Cover story
Since not only the jitter influence of the data signal, but also its clock signal is important, the first step is to check the clock stability of the bus system. The clock stability is very important in order to achieve good synchronization between the clock and the data transmission so that the data does not drift away from the clock. The results table displays the edge deviation from the ideal edge [TIE], the pulse width deviation of the following pulse [+/- width to +/- width] and the period deviation [Cycle- to-Cycle] compared to the following period measure. The probability density function of the jitter can be represented in the histogram. On the one hand, the histogram offers a graphic representation which, due to the symmetry, serves as an aid in assessing the type of jitter. However, the histogram shows the total jitter (convolution of the jitter components in the time domain). Individual jitter components cannot be 100% determined from the histogram, especially not when different components are dominant. Therefore the jitter measurement offers a further display function of the jitter deviation and characteristics with the trend graph.
In Figure 2, jitter is caused by another 10 kHz sinusoidal signal. The trend has the effect of integrating the interference clock signal. Since a sinusoidal characteristic can be seen in the trend, a single frequency disturbance can be concluded. The histogram display underlines this assumption through the greatest even distribution on the sides. I.e. the largest jitter excursions arise from the maxima/minima of the interference signal. The interference signal could arise e.g. from an oscillation in a PLL circuit. Noise components are not dominant on the influence of the jitter in this measurement.
The next step is to measure the influence of amplitude interferences and noise with the eye diagram for the influence of jitter. Several thousand transmission sequences can be recorded with the real-time eye diagram of the MSO8000 series. For example, the Q-factor can be represented by the measured values of the eye diagram. The Q factor is an important quality criterion in data transmission, which is used to judge the data signals and also allows a statement to be made for the BER (see formula 1, µi = is the average value and σi = the standard deviation of the amplitudes of the states i = 0 and 1). With the aid of the eye diagram, a transmission can also be checked for its robustness, in order, if necessary, to give external interference to the transmission medium and to analyze the behavior at the eye.
Figure 4: Pure data signal with low noise behavior, more bandwidth and low jiIer, which leads to a significantly beIer RF characteris*c with improved BER
Formula 1: Calculation of the BER using the Q factor
On the one hand, the noise behavior of the transmission can be tested. Another influence is the line attenuation of the transmission path, which should be chosen so that the eye is still largely open. The eye is not only used to measure vertical influences. Horizontal influences such as jitter can also be visualized and measured. In the eye diagram representation on e.g Channel 1 (see Figure 3) and the clock display on e.g. Channel 3 can also be seen whether the signal is drifting away from the clock. In Figure 3 it can be seen that a noisy jitter-afflicted signal with strong bandwidth limitation can lead to interference in the HF transmission (modulation on HF carrier: 2FSK). The RF transmission and the BER were measured with the vector signal analyzer mode and the spectrum with the
real-time spectrum analyzer mode of the RSA5065N. Another important component of digital transmission is bandwidth. However, optimizing the rise time has disadvantages. On the one hand, as described above, more bandwidth is required and, on the other hand, overshoots can be generated, which can also be undesirable. With the FFT in the MSO8000, the bandwidth requirements of the data signal with different rise times can be carried out with a very precise frequency analysis. At the same time, this measurement can be used in the time domain in order to achieve a compromise between the best possible rise time and the smallest overshoot. Figure 4 shows a data signal in which all noise components and spurious components for the jitter have been removed. The Q factor is very high and the spectral purity of the 2-FSK modulation on the RF carrier is significantly improved. The BER measurement no longer shows any errors and the modulation analysis measures significantly better parameters.
Summary
Different interferences (jitter, noise, coupled interferences) can strongly influence the quality of a data transmission. With the help of the 2 GHz oscilloscope from the MSO8000 series and the real- time spectrum analyzer from the RSA5000 series, it is possible to measure the complete signal chain from the embedded signals to the air interface and the receiver unit. Suitable regeneration and troubleshooting as well as successful recovery can be implemented using these measurement methods for successful data transmission.
www.electronicsworld.co.uk September 2021 07
Figure 3: Measurement of the eye diagram of a noisy data signal and its effect on the HF transmission and BER
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50
