“The first two articles mainly introduced the functional characteristics and related theoretical knowledge of Spectrum View. Compared with the traditional FFT spectrum test method of oscilloscope, Spectrum View has unique advantages. So which scenarios are the excellent performance Spectrum View mainly used for? This is what this article will focus on.
The first two articles mainly introduced the functional characteristics and related theoretical knowledge of Spectrum View. Compared with the traditional FFT spectrum test method of oscilloscope, Spectrum View has unique advantages. So which scenarios are the excellent performance Spectrum View mainly used for? This is what this article will focus on.
This article will use Tektronix’s new generation oscilloscope MSO64 as an example to explain the time-frequency domain signal analysis technology. MSO64 uses the new TEK049 platform, which not only achieves a high sampling rate of 25GS/s when 4 channels are turned on at the same time, but also achieves a high 12-bit vertical resolution. At the same time, due to the use of a new low-noise front-end amplification ASIC-TEK061, the noise level is greatly reduced. At 1mv/div, the measured RSM value of the noise floor is only 58uV, which is far lower than similar oscilloscopes in the market. These features are all MSO64 spectrum mode-Spectrum View has a strong guarantee for high dynamics and low noise floor.
Figure 1. MSO64 uses the new TEK049 platform and ultra-low noise front-end TEK061
Time-frequency domain synchronization analysis
In the process of mixed-signal debugging, it is often necessary to observe the time-domain waveform and signal spectrum at the same time. For such test requirements, an oscilloscope is an ideal choice. Although the test dynamics are not as good as the spectrum analyzer, the oscilloscope has its own advantages:
・ Waveform and spectrum analysis can be completed at the same time, and the two have time correlation;
・ Support the simultaneous analysis of multiple channels in the time and frequency domain to realize multi-point monitoring of the circuit;
・ It can analyze the frequency spectrum of periodic signals and the frequency spectrum of non-periodic signals;
・ It can analyze the spectrum of very low frequency (down to DC) signals, which is beyond the reach of a spectrum analyzer;
・ Supports a wealth of signal detection methods, which can be connected via a standard coaxial interface, or can be flexibly detected via matching voltage and current probes.
As a brand-new oscilloscope-based spectrum analysis method, Spectrum View perfectly realizes the parallel processing of signals in the time domain and frequency domain. For applications requiring high frequency resolution, the traditional FFT method needs to increase the horizontal time base. This not only reduces the measurement speed, but also makes it impossible to observe the details of the time-domain waveform. Spectrum View supports independent settings in the time and frequency domains. Even with a very small horizontal time base setting, you can still get a high frequency resolution, not only can observe the waveform details, but also have a higher spectrum refresh rate.
Figure 2 tested a 100MHz CW signal and captured 4 cycles of time-domain waveforms. In the figure, the Spectrum View and the traditional FFT (Math function) are used to test the frequency spectrum of the signal. Through comparison, it can be seen that the resolution of the traditional FFT spectrum is very low due to the short time domain capture time. On the contrary, the spectrum test result of Spectrum View is very good. It not only has high resolution, but also has very low noise floor. It can clearly observe the signal itself and its harmonics and spurs. At the same time, because the horizontal time base is set smaller, the detailed information of the time-domain waveform can also be observed.
Figure 2. Harmonic, spurious test: Spectrum View vs Conventional FFT
Figure 3. RF Chirp Pulse time domain parameters and spectrum test
In view of these advantages of Spectrum View, combined with other functions of the oscilloscope, it can also perform diagnostic tests on RF pulse signals, including time-domain envelope parameters and signal spectrum. Figure 3 tests a linear frequency modulation pulse signal with a 200MHz carrier, with a pulse period of 5us, a pulse width of 1us, and a bandwidth of 50 MHz. The time-domain waveform, envelope, and spectrum test results are also shown. During the test, Span and RBW can also be flexibly adjusted to observe the envelope spectrum or linear spectrum, so as to perform a more detailed analysis of the signal.
