2018

February 2018

# Why Measuring in the Time Domain and Frequency Domain Is the Same, but Not

Posted by nickben Feb 28, 2018

Like any RF engineer, there comes a time in your product’s design cycle that you need to test your device to make sure it’s behaving as you expect. There are different ways you can view your device’s signal, which brings us to why measuring signals in the time domain and frequency domain is the same, but not. This is because they both convey the same signal, but in a different way.

Figure 1. The time domain of a signal on the left, and the frequency domain of the same signal on the right. The time domain displays a signal in respect to amplitude vs. time whereas the frequency domain displays amplitude vs. frequency.

By properly combining spectrum, or a collection of sine waves, you can view the time domain of your signal. It shows your signal’s amplitude versus time. This is typically done using an oscilloscope. Why would you want to view your signal in the time domain, you ask? Basically, a time-domain graph shows how a signal changes with time. This lets you see or visualize instances where the amplitude is different.

Viewing your device’s signal in the time domain doesn’t always provide you with all the information you need. For example, in the time domain you can decipher that a signal of interest is not a pure sinusoid, however, you won’t know why. This is where the frequency domain comes in. The frequency domain display plots the amplitude versus the frequency of each sine wave in the spectrum. This may help you discern why your signal isn’t the pure sinusoidal wave you were hoping it to be.

Figure 2. Harmonic distortion test of a transmitter, which is most appropriately measured using a spectrum analyzer in the frequency domain.

The frequency domain can help identify questions about your signal that you wouldn’t be able to see in the time domain. However, this doesn’t mean that you can just scrap measuring signals in the time domain altogether. The time domain is still better for many measurements, and some measurements are only possible in the time domain. Examples include pulse rise and fall times, overshoot, and ringing.

But just like the time domain has its advantages, so does the frequency domain. For one, the frequency domain is better for determining the harmonic content of a signal (as seen in Figure 2). So, those of you in wireless communications who need to measure spurious emissions are better off using the frequency domain. Yet another example is seen in spectrum monitoring. Government regulatory agencies allocate different frequencies for various services. This spectrum is then monitored because it it is critical that each of these services operate at its assigned frequency and stay within the allocated channel bandwidth.

While measuring signals in the time domain and frequency domain is similar, it is also very different. Each domain conveys the same signal, but from different perspectives. This enables us engineers to get more insight into how our device is behaving and ultimately develop better products for our customers.

To build a stronger foundation in signal analysis that will help you deliver your next breakthrough, check out the Spectrum Analysis Basics app note. Please post any comments - positive, constructive, or otherwise - and let me know what you think. If this post was useful give it a like and, of course, feel free to share.

# Passing Along the Magic

Posted by benz Feb 20, 2018

Demystifying technology, and marking five years of The RF Test Blog

For several years, I co-coached two middle school robotics teams. It was a great experience, and I learned at least as much as I taught—though generally about different subjects!

Some of the kids gravitated toward the robot mechanisms, while others found a natural focus on the programming side. I suppose that’s part of the intent of robotics clubs, mixing hardware and software to increase the chances of inspiring kids to pursue STEM studies and careers.

Ironically, our success with ever-more-complex technology may create some barriers to getting kids interested in it. During a club meeting one afternoon, I was vividly reminded of Arthur C. Clarke’s famous quote, “Any sufficiently advanced technology is indistinguishable from magic.” While the kids were working with robots and laptops, virtually all of them were carrying a magical device that was even more advanced: their mobile phone.

These thin slabs of metal, glass, and plastic, invisibly connected to the rest of the universe, could be expected to do just about anything when equipped with the right app. Seeing something so magical being taken so thoroughly for granted, I understood why some kids weren’t all that captivated by the robots.

That realization left me a bit troubled, and I wondered about other ways to get the kids engaged.

A partial answer came later in the semester. My co-coach had the brilliant idea of devoting one club session to the dismantling of technology. She brought in some older devices, working or not, including an early digital camera, a portable CD player, and a slider-type mobile phone. We gave the kids some small screwdrivers and turned them loose to get a glimpse behind the engineering curtain.

I was amazed at the spike in enthusiasm and engagement, especially from some kids who had previously been marginal participants. Once they reasoned out how to open the devices and free the contents, they then delighted in showing others how they thought the parts actually worked. They got an especially big kick out of the tiny motor and attached eccentric that vibrated the phone. It was the one recognizable part of the device that moved!

My take-away: if we want to pass along our interest in creating the magic of new technologies—and solving the attendant problems—we need to keep our eyes open to new approaches to communicate and share.

That’s what we were thinking five years ago when we started this blog. Since then, it has been a delight to learn about RF technologies and share the results with you. I very much appreciate your indulgence as I’ve wandered from Loose Nut Danger (the first post) to MIMO to the technology of furry hoods.

It’s now time to pass along the writing of this blog to a new generation, with their own perspectives, insights, and peculiar interests.

Meet the new primary writers of Keysight’s Better Measurements: The RF Test Blog, clockwise from upper left: Eric Hsu, Vandana Duff, Nick Ben, and Tit Bin Teo

Nick Ben has already written several guest posts here, and I think this blog will benefit from the new writers’ wider range of interests and experience. I look forward to following where they lead.

As for me, I plan to pursue my interests in a direction that looks more like retirement, with increased opportunities to learn and to teach, coach, and share.

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