view is a great way to examine a signal to find the frequency and power that you could not normally see any other way. Make sure to take advantage of this powerful tool that next time you are trying to find elusive signals in your design.

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As an oscilloscope user you understand the importance of analyzing analog signals in a digital circuit. However, many users are missing out on one of the most powerful features of today’s oscilloscopes: the mixed signal oscilloscope (MSO). An MSO adds up to 16 digital channels to your 4 analog channel oscilloscope. This greatly expands the types of analysis that can be performed by this versatile engineering tool. Digital signals can be a simple chip select, or a communication bus. The ability to monitor these digital signals is often critical to properly analyze system operation.

Debugging a mixed signal design can be a difficult and somewhat daunting task to the engineer who is armed with a 4-channel oscilloscope since you often need to capture more than four signals. An mixed signal oscilloscope provides that capability, with the ability to examine the state of up to 20 signals all on the same timescale, while using the familiar controls of a basic oscilloscope.

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When needed, MSO digital channels provide just enough logic analysis for users whose home base is an oscilloscope. MSO digital channels serve as an extension to the oscilloscope capabilities and can provide valuable insight into the operation of your design.

Correlation of input/output of ADC/DAC is simple and straightforward. An MSO adds some very powerful and useful tools for analyzing a digital bus. In this one simple view we can see the state of the analog signal, the state of each of the digital signals, the hex representation of this digital bus, and I still have all of the measurements available on the oscilloscope to evaluate the signal quality. The controls and measurements are all still based on the oscilloscope operation, making it very easy to navigate and control.

At the same time the MSO provides the means to help analyze your digital bus. The grouping of digital signals to create a “bus” with easy-to-read hex values can be used in decoding the signals, or for triggering on specific addresses or values.

MSOs and logic analyzers have fundamental architectural differences in how they acquire and display signals. MSOs exclusively use asynchronous sampling, just like an oscilloscope. For many users, this makes setting up an acquisition on digital channels simpler, because it feels like a scope.

By applying some additional logic to the digital bus you can create a visualization of the bus operation. By using one of the signals as a clock, the MSO can chart the bus “state” to display a logical representation of the digital data.

Displaying an analog equivalent of the data that is being transferred across the bus can quickly identify errors in the digital data.


Low-Speed Serial Bus Support

Today’s designs incorporate digital communications between components and systems by using high- and low-speed serial digital communication, and microprocessor buses. Serial buses like I²C and SPI are frequently used for chip-to-chip communication, but cannot replace parallel buses for all applications. But here again, oscilloscopes add powerful troubleshooting capability by providing just enough protocol analysis.

A key difference between logic analyzers and MSOs is the latter’s ability to trigger on and decode serial buses. Low-speed serial buses are ubiquitous in electronic designs because of their ease and low cost of implementation. In fact, it is hard to find a design that does not include at least one I2C or SPI bus, or USB.

Mixed signal oscilloscopes excel at debug that includes low-speed serial buses. All good MSOs come with both triggering and decode options for serial buses. However, logic analyzers have not incorporated similar triggering and decode technology. Without protocol triggering, it’s impossible to set up the oscilloscope to trigger on specific packets. For example, you can set the MSO to trigger when an I2C read to a certain address with a certain data value happens. Alternatively, you can trigger on a certain SPI data packet or at the start of USB enumeration.

Since the blending of analog and digital information is so prevalent, oscilloscope users can take advantage of the ability of current MSOs to improve their troubleshooting capabilities and simplify their mixed-signal design debugging.

Keysight oscilloscopes have the added benefit that most existing DSOs (Digital Storage Oscilloscope) can be easily upgraded to add MSO capabilities.

Learn more about MSOs!