[Extra Speed] High-Speed CMOS Circuits For Optical Receivers
Click Here ->>> https://tlniurl.com/2tuL7K
How to Design High-Speed CMOS Circuits for Optical Receivers
Optical communication is a fast-growing field that requires high-speed and low-power electronic circuits to interface with optical fibers. Optical fibers offer high bandwidth and low loss, enabling the transmission of large volumes of data over long distances. However, the electronic circuits that process the optical signals need to operate at very high frequencies and consume minimal power.
One of the key components of an optical receiver is the clock and data recovery (CDR) circuit, which extracts the clock and data from the incoming optical signal. The CDR circuit needs to be able to handle different data rates, formats, and jitter levels, as well as provide a clean and stable output signal. The design of a CDR circuit involves many challenges, such as frequency acquisition, phase locking, jitter filtering, and power consumption.
In this article, we will review some of the techniques for designing high-speed CMOS circuits for optical receivers, based on the book High-Speed CMOS Circuits for Optical Receivers by Jafar Savoj and Behzad Razavi[^1^]. We will focus on the design of CDR circuits for 10 Gb/s applications using a pure CMOS process.
CDR Architectures
There are two main types of CDR architectures: analog and digital. Analog CDR circuits use continuous-time filters and voltage-controlled oscillators (VCOs) to recover the clock and data. Digital CDR circuits use discrete-time filters and phase-locked loops (PLLs) to perform the same function. Both architectures have advantages and disadvantages in terms of performance, complexity, and power consumption.
Analog CDR circuits can achieve fast locking, low jitter, and low power consumption, but they suffer from process variations, noise sensitivity, and limited frequency range. Digital CDR circuits can achieve wide frequency range, robustness to noise and variations, and easy integration with digital logic, but they require more power consumption, more area, and more complex circuitry.
The book High-Speed CMOS Circuits for Optical Receivers presents three examples of CDR circuits using different architectures: a linear half-rate CDR circuit using an analog phase interpolator[^2^], a bang-bang half-rate CDR circuit using a digital phase detector[^3^], and a linear full-rate CDR circuit using an analog phase detector. We will briefly describe each of these examples in the following sections.
A Linear Half-Rate CMOS CDR Circuit
The first example is a linear half-rate CMOS CDR circuit that uses an analog phase interpolator to adjust the phase of a half-rate clock signal according to the phase error between the incoming data and the recovered clock. The phase interpolator consists of two differential pairs that are cross-coupled by capacitors. By controlling the tail currents of the differential pairs, the phase interpolator can generate any phase between 0Â and 180Â with respect to the half-rate clock.
The phase error is detected by a linear phase detector that compares the incoming data with two delayed versions of the recovered clock. The phase detector generates two error signals that are proportional to the sine and cosine of the phase error. These error signals are then filtered by a loop filter that consists of a resistor and a capacitor. The loop filter provides both low-pass filtering and integration functions. The output of the loop filter drives two current sources that control the tail currents of the phase interpolator.
The half-rate clock is generated by dividing a full-rate clock by two using a divide-by-two circuit. The full-rate clock is obtained from a ring oscillator that consists of five inverters connected in a loop. The ring oscillator is biased by a current source that is controlled by an automatic frequency control (AFC) loop. The AFC loop compares the frequency of the ring oscillator with an external reference frequency using a frequency detector. The frequency detector generates an error signal that is filtered by another loop filter and drives another current source that controls the bias current of the ring oscillator.
The main advantages of this CDR circuit are its simplicity, linearity, and low power consumption. The main disadvantages are its limited capture range, sensitivity to noise and variations, and requirement for an external reference frequency.
A Bang-Bang Half-Rate CMOS CDR Circuit
The second example is a bang-bang ec8f644aee