When FFT isn't enough for RF Measurements on an Oscilloscope
A New Approach to Frequency Analysis on Oscilloscopes
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 power-on?
Mixed domain analysis can help answer questions like these by providing views of time domain waveforms and frequency domain spectra in a synchronized view. Until now, 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. It accomplishes this by incorporating a full spectrum analyzer with its own dedicated input channel.
To address the need for RF analysis in its latest 4 Series mixed signal oscilloscope, Tektronix is taking a new approach that does not require a separate input channel while still offering similar capabilities. Recently released firmware unlocks an analysis tool called Spectrum View that takes advantage of patented hardware already in the instruments. With this tool, the scopes can now provide simultaneous analog waveform views as shown on left in Figure 1 and spectral views as shown on the right, with independent controls in each domain.
For more complex troubleshooting, instruments with the upgraded firmware can perform mixed-domain analysis on more than one channel, as shown in Figure 2. Each of the two color-coded analog waveforms has a corresponding spectrum. Note that below each waveform is a small bar. This indicates the time at which the spectrum occurs within the time domain. This spectrum time can be moved through the waveform to see the synchronized spectrum at any point in the waveform. In this example, the two channels show the startup of a clock signal from two different points in a circuit.
Scope FFTs vs. Spectrum View
FFTs are notoriously difficult to use for two reasons :
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 right view.
Second, FFTs are driven by the same acquisition system as that used for the analog time domain view. This means that speeding up the time scale in the time domain reduces resolution in the frequency domain. As a result, with conventional FFTs, it’s virtually impossible to get optimized views in both domains. For example, in the screenshot in Figure 3 taken from a TDS3000, the time domain waveform is seen clearly, but FFT resolution is inadequate to see meaningful details.
Check out the attached video for a brief overview of Spectrum View on the latest Tektronix 4 Series MSO.
The Tektronix 4 Series Oscilloscope is available in 4 or 6 channels, ranging from 200MHz to 1.5GHz.