The latest versions of USB offer up to 2.5 Gb/s if the host is in close proximity to the peripheral. In applications that use USB over long distances, designers must find some way to counteract signal degradation to maintain USB-specified data rates.
While equalization, emphasis, and DC gain techniques can also be employed, designers can achieve greater success and reduce time-to-market with USB redrivers. Redrivers are integrated devices that include all the electronics needed to address signal attenuation.
This article first describes the operation of the redriver, then introduces some example devices and explains how they can be used.
USB can stretch distances, but at a cost
The USB specification was developed with the assumption that connections would only be made between devices within a few meters, such as between a computer and an external hard drive. The USB 3.0 specification states that cable lengths should be limited to less than 3 meters to maintain signal integrity. But the success of USB technology is that it can now be used in applications where longer cables are practically necessary. Examples include connecting servers to display panels installed in large stores.
Unfortunately, longer cables combined with high frequency signals common to Hi-Speed USB versions create signal integrity challenges such as channel insertion loss, crosstalk, intersymbol interference (ISI), and consequent reduced throughput .
There are several techniques that USB system designers can employ to overcome signal degradation. For example, equalization and emphasis can be used to limit the effects of channel insertion loss and ISI. Increasing the DC gain helps overcome losses due to crosstalk.
However, designing signal conditioning circuits adds complexity to a USB system and makes the challenge even more challenging because USB technology uses separate signal pairs for transmission and reception, doubling the circuitry required. The advent of USB redrivers has been a boon for designers.
Causes of Signal Attenuation
The signal attenuation problems that Hi-Speed USB has to overcome are not unique to the technology; designers of all high-speed communication link products are familiar with such problems. They’re not unique to long cable USB installations either, but since there’s less signal attenuation in short cables, the problem isn’t as noticeable.
Signal attenuation in high-speed communication systems is mainly due to a combination of insertion loss, crosstalk, and ISI.
Insertion loss is the result of signal power attenuation caused by the cable. Loss is proportional to cable length. Crosstalk is the capacitive, inductive, or conductive “coupling” of adjacent signal carriers, which reduces the integrity of the signals in both. ISI occurs when a symbol (a discrete signal that carries data and repeats according to the carrier frequency) interferes with the previous symbol, adding noise and distortion. ISI is proportional to the carrier frequency (since the time interval between signals decreases with increasing frequency) and cable length (since the signal-to-noise ratio (SNR) decreases in longer cables). Noise is the part of a signal that does not carry useful information.
Hi-Speed USB systems will also include a certain amount of deterministic and random jitter, which can be understood as small deviations from the signal’s nominal periodicity, which can compromise signal integrity. The higher the system communication frequency, the greater the impact of jitter.
Overcome signal attenuation
Some signal attenuation is inevitable in high-speed communication systems, but it only becomes a problem when the SNR becomes so poor that some of the data sent cannot be decoded at the receiver. This can result in compromised throughput and, in extreme cases, communication failures.
Engineers have developed four techniques to improve SNR (or implement “signal conditioning”) to increase the throughput of high-speed communication systems:
Emphasis/De-emphasis Amplifies the transmit frequencies most likely to be affected by noise and then de-emphasizes them at the receiver to reconstruct the original signal.
Equalization uses filtering to ensure that the frequency characteristics of the received and transmitted signals match, effectively maintaining a flat frequency response over the entire length of the cable.
The DC gain compensates for the linear attenuation of a given length of cable.
Output swing control configures the USB differential voltage to ensure it meets the 0.8 to 1.2 volt specification.
Optimizing communications for a particular configuration requires extensive testing to determine the amount of equalization, emphasis, DC gain, and output swing control required for a range of operating conditions. This information can then be used to adaptively change each parameter during operation to maintain the ideal signal. However, it is not practical to perform adaptive signal conditioning on all systems, not just the most critical communication systems.
Passive signal conditioning, where a single setup meets all operating conditions, can indeed achieve reasonable results at a much lower cost. The downside is that it doesn’t always ensure optimal conditions. Designers can ensure consumer satisfaction by supplying cables of specific lengths whose designs have been tested for use, or by specifying maximum cable lengths.
Signal conditioning is required from the USB host (microprocessor) to the redriver channel, and from the redriver to the peripheral channel (via connectors and cables). Typically, each side requires different signal conditioning parameters.
Redesigning the redrive
A USB redriver is a convenient and relatively low-cost way to implement transparent (without affecting data transfer) signal conditioning to the USB channel. Products such as Diodes Incorporated’s PI3EQX1001XUAEX, a 10 Gb/s, 1-channel USB 3.1 linear redriver, restore Hi-Speed USB signals to their original state before being received by the endpoint device (Figure 1).
Figure 1: A USB redriver, such as the PI3EQX1001XUAEX from Diodes Incorporated, is a convenient way to restore signal integrity over long cables. (Image credit: Diodes Incorporated)
Since the redriver allows various configuration parameters, the chip can be mounted on the host USB printed circuit board, as close as possible to the connector, or on the far end of the cable, close to the connector of a peripheral or endpoint device (as shown in Figure 1). Show). However, most applications use redrivers on the host USB end of the cable.
