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Optical Dispersion Compensation and Gain Flattening
Published in Le Nguyen Binh, Photonic Signal Processing, 2019
An eye pattern diagram is usually used to visually assess the system performance in a digital communication system. Initially, random data signal patterns are generated. For a 3-bit-long sequence, for instance, we could have ′111′,′011′,′110′…, and so on When these combinations are superimposed simultaneously, an eye pattern diagram is formed. The eye pattern diagrams for the intensities before and after the equalizer corresponding to the above case are shown respectively in Figure 5.40a and b respectively. Recall that the operating point of the equalizer here is 2.8225 rad.
Applications of Shaped Pulses to Communications
Published in Marcos Dantus, Femtosecond Laser Shaping, 2017
We start by realizing that the ability to read data being transmitted through an optical fiber requires the symbols or pulses to remain distinguishable. If the pulses get broadened while being transmitted, they can start to blend in and this affects their readability (Figure 14.2). The key source for pulse broadening is spectral dispersion as discussed in Chapters 4 and 5. Broadening is proportional to the inverse pulse duration squared. That implies that for relatively long pulses, for example, half a nanosecond, dispersion causes a million times less broadening than for half a picosecond pulse. This is important because higher transmission data rates are very desirable. For example, 1 Gb/s data transfer rate implies 1 bit is transmitted with subnanosecond time, and 1 Tb/s requires each bit to be transmitted with subpicosecond time. However, when the pulses being transmitted exceed the bandwidth of the fiber bandwidth, the communicated signal becomes distorted because of temporal overlap and interference between pulses. Optical communication systems are evaluated using the “eye pattern” measurement, which is an oscilloscope trace showing the detected signal as a function of the transmission rate. When the signals come in cleanly, an open eye pattern is observed. As the interpulse interference increases because of jitter, or dispersion, the eye pattern becomes fussier and the open regions decrease. The transition from Gb/s to Tb/s required a number of important developments in ultrafast lasers and pulse shaping.
Project 10: BPSK Digital Receivers
Published in Thad B. Welch, Cameron H.G. Wright, Michael G. Morrow, ® to C with the TMS320C6x DSPs, 2016
Thad B. Welch, Cameron H.G. Wright, Michael G. Morrow
Before we commence our discussion of the timing recovery loop, we need to briefly review the concept of the eye-pattern. As previously mentioned, the BPSK receiver must remove the effects of the carrier (also called “down conversion”), filter the down-converted signal, and then sample the resulting signal at just the right time (timing recovery process) to convert (in this case) 20 samples (the number of samples in a transmitted symbol) back into a message symbol (+1 or −1). This process is equivalent to creating what is commonly called an eye-pattern and sampling this pattern at the point of maximum average eye opening. An example of an eye-pattern created using an oscilloscope and a recovered symbol timing signal to trigger the display is shown in Figure 19.4. In this figure, 100 ms of the matched filter’s output is displayed. The eye opening is labeled, but this opening is not symmetric. To achieve a symmetric eye opening, considerably more data (gathered over time) is required to be displayed. In Figure 19.5, a full second of matched filter output is displayed. The eye is now symmetric, and the symbol period is labeled on the horizontal axis. The symbol rate is 2400 symbols per second, which results in a symbol period of 1/2400 = 416.67 μs. Considering that we wish to display three eye openings (i.e., two symbol periods), the time scale (the horizontal axis of the oscilloscope) needs to be set to (1/2400) * 2/10 = 83.33 μs/div, so the oscilloscope time-base was set to the nearest value of 84 μs/div.
Signal integrity analysis on a microstrip ultra-wideband coupled-line coupler
Published in International Journal of Electronics, 2019
Saffrine Kingsly, Sangeetha Velan, Malathi Kanagasabai, Sangeetha Subbaraj, Yogeshwari Panneer Selvam, Bhuvaneswari Balasubramaniyan
Signal integrity can also be analysed by visualising the eye pattern which is the common basis for interpreting the accuracy of signals in the high-speed systems. To form an eye pattern, the received signal is periodically sampled and applied to the vertical input, at the same time the data rate is applied to the horizontal scale. The vertical eye opening corresponds to the SNR and BER (which is inversely proportional to SNR), which represents the range in which we sample the signal with accuracy. The time variations (horizontal axis) at zero crossing on one side of the eye denotes the jitter. The slope is obtained as a result of timing error which is desired to be small. These parameters are denoted in Figure 12. The snap shots of the eye pattern for the two selected frequencies and the two modulation schemes are depicted in Figure 13.