Cover story
Characterization of the Signal Integrity of Data Signals
Author: Boris Adlung, Sales/Marketing Manager, Rigol Technologies, Gilching W
hen transmitting digital data on digital circuits, it is increasingly a challenge to achieve clean and ever faster data transmission. The smallest deviations and disturbances that occur during signal generation and in the internal
transmission can have a negative effect on the RF transmission of the signal. These disturbances can ultimately be fatal for the entire data transmission, since they add to the disturbances via the air interface. In the following text, the different signal components of a digital circuit are considered, which also transmit their information via the air interface. The aim here is to measure disruptive factors in the individual subcomponents by means of targeted error analysis. In addition, the effects of the errors of embedded signals on the RF transmission quality are measured.
The main goal of digital data transmission is to achieve a complete regeneration of the digital signal at the receiver. The regeneration must be at least so good that a threshold value decision of the digital units is possible. Unfortunately, the transmission can be negatively influenced by many different factors. The higher the data rate, the more the quality is influenced by plug connections, the cable or the material used. The more impure a data sequence is, the more interference and unwanted frequency components may occur during a possible HF transmission. The planning of a digital transmission is to be interpreted in such a way that the BER is below the value defined in the specification sheet. The BER is also the essential quality benchmark for digital transmission. Thus, the goal when planning a data transfer is that the BER should be very small. The new 2 GHz oscilloscope out of Rigol’s MSO8000 series is used to characterize the signal integrity. The architecture of the oscilloscope is based on the chipset developed by Rigol itself. With MSO8000 it is possible to carry out the measurement with the real- time eye diagram and the jitter analysis which represents a significant add-on value in the analysis of signals. This device also looks at the influence of bandwidth on rise time and overshoot. The influence of the signal integrity is also compared after the modulation on an RF carrier. On the one hand, the frequency response can be performed using the FFT (1 million points) integrated in the MSO8000. On the other hand, the real-time spectrum analyzer of the RSA5000N series with the vector signal analyzer module is used for demodulation and for testing the bit error rate [BER].
Characterization of the signal integrity in a serial data stream For the first analysis with the oscilloscope, the correctness of the individual bus systems that are used for digital transmission is checked. In the example in Figure 1, a parallel bus (clock on channel 3, data on channel 1) was measured with a PRBS7 test signal. The clock signal is present on channel 2. Triggering can take place on the
06 September 2021
www.electronicsworld.co.uk
Figure 1: Data signal (yellow) with clock (purple) and decoding, stabilized with dura1on trigger
Figure 2: Ji[er measurement of the clock signal with histogram and trend display
rising edge of the data signal (Edge). Alternatively, other more sensible trigger methods, such as triggering on the longest state (e.g. “0” or “1”) with the duration trigger. A third possibility is to use two zone triggers at the same time.
Instead of the analog inputs on the oscilloscope, the 16 digital inputs of the MSO8000 can also be used to test how a digital receiver interprets this bus and carries out the threshold value decision. An essential component of digital transmission is the jitter and noise
behavior, which significantly influences the threshold value decision. Jitter occurs when there are phase variations in the individual bits to be transmitted compared to the optimal bit edge. For high-quality data transmission, it is important to know the type of jitter in order to effectively minimize the causes. Impulse interference, crosstalk, or noise have the effect of non-symmetrical or random jitter. Unintentional influencing by another clock signal, on the other hand, is referred to as symmetrical or deterministic jitter, the influence of which then also dominates. This can have the effect of a data-dependent or periodic jitter.
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