New time-frequency domain signal analysis technology based on Tektronix MSO64 [Application]

The first two articles mainly introduced the functional features and related theoretical knowledge of Spectrum View. Compared with the traditional FFT spectrum test method of oscilloscope, Spectrum View has unique advantages. So, in what scenarios is the excellent Spectrum View mainly used ? This is what this article will focus on.

This article will use the new generation of Tektronix 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 opened simultaneously , but also achieves a high vertical resolution of 12-bit . At the same time, due to the use of the new low-noise front-end amplifier ASIC-TEK061 , the noise level is greatly reduced. At 1mv/div, the measured background noise RSM value is only 58uV, which is much lower than similar oscilloscopes on the market. These characteristics are a strong guarantee for the high dynamic and low noise floor of MSO64 spectrum mode-Spectrum View .

During mixed signal debugging, it is often necessary to observe both the time domain waveform and the signal spectrum at the same time . For such testing requirements, an oscilloscope is an ideal choice. Although the test dynamics are not as good as a spectrum analyzer, the oscilloscope has its own advantages:

• Waveform and spectrum analysis can be completed simultaneously, and the two are time-correlated;

• Supports synchronous analysis of multiple channels in the time and frequency domains to achieve multi-point monitoring of the circuit;

• It can analyze the spectrum of periodic signals and the spectrum of non-periodic signals;

• It can analyze the spectrum of extremely low frequency (down to DC) signals, which is beyond the reach of spectrum analyzers.

• It supports a variety of signal detection methods. It can be connected through a standard coaxial interface, or flexibly detected through the matching voltage and current probes.

As a new spectrum analysis method based on oscilloscope, Spectrum View perfectly realizes the parallel processing of time domain and frequency domain of signals . For applications requiring high frequency resolution, the traditional FFT method requires increasing the horizontal time base to achieve this, which not only reduces the measurement speed, but also fails to observe the details of the time domain waveform. Spectrum View supports independent settings of time and frequency domains. Even with a very small horizontal time base setting, it can still obtain a high frequency resolution, which can not only observe the waveform details , but also has a high spectrum refresh rate .

Figure 2. Harmonic and spurious test:

Spectrum View vs Conventional FFT

Figure 2 tests a 100MHz CW signal and captures a 4-cycle time domain waveform . The figure uses Spectrum View and traditional FFT (Math function) to test the spectrum of the signal. By 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 results of Spectrum View are very good. Not only does it have high resolution, but the background noise is also very low, and the signal itself and its harmonics and spurs can be clearly observed. At the same time, because the horizontal time base is set to a small size, the detailed information of the time domain waveform can also be observed.

Figure 3. RF Chirp Pulse time domain parameters and spectrum test

Given these advantages of Spectrum View, combined with other functions of the oscilloscope, it is also possible to 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. It also gives the time domain waveform, envelope, and spectrum test results. During the test, the Span and RBW can be flexibly adjusted to observe the envelope spectrum or line spectrum, thereby performing a more detailed analysis of the signal.

The oscilloscope has multiple analog channels, and each channel can activate the Spectrum View function, so it supports multi-channel spectrum testing. In the complex debugging process, it is possible to monitor the waveform and spectrum of multiple points . Similar to the multi-channel time domain waveform display mode of MSO64, the activated spectrum can be displayed in either "stacked" or "overlay". Figure 4 observes the time domain waveform and spectrum of two channels at the same time, and uses an overlapping display to facilitate the comparison between the spectrums.

The spectra of all channels share the same span, RBW, and FFT window, 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. ↘↘↘

Figure 4. Spectrum View supports multi-channel waveform and spectrum testing

As mentioned above, Spectrum View supports sliding the position of Spectrum Time to perform spectrum tests on signals in different time periods, which makes it possible to perform multi-domain linkage tests on signals.

The following tests are respectively conducted on the Chirp Pulse and the Hopping Signal. Combined with the Spectrum View and Frequency Time Trend test functions, the signal linkage test in the time domain, frequency domain, and modulation domain is realized .

1. Chirp Pulse multi-domain linkage analysis

As a pulse compression technology, linear frequency modulation has very high time resolution and is widely used in radar applications. Whether it is a linear frequency modulated pulse or a frequency modulated continuous wave, the performance of the signal needs to be verified during the product development stage, and the time domain parameters, amplitude parameters and modulation domain parameters of the signal need to be tested.

Figure 5. Chirp Pulse time domain, frequency domain and modulation domain linkage analysis

In this example, a chirp pulse is measured. The time domain parameters can be tested using an oscilloscope, and the spectrum can be tested in Spectrum View. The modulation domain parameters of the chirp pulse, the frequency modulation curve, can be tested using Frequency Time Trend, and the chirp rate and linearity can be derived from the frequency modulation curve.

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 testing can also be completed. As shown in Figure 6, Frequency Time Trend tests the frequency hopping state sequence, which can observe the frequency hopping process and use Cursor to calibrate the frequency switching time and frequency dwell time.

Spectrum Time is located at the red mark in Figure 6. Its position can be moved, and the tested spectrum is the spectrum corresponding to the current position. By dragging the position of Spectrum Time, you can observe different frequency points respectively, and you can also observe the spectrum changes during the frequency switching process, 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 dedicated spectrum analyzers and traditional FFT functions of oscilloscopes, Spectrum View has unique advantages. This function can not only complete ordinary spectrum tests, but also realize synchronous testing of time domain waveforms and spectrum, and supports multi-channel linkage testing.

The mobility of the Spectrum Time position, combined with the Frequency Time Trend function, enables the oscilloscope to have the function of multi-domain linkage analysis. The feasibility of multi-domain linkage analysis is verified by testing linear frequency modulation pulse and frequency hopping sequence signals.