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

Blog Post created by nickben on 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.