Circuit board traces should be designed in accordance with best practice guidelines for high-speed signal design. For example, the traces should be matched, impedance-controlled differential pairs. Routing should avoid vias and sharp turns (kept at 135° or greater), and traces should be referenced to a solid ground plane without cuts and bifurcations to prevent impedance discontinuities (Figure 2).
Figure 2: The traces connecting the USB host to the redriver and connector should use high-speed signal design best practices. For example, turns should be limited to 1350 to limit interference. (Image credit: Texas Instruments)
Once the printed circuit board and components are assembled, the developer can configure the signal conditioning parameters to meet the specific characteristics of a specific channel.
The PTN36043BXY USB 3.0 redriver from NXP Semiconductors is an example of a modern product. The chip is a compact, low power, dual differential channel product using a 2-to-1 active switch with an integrated USB 3.0 redriver. The switch steers the two differential signals to one of two positions and is designed to minimize crosstalk (Figure 3).
Figure 3: NXP Semiconductors’ USB 3.0 redriver integrates emphasis, equalization, DC gain, and output swing control. Because the cable characteristics are different in different directions, the transmission line and the receiver line need to be controlled separately. This redriver is used in conjunction with a USB Type-C connector, so it has two transmit and receive twisted pairs on the connector side. (Image credit: NXP Semiconductors)
The NXP USB 3.0 redriver allows developers to adjust emphasis/de-emphasis, equalization and output swing per channel (USB host to redriver and redriver to peripheral). In addition, the device compensates for cable attenuation by increasing the DC gain.
Each channel is connected to two control pins, allowing the designer to select signal conditioning parameters for a given setup. For the TX/RX lines on each channel, developers can choose from nine signal conditioning combinations (table)
Table: When using an NXP redriver, developers can choose from 9 signal conditioning parameters for the TX/RX lines on the USB host to redriver channel. Similar options are available for redrivers to peripheral channels. (Table source: NXP Semiconductors)
Evaluate redriver designs
The prototype needs to be evaluated under a range of operating conditions to determine the best options for emphasis, equalization, DC gain, and output swing control. The designer’s task is made easier by the availability of evaluation kits.
For example, Texas Instruments offers the USB-REDRIVER-EVM USB 3.0 Redriver Evaluation Module (EVM). This module is based on the company’s TUSB501DRFR USB 3.0 3.3 V single-channel redriver.
When the USB system is active, the TUSB501 periodically performs receiver detection on the TX pair. If it detects a SuperSpeed USB receiver, the RX termination becomes enabled and the TUSB501 is ready to redriver.
The chip employs a receiver equalizer with three gain settings (3, 6 and 9 dB) controlled by pin “EQ”. The chip also supports de-emphasis and output swing on pins “DE” and “OS”. The de-emphasis value depends on the output swing selection. When the output swing is set to “low”, the de-emphasis can be set between 0 and -6.2 dB. When set to High, EM supports de-emphasis between -2.6 and -8.3 dB.
The EVM acts as a USB adapter and includes two TUSB501 redrivers (plus a USB 2.0 redriver). The adapter is powered from the USB host VBUS pin and passes the supply voltage to downstream ports to power peripherals.
One TUSB501 redriver on the EM boosts the host TX line performance, while the other controls the RX line. The default configuration values for equalization and de-emphasis are typically the same as the transmit and receive configuration values for USB 3.0 systems with 3 to 5 meter long cables and 20 to 25 cm circuit board traces. DC gain is implemented by choosing the appropriate resistors.
The EVM allows developers to test how changes to redriver configuration parameters affect the signal integrity of the TX and RX pairs of a Hi-Speed USB system. The EVM can also be used as a reference design, modified for any intended application. It comes with a USB Type-A plug and socket.
Test the system using a USB redrive connection
When testing a physical system, it’s important to remember that redrivers modify the USB signal, causing system jitter. This jitter should be measured to check its effect on the signal conditioning settings.
TI recommends using a test system with a 3-meter cable and a host USB printed circuit board with 24-inch traces and placing the redriver 4 inches from the connector. At the far end of the cable, peripherals are represented by a printed circuit board with traces ranging from 1 to 6 inches in size (Figure 5).
Figure 5: Hi-Speed USB jitter test setup using the TUSB501 redriver. This setup replicates an application such as a PC connected to a peripheral flash drive using a 3m cable. (Image credit: Texas Instruments)
An ideal design would exhibit zero jitter, ensuring that compensation such as de-emphasis is fully applied immediately after a high-to-low/low-to-high transition. Since this is not practical, TI recommends that the design limit the jitter so that full compensation is applied within 200 picoseconds (ps) of the transition (Figure 6).
Figure 6: Jitter in Hi-Speed USB systems using redrivers should be limited so that full compensation is applied within 200 ps of signal transitions. (Image credit: Texas Instruments)
The original form of USB 3.0 was suitable for cables with a maximum length of 2 meters, but many applications today use longer cables. Because the technology requires the use of high-frequency signals, extending the cable length beyond 3 meters can introduce signal integrity issues that can affect throughput. Low-cost and compact USB 3.0 redrivers provide a relatively simple solution, enabling developers to increase the equalization, emphasis, and DC gain to improve high-speed USB signal quality.
As mentioned above, silicon suppliers now offer off-the-shelf EVMs based on redrivers, making it easy to try out target devices in proposed applications. The datasheet includes component and printed circuit board layout information, allowing the EVM to be used as a reference design for the final product.