As data rates exceed the 1Gbps level, designers must address new issues in the design of their backplane systems. The signal integrity of these backplates is affected by skin effect, dielectric loss, greater noise caused by crosstalk, ISI, and other factors. The skin effect is the phenomenon that, with an increase in frequency, most of the current will be concentrated on the outer conductor.
The loss due to skin effect is proportional to the square root of the frequency, the width, and the height of the line.
The dielectric loss is caused by the heat loss of plate dielectric and increases linearly with frequency. At higher frequencies, dielectric loss becomes a more serious problem. These losses not only reduce the amplitude of the signal but also slow down the edge velocity of the signal, resulting in poor signal divergence and shaking tolerance.
Because the low-frequency components, which are less attenuated, are added to the high-frequency components, which are more attenuated, on the receiver, signal divergence will result in intersymbol interference. As a result, the aperture of the eye chart is smaller, making it more difficult to recover at the receiving end, resulting in an unacceptable bit error rate.
This limits the maximum bit rate. Another way of explaining this is that the signal “gets dirty” or diverges, causing the energy to drop bit by bit, which in turn creates an error code.
At lower rates, ISI can be corrected because there is a sufficient timing margin. But at higher rates, ISI is no longer limited to the signal boundary but can affect the whole bit width.
The main source of noise is crosstalk caused by high-density connectors and backplane wiring.
Crosstalk is a major source of noise caused by high-density connectors and backplane layout.
There are two types of crosstalk: proximal crosstalk (NEXT) and distal crosstalk (FEXT). A signal from a transmitter close to the victim receiver interferes with the received signal, causing NEXT.
FEXT is caused when the received signal is interfered with by a “remote transmitter” connected to the victim receiver.
All of these channel damage can be compensated for or eliminated by special signal adjustment circuits such as preweighting and equalization in backplane interconnect devices. These circuits compensate for the signal loss by attenuating low-frequency components and amplifying high-frequency components.
The key role of backplane interface devices is to solve the channel damage problems such as loss and crosstalk and thus extend the service life of the backplane. The interface transmitter has amplitude control and a pre-weighted signal adjustment circuit.
Similarly, the backplane interface receiver USES equalization technology to control losses. In addition, these devices are required to have testable characteristics such as JTAG and BIST so that system-level testing can be performed at the time of manufacture. National Semiconductor’s Four-way 5Gbps SerDes meets all of these requirements. The following is a detailed description of the signal integrity technology used by the four-channel 5Gbps backplane transceiver SCAN50C400 pair and other high-speed backplane interface devices.
Preweighting and de-weighting: This technique distorts a signal before it is sent so that the quality of the signal on the receiver is the same as that of the original transmission. When the signal remains at the DC level for more than one bit, preweighting raises the high-frequency component and lowers the low-frequency component. In designing these methods, the system designer must carefully control the output amplitude to limit the output power.
Receiving equalization: Receiving equalization compensates for channel loss characteristics by applying relative frequency characteristics to input data. There are two types of equalization circuits: stationary and adaptive. Fixed equalizers set compensation features manually, while adaptive equalizers use adaptive algorithms to set the best compensation features, enabling users to apply one device to a variety of different channels. It can also automatically compensate for the variation of channel characteristics caused by manufacturing deviation and environmental change.
The receiver equalization function can be integrated into the backplane interface device or performed in a stand-alone device. The advantage of performing receiver equalization in a stand-alone device is that it provides optimal wiring advantages and design flexibility.
Crosstalk elimination: In addition to preweighting and receiving equalization, crosstalk elimination is also used in some systems. These chips use a noise-canceling mechanism that samples noise from adjacent channels and subtracts it from the signal.
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