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Spectrum View: A New Approach to Frequency Domain Analysis on Oscilloscopes
Spectrum View is a new way of performing spectrum analysis on an oscilloscope. This application note shows and explains how Spectrum View operates and how it differs from traditional oscilloscope FFT functions.
- Any analog signal may be presented as a waveform, spectrum, or both
- Time domain waveforms and spectrum are synchronized
- Waveform and spectrum display may be independently adjusted
- Settings are made with spectrum analyzer controls such as Center Frequency, Span, and Resolution Bandwidth (RBW)
Spectrum View: A New Approach to Frequency Domain Analysis on
FIGURE 1. Spectrum View enables simultaneous analog and spectrum views with independent controls in each domain.
Debugging embedded systems often involves looking for clues that are hard to discover just by looking at one domain at a time. The ability to look at time and frequency domains simultaneously can offer important insights. Mixed domain analysis is especially useful for answering questions such as:
•What’s going on with my power rail voltage when I’m transmitting wireless data?
•Where are the emissions coming from every time I access memory?
•How long does it take for my PLL to stabilize after
Mixed domain analysis can help answer questions like these by providing views of time domain waveforms and frequency domain spectra in a synchronized view. Up until recently, the Tektronix MDO4000C mixed domain oscilloscope has been the only oscilloscope to offer synchronized time and frequency domain analysis with independent control over waveform and spectrum views.
To address this need, the 4, 5 and 6 Series MSO mixed signal oscilloscopes offer an analysis tool called Spectrum View. It is an option in the 4 Series MSO and a standard feature in the 5 and 6 Series MSOs. It delivers several key capabilities:
•Enables the use of familiar spectrum analysis controls (Center Frequency, Span and RBW)
•Allows optimization of both time domain and frequency domain displays independently
•Enables a signal to be viewed in both a waveform view and a spectrum view without splitting the signal into different inputs
•Enables accurate correlation of time domain events and frequency domain measurements (and vice versa)
•Significantly improves achievable frequency resolution in the frequency domain
•Improves the update rate of the spectrum display
FIGURE 2. Digital down converters implemented on a custom ASIC enable simultaneous waveform and spectrum views with independent controls in the Tektronix 4, 5 and 6 Series MSOs.
A new architecture
Spectrum View uses patented hardware built into the instruments. To understand how it works, it is important to note that digital oscilloscopes generally run their
In the 4, 5 and 6 Series MSOs, behind each FlexChannel input is a
FIGURE 3. With the time domain optimized using conventional FFTs, frequency domain detail is lacking on this
Spectrum View with independent controls vs. conventional FFT
Although spectrum analyzers are designed specifically for viewing signals in the frequency domain, they are not always readily available. Scopes, on the other hand, are almost always nearby in the lab so engineers tend to rely on scopes as much as possible. For this reason, oscilloscopes have included
First, for frequency domain analysis, spectrum analyzer controls like center frequency, span and resolution bandwidth (RBW) make it easy to define the spectrum of interest. In most cases, however, oscilloscope FFTs only support traditional controls such as sample rate, record length and time/div, making it difficult to get to the desired view.
Second, even if the scope offers spectrum analyzer style controls, the FFT is driven by the same acquisition system as that used for the analog time domain view. Changing the center frequency, span, or resolution bandwidth will change the scope’s horizontal scale, sample rate and record length in unanticipated and undesired ways. Once the desired frequency domain view is achieved, the time domain view of other signals is no longer usable. When adjustments are made to horizontal scale, sample rate, or record length to again achieve the desired time domain view, the FFT view is no longer usable. For example, the next two screenshots taken from an MDO3000 illustrate the time domain and FFT views of a spread spectrum clock that moves from 97 MHz to 100 MHz. In Figure 3, the time domain view enables easy visualization of the clock, but the FFT doesn’t have adequate resolution to be useful. In Figure 4, the FFT shows the spread spectrum nature of the clock, but the time domain view is no longer helpful.
FIGURE 4. With the FFT view optimized, the time domain view of the clock signal is now not useful.
Spectrum View provides the ability to adjust the frequency domain using familiar center frequency, span and RBW controls.
And because these controls do not interact with the time domain scaling, it is possible to optimize both views independently as shown in Figure 5.
FIGURE 5. Looking at the same
In Figures 6 through 9, we’ve captured the startup sequence of the spread spectrum clock discussed earlier. The Spectrum Time indicator appears very narrow in the screenshots and is highlighted by a red box to assist the reader. In this case, Spectrum Time is 1.9 (Window Factor) / 10,000 (RBW) = 190µs wide.
FIGURE 6. Spectrum Time (highlighted with a red box) is placed early in the acquisition, before the trigger event. As expected, there aren’t any strong signals in the frequency domain as the clock has not turned on yet.
FIGURE 7. Spectrum Time placed approximately 20 ms after the clock turned on. Notice that the clock is not spread spectrum yet, it’s merely sitting at 94 MHz.
FIGURE 8. Spectrum Time placed approximately 300 ms after the clock turned on. Notice that the clock is now exhibiting spread spectrum behavior, but it’s using more of the spectrum than intended. The cursors show the expected spread spectrum width.
FIGURE 9. Spectrum Time placed approximately 324 ms after the clock turned on. Notice that the clock is now exhibiting spread spectrum behavior and is within the intended spectrum operating range.
RF vs. Time Waveforms
The underlying I&Q data that’s used to create the spectrums shown by Spectrum View can also be used to calculate RF vs. Time waveforms that show how various characteristics of the RF waveform vary over the entire acquisition, not just where Spectrum Time is positioned. Three types of waveforms are available:
•Magnitude – The instantaneous magnitude of the spectrum vs. time
•Frequency – The instantaneous frequency of the spectrum relative to the center frequency vs. time
•Phase – The instantaneous phase of the spectrum relative to the center frequency vs. time
Each of these traces can be turned on and off independently, and all three can be displayed simultaneously.
These waveforms are shown in Figures
in each of the images. The top one is the analog view of the signal. Next is the RF Magnitude vs. Time waveform, then the RF Frequency vs. Time waveform and finally the RF Phase vs. Time waveform.
FIGURE 10. It’s very easy to see what’s going on with this spread spectrum clock by looking at the Magnitude and Frequency vs. Time waveforms. The Magnitude vs. Time trace shows the signal turning on at the trigger point at a very low level and the Frequency vs. Time trace shows the signal staying at a single frequency for the first ~300ms. At that point, we see the amplitude of the signal increase significantly and the frequency begin changing.
FIGURE 11. We’ve now zoomed in on the period of interest (roughly
FIGURE 12. We’ve now zoomed in even further and can easily view the triangular frequency modulation in use and can confirm through the automatic measurement in the results bar that we are getting the correct modulation rate of 39.07 kHz.