Multi-channel spectrum analysis
The oscilloscope has multiple analog channels, and each channel can activate the Spectrum View function, so it supports multi-channel spectrum testing. In the process of complex debugging, multi-point waveform and spectrum monitoring can be realized. Similar to the multi-channel time-domain waveform Display mode of MSO64, the activated spectrum can be displayed either “Stacked” or “Overlay”. Figure 4 simultaneously observes the time-domain waveforms and frequency spectra of the two channels, and uses an overlapping Display to facilitate the comparison between the frequency spectra.
Spectrum View supports moving the position of Spectrum Time, as shown at the marker in Figure 4, to observe the spectrum at different moments. The Spectrum Time position of each channel is linked by default, which ensures the correlation of the test spectrum of each channel. When the linkage setting is canceled, the Spectrum Time position of each channel can also be set independently.
The frequency spectrum of all channels share the same Span, RBW, and FFT Window, which is similar to the time domain requirements for sharing sampling rate, horizontal time base and triggering among multiple channels. Nevertheless, the center frequency of each channel can be set independently, which is linked by default, or can be set to different values as needed.
Figure 4. Spectrum View supports multi-channel waveform and spectrum testing
Multi-domain linkage test
As mentioned earlier, Spectrum View supports sliding the position of Spectrum Time to perform spectrum testing on signals at different time periods, which makes it possible to perform multi-domain linkage testing on signals.
The following tests the Chirp Pulse and Hopping Signal respectively, combined with the Spectrum View and Frequency Time Trend test functions, to achieve the linkage test of the signal in the time domain, frequency domain and modulation domain.
1. Chirp Pulse multi-domain linkage analysis
As a pulse compression technology, chirp has a very high time resolution and is widely used in radar applications. Whether it is chirp pulse or FM continuous wave, the signal performance needs to be verified in the product development stage, and the time domain parameters, amplitude parameters and modulation domain parameters of the signal need to be tested.
In this example, a chirp pulse is actually measured. The time domain parameters can be tested with an oscilloscope, and the spectrum can be tested in the Spectrum View. The modulation domain parameter of Chirp pulse-frequency modulation curve, you can use Frequency Time Trend test, and the chirp rate and linearity can be derived from the frequency modulation curve.
In addition, Frequency Time Trend supports the introduction of a low-pass filter, which can filter out broadband noise superimposed on the frequency modulation curve, thereby improving test accuracy. The FM curve data can also be saved, so that developers can correct the transmitter.
Figure 5. Chirp Pulse time domain, frequency domain and modulation domain linkage analysis
Figure 6. Hopping Signal time domain, frequency domain and modulation domain linkage analysis
2. Hopping Signal multi-domain linkage analysis
For frequency hopping signals, multi-domain linkage tests can also be performed on them. As shown in Figure 6, Frequency Time Trend tests the frequency hopping state sequence, can observe the frequency hopping process, and use Cursor to calibrate the frequency switching time and frequency dwell time, etc. .
The Spectrum Time is located at the red mark in Figure 6, and its position can be moved. The tested spectrum is the spectrum corresponding to the current position. Drag the position of Spectrum Time, you can observe different frequency points separately, you can also observe the spectrum changes during frequency switching, as shown in Figure 7.
Figure 7. Hopping Signal time domain, frequency domain and modulation domain linkage analysis
This article focuses on the application of Spectrum View, a new spectrum analysis function of Tektronix oscilloscopes. Compared with specialized spectrum analyzers and traditional FFT functions of oscilloscopes, Spectrum View has unique advantages. This function can not only complete the ordinary spectrum test, but also realize the synchronization test of the time-domain waveform and the spectrum, and support multi-channel linkage test. The mobility of the Spectrum Time position, combined with the Frequency Time Trend function, enables the oscilloscope to have a multi-domain linkage analysis function. The paper verifies the feasibility of multi-domain linkage analysis by testing the chirp and frequency hopping sequence signals.