Why digital RF engineers need knowledge?

In many high-speed applications such as computers, communications and other fields, many digital bus data rate reached Gb / s or more even higher. Ideal digital signal 0,1 we traditionally think of starting more demonstration of its RF characteristics. True digital signal during transmission, but also more and more demonstration of the characteristics of microwave circuits.

When these high-speed signal analysis, the traditional time domain analysis facing lack of precision and the lack of analytical tools and other issues, and analytical tools in the field of RF and microwave frequency domain is very mature and improve. Therefore, for analyzing and measuring the high-speed digital signals are increasingly began to use analysis of some radio frequency or microwave. Digital design engineer requires more radio frequency by means of a number of methods to analyze the digital signal or concepts, such as the means to use the frequency domain analysis of the signal spectrum, analyzing the reflected transmission path loss, etc. using S parameters.

In order to help the majority of the digital test engineers to understand the basic concepts of frequency-domain analysis means, I deliberately put together this "digital RF engineers need to have knowledge," the article, which was first published in 2013 in the EDI CON conference, now take come out and share.

The digital signal to be analyzed, the primary reason is that the real high-speed digital signal transmission has been far from ideal textbook 0/1 level. True digital signal transmission must be some (or even very serious) distortion and deformation. As shown below in red is what we expect ideal digital signal waveform, while it may be true yellow signal waveform, you can see the signal has been due to the shock (usually due to poor impedance matching) has large deformation occurred. In fact, this is already the case at high speed is relatively good signal waveforms, and waveform signals a lot of time will be more severe than this.

Research to conduct a digital signal, the first to get the true digital signal waveform, which involves the problem of measuring instruments used. The best tool to observe the electrical signal waveform oscilloscope, when the signal rate is relatively high, generally require higher bandwidth oscilloscope. If the oscilloscope bandwidth is not enough, in the high-frequency component signal will be filtered out, the observed digital signal will produce distortion. Many digital engineers accustomed to estimate harmonic signal bandwidth, but this method is not accurate.

For an ideal square wave signal, which is infinitely rising steeply, from the frequency domain point of view it is an infinite number of odd-numbered harmonics constituted therefore an ideal square wave can be considered as an infinite number of odd harmonics of sine Overlay.

But for the true digital signal, its steep rising not unlimited, so the higher harmonics of the energy will be limited. For example, the following figure is 50Mhz and spectrum clock signal with the same 250MHz clock source generated respectively, we can see that although the output clock frequency is not the same, but the main spectral energy signals are concentrated in less than 5GHz, and not necessarily 250MHz spectrum Distribution will certainly larger than five times of 50MHz.

For real data signals, the spectrum will be more complicated. The spectral envelope of such a pseudo-random sequence (PRBS) stream is a Sinc function. The figure is 800Mbps PRBS signal and spectrum with the same transmitter 2.5Gbps generated respectively, we can see that although the output data rate is not the same, but the main spectral energy signals are concentrated in less than 4GHz, also does not necessarily 2.5Gbps high frequency energy of the signal is much higher than 800Mbps.

The above two graphs are measured by means of the spectrum analyzer. While modern digital oscilloscopes already have digital FFT functions can help the user to observe the signal spectrum, but due to limitations ADC bits and dynamic range of the spectrum analyzer is still the frequency of the signal energy distribution of the most accurate tools for analysis, so Engineers can by means of a digital spectrum analyzer for spectrum distribution of the measured digital signal analysis. When no analyzer is available, we usually go to estimate the spectral energy of the signal in accordance with the rise time of the digital signal.

Maximum signal frequency content = 0.4 / fastest rise or fall time (20 - 80%)

Or

Maximum signal frequency content = 0.5 / fastest rise or fall time (10 - 90%)

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