Tektronix Oscilloscope Spectrum View Function - Frequency Domain Analysis Tool

Oscilloscopes and spectrum analyzers are both essential test equipment in electronic testing and measurement, used to observe the time domain waveform and spectrum of signals respectively. The time domain waveform is the most original information of the signal, and the introduction of the spectrum is mainly to facilitate the analysis of the signal. For example, the test of harmonics and spurious is difficult to observe in the time domain, but can be clearly distinguished in the frequency domain.

In addition to the basic function of collecting signals, oscilloscopes can also perform FFT transformation on signals to obtain spectrum, thus having spectrum analysis function. Almost all mid-to-high-end oscilloscopes support FFT spectrum analysis. In this article, Antai Test will introduce the spectrum analysis function of Tektronix oscilloscopes - Spectrum View, which is a powerful spectrum analysis tool. Its introduction has opened up a new time-frequency domain signal analysis.

Spectrum View, which combines the spectrum mode of the TEK049 ASIC innovation platform and the TEK061 low-noise front-end amplifier chip, is a strong guarantee for obtaining high dynamics and low noise floor.

Figure 1. TEK049 platform and ultra-low noise front end TEK061

SpectrumView Features at a Glance

In terms of implementation, Spectrum View also uses FFT, but it does not directly process the collected samples. Instead, it first obtains IQ data through digital down-conversion (DDC technology), and then obtains the signal spectrum through FFT. This is also a major feature compared to traditional FFT. Compared with the original collected signal, the frequency carried by the IQ signal is much lower, and the resampling of the IQ data does not require a very high sampling rate, which greatly reduces the amount of data and improves the processing speed.

Whether compared with a spectrum analyzer or the traditional FFT method of an oscilloscope, Spectrum View has its own characteristics:

Spectrum View makes the capture time of the time and frequency domains independent of each other, and the processing speed of the oscilloscope can still be guaranteed when a smaller RBW is set. Traditional FFT testing requires changing the RBW by adjusting the horizontal time base. In test scenarios where a very small RBW is required, the horizontal time base needs to be increased, which seriously affects the processing speed of the oscilloscope.

Spectrum View has the same operating settings as a standard spectrum analyzer, as shown in Figure 2, but with a more user-friendly interface that allows you to directly set the center frequency, span, RBW, and time domain window type.

Spectrum View supports a variety of detection methods and can test the spectrum of extremely low frequency signals, which is beyond the reach of ordinary spectrum analyzers;

Supports multi-channel time and frequency domain joint analysis, and supports trigger capture, making it possible to analyze the spectrum of transient or intermittent signals;

Supports multiple time window types, which can be selected according to the characteristics of the signal to be tested to ensure the accuracy of the test results;

Supports automatic peak search function, can set the peak search threshold, and can display up to 11 Peak Markers;

Supports four trace display modes: Normal, Max. Hold, Min. Hold and Average;

When displaying the spectrum of multiple channels, it can be displayed in "Stacked" or "Overlay" mode;

All channels have the same Span, RBW, FFT Window and Spectrum Time, but the center frequency can be set independently.

Figure 2. Spectrum View operation interface

Parallel analysis in time and frequency domains

Figure 3 shows a schematic diagram of the signal acquisition and processing architecture. After the analog signal is converted to a digital signal by the ADC, the time domain and frequency domain are processed in parallel, so that the time domain and frequency domain capture time can be set independently. SpectrumView supports sliding the position of Spectrum Time to perform spectrum tests on signals in different time periods, which makes it possible to perform time-frequency domain linkage tests on signals.

Figure 3. Schematic diagram of signal acquisition and analysis architecture

As an example, Figure 4 shows the analysis result of a frequency hopping signal, and also shows the results of the time domain waveform, spectrum, and frequency hopping sequence. The red mark in the figure is Spectrum Time, which is the time used for FFT analysis. Its position can be moved, and the tested spectrum is the spectrum corresponding to the current position. By dragging the position of Spectrum Time, different frequency points can be observed separately. The current observation is the spectrum change during the frequency switching process.

Figure 4. Linked analysis of time domain, frequency domain, and modulation domain

Multi-channel spectrum testing

In the spectrum application process, the data processing process of Spectrum View is the same as that of the spectrum analyzer in FFT mode. Although the test dynamics are not as good as the spectrum analyzer, Spectrum View has its own advantages, such as the ability to test very low frequency signals, rich and flexible detection methods, and the correlation of time-frequency analysis. In addition, Spectrum View also supports multi-channel spectrum testing, thanks to the fact that TEK049 supports spectrum analysis and processing of signals in each channel at the same time.

Similar to the multi-channel time domain waveform display mode of TEK049, the activated spectrum can be displayed in either "stacked" or "overlay". Figure 5 observes the time domain waveform and spectrum of two channels at the same time, and uses an overlay display to facilitate comparison between the spectra.

The spectrum of all channels shares the same Span, RBW, FFTWindow and Spectrum Time, which is similar to the time domain requirement that multiple channels share the same sampling rate, horizontal time base and trigger. However, the center frequency of each channel can be set independently, and is linked by default, but can also be set to different values ​​as needed.

Spectrum View supports automatic peak search and supports up to 11 PeakMarkers. The frequency point with the largest amplitude is automatically marked as "Ref.Marker". The frequency points and amplitudes of other markers can be displayed as absolute values ​​or relative values ​​to "Ref.Marker". If the number of markers required exceeds the limit, the frequency and amplitude can also be determined by using the frequency domain cursor.

Figure 5. Simultaneous observation of the time domain waveform and spectrum of two channels