Wu Xiongwen, member of the expert group of dingyang hardware design and testing think tank

For most electronic engineers, the signal in time domain is very intuitive and easy to understand, because time domain is a real world and a real domain. All of our experiences are developed and verified in time domain, and we are used to the time sequence of events, which are recorded and preserved.

Dingyang technology provides a series of different oscilloscopes for electronic engineers to capture, observe, measure, analyze and archive the time domain signals of interest to us. I believe you can easily understand the basic working principle of oscilloscope and master the use and operation method of oscilloscope. But is a signal really as “pure” and “perfect” as it seems? Sometimes it is difficult to do real and quantitative analysis with oscilloscope. Is there any harmonic or non harmonic component in a sine wave that looks very smooth to the naked eye? A corner of a square wave, why use oscilloscope to observe always can’t show a perfect right angle? These problems often trouble some users who have not contacted the spectrum analyzer. The frequency domain, the role hidden behind the time domain, seems to be covered with a mysterious veil, which makes it easy for some electronic engineers to have a sense of distance. Moreover, the concept of bandwidth, amplifier, attenuator and filter on spectrum analyzer often makes us have no way to start, and it is difficult to get a correct result. Dingyang technology, as the industry leader of general electronic testing and measuring instruments, always takes high efficiency, authenticity, accuracy, convenience and quickness as the core idea of our product design, ssa3000x Plus Series High-Performance spectrum analyzer and sva1000x series spectrum & vector network analyzer perfectly explain the customer-oriented product concept, making our spectrum testing very simple.

As shown in Figure 1 below, we use a sdg6052x function / arbitrary waveform generator, a sds5054x digital fluorescence oscilloscope and a sva1032x spectrum & vector network analyzer to build a comparative demonstration system of time domain and frequency domain.

From time domain to frequency domain

Figure 1 Comparison Test System of vs in time domain and frequency domain

We connect channel 1 and channel 2 of sdg6052x signal source with BNC cable directly to channel 1 of sds5054x oscilloscope and RF input of sva1032x spectrum analyzer, and then turn on channel tracking function of sdg6052x signal source. In this way, channel 2 can output exactly the same waveform only by setting the parameters of channel 1 of signal source, so as to ensure that oscilloscope and spectrometer can simultaneously output the same waveform Received exactly the same signal.

First of all, set the signal source to output a 1MHz square wave signal. It is easy to capture a standard square wave signal with the auto setup button of the oscilloscope, as shown in Figure 2. Our spectrometer is also designed with auto tune button. By simply pressing the button, the spectrometer will automatically search for signals in the whole frequency band, and adjust the frequency and amplitude parameters to the optimal state. One key can realize signal search and parameter automatic setting, and a 1MHz fundamental signal as shown in Figure 3 can be obtained soon.

From time domain to frequency domain

Figure 2 1MHz square wave signal measured by oscilloscope

From time domain to frequency domain

Fig. 3 1MHz fundamental signal automatically captured by spectrometer

However, as we all know, square wave is a wave form with rich harmonic components. At this time, we only need to set the termination frequency of the spectrometer to a larger value, such as setting it to 18mhz, to perfectly present part of the square wave harmonic, as shown in Figure 4. We can also use the cursor list function of spectrum analyzer to display the frequency and amplitude of each harmonic in real time, as shown in Figure 5. Even, we can set the termination frequency to 100MHz and find that even in the harmonic component of nearly a hundred times, it still has an amplitude of nearly 40dbc (relative to the fundamental wave). This fully explains the reason why the distortion of square wave signal over 1MHz will be relatively large when we use the oscilloscope to test the square wave signal, because the bandwidth of the oscilloscope itself and the bandwidth limit of the test cable, the square wave signal is filtered out High order harmonic component of.

From time domain to frequency domain

Fig. 4 1MHz square wave spectrum after the termination frequency is set to 18mhz

From time domain to frequency domain

Figure 5 shows the frequency and amplitude of each harmonic using the cursor list

From time domain to frequency domain

Fig. 6 1MHz square wave spectrum after the termination frequency is set to 100MHz

Then, we let the signal source output an FM signal, set the carrier frequency to 10MHz, the frequency offset to 5MHz, and the modulation frequency to 200MHz. A sine wave signal with changing frequency can be captured on the oscilloscope. By opening the measurement and statistics function of the oscilloscope, the minimum frequency and maximum frequency can be accurately measured as 5MHz and 15MHz, as shown in Figure 7. If you want to see the performance of FM signal in the spectrum analyzer, you just need to click the auto tune button of the spectrum analyzer to quickly measure the spectrum of the carrier signal, and then turn on the modulation switch of the signal source, we can clearly see a spectrum reciprocating from 5MHz to 15MHz, and the cycle is 5 seconds, as shown in Fig. 8, 9 and 10. This allows us to intuitively understand the meaning of various indicators of FM signal: carrier frequency – frequency offset = the minimum frequency of output; carrier frequency + frequency offset = the maximum frequency of output; the reciprocal of modulation frequency is the modulation period, which is the time to complete a frequency change from the minimum to the maximum, and then back to the minimum frequency.

From time domain to frequency domain

Figure 7 FM signal measured by oscilloscope measurement and statistics function

From time domain to frequency domain

Figure 8 FM signal measured by spectrum analyzer

From time domain to frequency domain

Fig. 9 FM signal measured by spectrum analyzer

From time domain to frequency domain

Fig. 10 FM signal measured by spectrum analyzer

Finally, we let the signal source output an AM modulation signal, the carrier is 10MHz, the modulation signal is 1MHz, the modulation depth is 50%. Setting the time base, amplitude resolution and trigger level of the oscilloscope reasonably, we can see an intuitive AM signal, as shown in Figure 11. For the spectrum analyzer of dingyang technology, we can quickly capture a graph of three spectral lines by simply pressing the auto tune button. As shown in Figure 12, the center frequency is 10MHz, and the left and right spectral lines are 9MHz and 11mhz respectively. By changing the frequency or modulation depth of AM modulation signal, a simple and intuitive performance can be obtained on the spectrum analyzer, so as to understand the meaning of various parameters of AM modulation signal.

From time domain to frequency domain

Figure 11 AM modulation signal measured by oscilloscope

From time domain to frequency domain

Fig. 12 AM modulation signal measured by spectrometer

Through the time and frequency domain comparison test of the above three kinds of signals, we can find that the use of spectrum analyzer is not as complicated as expected. We can even easily peel off the complex appearance of the signal and see the essential information hidden behind the time. In particular, dingyang technology ssa3000x Plus Series High-Performance spectrum analyzer and sva1000x series spectrum & vector network analyzer not only have excellent hardware performance, but also are equipped with 10.1 inch multi touch capacitive screen, which can support many functions, such as one key automatic search test, one key storage, one key help, one key recovery factory settings and one key automatic coupling. At the same time, the instrument can be aligned based on computer or handheld terminal network browser Remote monitoring makes our spectrum testing easier and faster.

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