Cross-domain analysis using hybrid domain analyzer

The MDO4000 series mixed domain analyzer (Figure 1) is an innovative analyzer that has won more than a dozen best innovation awards at home and abroad. The reason is the "five-in-one" feature of the MDO4000, namely: a four-channel 500MHz/1GHz bandwidth digital phosphor oscilloscope, a 16-channel logic analyzer, multiple bus protocol analyzers, a 3GHz/6GHz spectrum analyzer, and a modulation domain analyzer with a bandwidth greater than or equal to 1GHz. These five functions work under the same clock and the same trigger mechanism, giving the MDO4000 an innovative cross-domain analysis function of time domain, frequency domain, and modulation domain time correlation.

Figure 1: MDO4000 Series Mixed Domain Analyzer.

Introduction to Cross-Domain Analysis

With the development of embedded technology, software radio technology and digital RF technology, when designing, developing and debugging the RF drive circuits of these systems, testing only in the frequency domain or time domain can no longer meet their needs for high efficiency and high reliability. This requires a test tool that can treat the RF signal as a channel of an oscilloscope and organically combine the baseband signal, control signal, bus signal, spectrum and modulation domain characteristics of the RF signal in the time domain. This is the cross-domain analysis of time domain, frequency domain and modulation domain time correlation. At present, the Tektronix MDO4000 series mixed domain analyzer is the only analyzer that can perform cross-domain analysis.

Figure 2 (a) is a typical schematic diagram of the time-correlated cross-domain analysis of the MDO4000 series hybrid domain analyzer. The upper part of the figure is the time domain waveform, and the lower part is the frequency domain spectrum. This figure shows the process of ASK modulation of the 2.4GHz carrier by the baseband data. In the figure, yellow is the baseband signal and blue is the bit reference signal. There are seven bit intervals between the two wide pulses of the bit reference signal, which are used to separate 8 serial binary numbers. Therefore, the yellow baseband data is presented as data 0~7 under the separation of the bit reference signal. So where is the time-correlated characteristics of the time domain and frequency domain reflected? In the upper part of Figure 2 (a) and 2 (b), the orange bar framed by the red circle indicates the moment of the spectrum on the time axis, that is, the spectrum shown in Figure 2 (a) and 2 (b) is the spectrum of the moment of the orange frame in the upper part on the time axis. By moving the position of the orange frame on the time axis, the spectrum in the lower part will change accordingly.

On the other hand, in Figure 2 (a), the upper time domain display also has an orange waveform, which is almost identical to the yellow baseband signal. In fact, this waveform is the amplitude variation of the 2.4GHz carrier signal over time, that is, the modulation domain waveform of the RF signal. It can be seen that the causal relationship between the baseband signal, control signal, RF spectrum and modulation domain characteristics of the RF signal can be easily tested by using time-correlated cross-domain analysis.

Figure 2: Schematic diagram of MDO4000 series cross-domain analysis.

How to implement cross-domain analysis

Since cross-domain analysis refers to the coordinated analysis of the time domain, frequency domain, and modulation domain, can an oscilloscope used for time domain analysis and a spectrum analyzer or vector signal analyzer used for frequency domain and modulation domain analysis form a test system for cross-domain analysis? The answer is no! Because the oscilloscope and spectrum analyzer or vector signal analyzer are different instruments, even if they are synchronized with an external clock, their independent triggering mechanisms make it difficult for them to obtain the same time base; even if the time base error caused by the uncertainty of the triggering of each instrument is ignored, the results displayed by each instrument are difficult to correspond on the time axis.

Perhaps someone will raise such a question: since the oscilloscope can test the timing relationship of different signals in each channel, if we sacrifice one oscilloscope channel, connect the RF signal to this channel, and then use the oscilloscope's FFT to display the frequency spectrum of this channel, wouldn't it be possible to perform cross-domain analysis? The answer is also no.

Figure 3 is a screenshot of the oscilloscope. The yellow signal of channel 1 is the control pulse, the blue signal of channel 2 is the data, the fan signal of channel 3 is the connected RF signal, and the red curve is the FFT spectrum of the RF signal of channel 3. The RF signal transmits a 900MHz carrier before the control pulse is sent, and transmits a 2.4GHz carrier after the control pulse is sent. From this screenshot, we can find the following problems: the bandwidth of the oscilloscope is 1GHz, so channel 3 cannot correctly display the 2.4GHz carrier after the control pulse; the red FFT spectrum is the FFT of all sample points of channel 3, and there is no information on the time axis. In view of the above two points, the red FFT spectrum only displays the 900MHz carrier spectrum and cannot display the entire spectrum change process, so this method cannot be used for cross-domain analysis.

Figure 3: Oscilloscope screenshot.

For comparison, the 3-channel RF signal in the above figure is connected to the spectrum analyzer, and its displayed spectrum is shown in Figure 4 (a). In this spectrum, the 900MHz and 2.4GHz signals can be seen at the same time. The spectrum display of Figure 4 (a) is changed to the maximum hold mode to obtain the spectrum of Figure 4 (b). We found that a signal appears intermittently at 2.5GHz.

Figures 4 (a) and 4 (b) show the maximum information that can be obtained from a spectrum analyzer. We cannot see how the spectrum changes over time, let alone the timing relationship between the RF signal and the control pulse. The emergence of the MDO4000 analyzer has solved the synchronization problem of time and triggering, allowing the frequency domain, time domain, and modulation domain waveforms to be displayed synchronously on the time axis, providing an innovative means for the design, development, and debugging of embedded and digital RF systems.

Figure 4: Maximum information available from a spectrum analyzer.

The following Figures 5 (a) to (c) illustrate the cross-domain analysis capabilities of the MDO4000 analyzer. Channel 1 of the MDO4000 is connected to the control pulse signal (yellow), and the other channel signals will be further explained below. Next to each screenshot, red arrows are marked, pointing to the current spectrum moment and the corresponding carrier of the spectrum display. In Figure 5 (a), the spectrum moment is located on the left side of the control pulse, and the RF signal frequency is 900MHz; in Figure 5 (b), the spectrum moment is located at the control pulse moment, and there is no RF signal at this time; in Figure 5 (c), the spectrum moment is located far to the right of the control pulse, and the RF signal frequency is 2.4GHz.

Figure 5: Cross-domain analysis capabilities of the MDO4000 analyzer.

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