Preface - How to use an oscilloscope to measure RF signals (Part 1)

    After launching the series "RF Knowledge Digital Engineers Need to Master", it received a strong response. Some engineer friends contacted me and said that in addition to digital engineers using RF instruments, some RF engineers also use oscilloscopes to test RF signals, but they are not sure about the accuracy and the difference between oscilloscopes and traditional instruments such as spectrum analyzers. I hope I can explain this aspect.

 

    To this end, I have compiled some application cases and precautions for oscilloscopes to test RF signals, which will be serialized in the future, hoping to provide some help to everyone

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    One of the main reasons for performing time domain measurements on RF signals is that they are intuitive. For example, the following figure shows four radar pulse signals of different shapes. The carrier frequency and pulse width of the signals are not much different. If only the frequency domain is analyzed, it is difficult to infer the time domain shape of the signal. Since the different shapes of these four time domain pulses are crucial to the final convolution processing algorithm and system performance, it is necessary to accurately measure the pulse parameters of the signal in the time domain to ensure that the system design requirements are met.

The need for higher analysis bandwidth

    In traditional RF microwave testing, some oscilloscopes with low bandwidth (<1GHz) are also used to test time domain parameters, such as using a detector to detect the RF signal envelope before testing parameters, or down-converting the signal before collecting it, etc. At this time, since the RF signal has been filtered out or the signal has been converted to the intermediate frequency, the bandwidth requirement of the oscilloscope used for measurement is not high.

 

    However, with the development of communication technology, the modulation bandwidth of signals is getting wider and wider. For example, in order to balance power and distance resolution, modern radars will use frequency or phase modulation inside the pulse. The modulation bandwidth of a typical SAR imaging radar may reach more than 2GHz. In satellite communications, in order to miniaturize and increase the transmission rate, the crowded C-band and Ku-band are also avoided, and the Ka-band with higher spectrum efficiency and available bandwidth is used. The actual available modulation bandwidth can reach more than 3GHz or even higher.

 

    With such a high transmission bandwidth, traditional detection or down-conversion measurement methods will encounter great challenges. Since it is difficult to find a detector or down-converter with a bandwidth of more than 2GHz and ideal amplitude-frequency/phase-frequency characteristics on the market, it will cause serious distortion of the test results.

 

    At the same time, if you need to demodulate the internal modulation information of radar pulses or satellite communication signals, you also need a very high real-time bandwidth. Traditional spectrum analyzers have high measurement accuracy and frequency range, but the real-time analysis bandwidth is not yet above GHz. Therefore, if you want to analyze and demodulate broadband signals above GHz, the most commonly used method is to use a broadband oscilloscope or a high-speed data acquisition system.