With extremely high-speed signals, it’s necessary to augment redriver functionality with retimer capability.
The first crystal oscillator, which was implemented using a piece of Rochelle salt, was created by American scientist Alexander M. Nicholson in 1917 while working at Bell Telephone Laboratories. A key initial use of these early components was in stabilizing the frequency of amplitude modulation (AM) radio transmitters in the 1920s. In the past few decades, crystals have been used to create accurate transmit frequencies for of all of the key computer interfaces, including USB, PCIe, and Ethernet.
Of course, things are more complicated that they used to be in the 1920s. Today’s high-end interfaces may require the use of both redrivers (a.k.a. repeaters) and retimers. Since the quality of a signal degrades as it propagates down a wire, redrivers are used to regenerate signals, thereby boosting the signal quality of high-speed interfaces.
Unfortunately, simply boosting the signal is no longer sufficient in the case of modern high-speed protocols in which both the data and the clocking information are encoded in a single differential signal. Although a redriver amplifies the signal and “sharpens its edges,” it fails to address jitter, which refers to any deviation from true periodicity of a periodic signal. Thus, in the case of extremely high-speed signals, it’s necessary to augment redriver functionality with retimer capability.
USB4 is the latest version of the 25-year-old USB family of interfaces. USB4 is particularly useful in high-bandwidth display and storage applications. However, the advent of USB4 has brought the need to use retimers in all but the smallest form-factor systems. This is because the higher speed of USB4 as compared to earlier generations causes excessive frequency dependent loss when traversing the system’s printed circuit board. Earlier generations of USB could often get by using analog redriver circuits, but these have been found to be insufficient for USB4 because redrivers improver only the eye height and not the eye width.
Crystals are used in USB4 retimers because these circuits must generate a clean transmit clock when operating in certain modes. USB4 retimers must operate in a long list of different modes that are determined during operation via negotiation. This is because the USB-C connector over which USB4 operates is sometimes known as “the one connector to rule them all.” The USB-C connector supports all the speeds and widths of USB as well as the DisplayPort and Thunderbolt protocols, among others.
The modes that need a new transmit clock are the lower speeds of USB, that is USB1 and USB2 when operating in Separate Reference Clock with Independent Spread spectrum clock (SRIS) mode. This is because these two rates operate in a half-duplex mode and—as a result—do not have a continuous receive clock to work with.
USB4 retimers are generally protocol-aware, so they can handle the clocking differences found in SRIS mode by utilizing the “SKP” add/drop opportunities found in the protocol. USB3 and USB4 operate in full-duplex mode with a continuous signal, so the opportunity arises to use lower-latency bit-level retiming based on the recovered clock signal. The crystal source is also used for the link for all operating speeds in certain training and calibration modes as well as in some test modes. The crystal also typically provides the clock to internal portions of the retimer device.
Crystals create an accurate and stable frequency because their oscillation results from the flexing of the physical crystalline material in an electric field. Since a crystal has a specific size, shape, and mechanical stiffness, it can flex only within a specific small frequency range.
Efficient sustaining crystal oscillator circuits for USB4 applications include both single-ended and differential crystal oscillator structures. With differential oscillators, capacitively coupling the two sources adds DC stability.
The crystals that are used in USB4 retimers are the same as, or similar to, those that are used for USB4 sources. They must be cost effective as they carry the economics of the retimer device instead of that of the source device. These components typically have small form factors with an example being a 2mm x 1.6mm part. Typical specifications for such a crystal would have a nominal frequency of 25 MHz, would operate in a fundamental mode, and would have a clock accuracy of +/- 300 parts per million (PPM) or better.
The drive for ever smaller and less expensive future solutions will likely see the oscillator integrated or otherwise brought inside the package via a microelectromechanical system (MEMS) device or small crystal. The possible use of retimers in USB-C cables further heightens the need for size-constrained and low power solutions.
To address a wide variety of use models, Kandou is currently shipping USB4 retimers in volume.