How to observe a serial data transmission system

Figure 3.19 illustrates a 100MBPS data transmission system. Due to inter-symbol interference and additional noise, the signal waveform output at point D shows more jitter than the waveform at point A. The main point here is how to correctly display the jitter characteristics of the output waveform.

The first step is to connect signal D to oscilloscope channel 1. We select channel 1 as the trigger channel and adjust the oscilloscope to trigger on the rising edge of the signal. We will see the pattern shown in Figure 3.20.

Note that the waveform shows no jitter at the trigger point, which is a key indicator that something is wrong. The oscilloscope waits for the positive transition of the signal and then shifts the data waveform to align with the trigger point at the left cursor. The first pulse correctly shows the positive transition of the signal and then shifts the data waveform to align with the trigger point at the left cursor. The first pulse correctly shows the minimum time interval of the signal transition, while the jitter error around the subsequent clock points is twice the actual data clock jitter time.

Figure 3.21 shows the correct display of the measurement. The trigger of the signal in Figure 3.21 uses the clock source as the absolute reference. Obviously, the jitter displayed here is half of the previous jitter value. The previous method makes all waveforms shifted, and all rising edges are aligned at the same point. This displacement will be superimposed on all other jump areas. The clock source signal is relatively stable and has no jitter. It is a reliable reference point for all data signal measurements.

Someone may ask, "Why can't we use the technique in Figure 3.20 and then divide the result by 2?" The answer is: jitter measurements can only be made if the eye diagram in Figure 3.20 is open enough, and we are not always so lucky. Sometimes the eye diagram will not open at all if we do not use the advanced triggering technique shown in Figure 3.21.

When there is no source data clock, you can try to use the source data signal for triggering (position B or A in Figure 3.19). The data signal at the source end has almost no jitter.

Some oscilloscopes, especially most new digital sampling oscilloscopes, have poor triggering capabilities, especially for non-periodic signals such as data waveforms. The trigger circuit may not be able to trigger even though the vertical input has the ability to display high-speed signals. When faced with such an oscilloscope with poor triggering characteristics, you should first construct a digital circuit to divide the system clock and then use this secondary waveform for triggering. As the trigger stability improves, you may find that the rise time of the signal displayed on the oscilloscope is reduced.