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Clock and Data Recovery Circuits
Published in Krzysztof Iniewski, Circuits at the Nanoscale, 2018
The task of the CDR circuit is to regenerate the data arriving at the receiver as shown in Figure 22.1a. The received signal experiences attenuation and distortion from the communication channel and is further corrupted by deterministic and random sources of disturbance such as crosstalk, reflection, and supply noise. The regeneration circuit inside the receiver is synchronized with the incoming data and captures the data at the instant of maximum eye opening, typically at the center of the bit period. The sampling clock signal should achieve high purity (in terms of timing jitter) because the purity of the regenerated signal can never exceed that of the sampling clock signal. Therefore, clock recovery focuses on synchronizing the sampling clock with the data, positioning the sampling edge of the clock at the center of the data eye, and reducing the timing jitter of the clock signal to the smallest possible.
Modulation for Short-Reach Access Optical Transmission
Published in Le Nguyen Binh, Optical Modulation, 2017
For the higher 56 Gb/s data rate, equalization is mandatory. Due to the increased signal rate, the channel the signal travels through distorts the signal at the receiver. The result can be seen using an oscilloscope and show a partially or completely closed eye diagram (Figure 9.6) that will prevent the receiver's ability to extract the clock and/or data. Equalization needs to be applied to reopen the eye diagram correcting for the inter-symbol interference (ISI) and recover the clock or data. Equalization methods used include feed forward equalization (FFE) for the transmitter and continuous-time-linear equalizer (CTLE), decision feedback equalizer (DFE) and CDR for the receiver. Clock recovery tracks low-frequency jitter and is utilized by either a real-time oscilloscope or a sampling oscilloscope when characterizing a transmitter. Current commercial oscilloscopes include software that can be used to model equalization such as CTLE, DFE, and FFE. An example is the Keysight N5461A Infiniium Serial Data Equalization Software, which allows users to choose which equalizer, or combination, to implement.
Voice Transmission
Published in Goff Hill, The Cable and Telecommunications Professionals' Reference, 2012
Stuart D. Walker, Rouzbeh Razavi
There are different ways to compensate for the impact of jitter. The most common is to use a Phase Lock Loop (PLL) for clock recovery. In its simplest form, a PLL consists of a phase comparator, a lowpass filter and a Voltage Controlled Oscillator (VCO) in a closed feedback loop. By employing a PLL when a gap occurs between the reference timing signals, the extracted clock signal can be used to stimulate the phase comparator. The performance of a PLL is evaluated based on two main metrics: phase noise and lock time. For a carrier frequency at a given power level, the phase noise of a PLL is defined as the ratio of the carrier power to the power found in a 1-Hz bandwidth at a defined frequency offset. The lock time of a PLL is also defined as the time it takes to move from one specified frequency to another specified frequency within a given frequency tolerance. After all, dejitterizers can be employed to reduce the effect of jitter in digital signals. Dejitterizers consist of an elastic buffer in which the signal is temporarily stored and then retransmitted at a rate based on the average rate of the incoming signal.
Design and implementation of a virtual on-line lab on optical communications
Published in European Journal of Engineering Education, 2023
Dimitris Uzunidis, Gerasimos Pagiatakis
The main building blocks of an optical receiver are depicted in Figure 2. The incoming optical signal is converted into an electrical signal with the use of a PIN photodiode while the impact of an additive white Gaussian noise source is also considered. The low-pass filter (LPF) module filters the signal in order to select the desirable frequencies and cut the unwanted ones off. In this way, by selecting the optimum cutoff frequency, the impact of receiver-induced noise is minimised. The clock recovery module synchronises the received signal with the transmitted one by removing the time delay between them. In other words, it regenerates the signal, exploiting the logical information which is attached to the incoming signal and pre-specified at the optical transmitter. Finally, the signal is inserted into the BER and Q-factor estimation module which performs sampling in the optical signal and uses a decision threshold in order to determine if the value of the received data is zero or one. This module also performs a Gaussian approximation for the noise distribution on both the one and zero levels and estimates the overall system performance in terms of Q-factor and Bit Error Rate (BER).