The Longer the PCB Trace, the Greater the Signal Distortion. Redrivers Can HelpApril 10, 2020 by Steve Arar
Laptop or desktop PC designers know the woes of USB signal loss down a 12-inch PCB trace. The good news is there's a device purpose-built to strengthen signals the whole way across.
The Diodes Incorporated's PI3EQX10612 is a recently-released two-port linear redriver for strengthening the USB type-C signals that need to run across long PCB traces. In a large system, such as a laptop or desktop PC, we might need to send USB signals down a 12-inch path. The loss introduced by such a long path can significantly distort data and lead to system failure.
The PI3EQX10612 amplifies and conditions the signal to make data transfer over long traces possible. It supports a data rate of 10 Gbps and includes 1:2/2:1 mux/demux functionality. The adaptive power management of the device can maximize battery life in power-sensitive consumer applications.
In addition to its applications in laptops and desktop computers, the PI3EQX10612 finds use in gaming consoles, mobile phones, docking stations, and other consumer devices. In this article, we’ll take a look at the distortion that a long PCB trace causes and how a redriver, such as the PI3EQX10612, can compensate for this distortion.
Loss Mechanisms of a PCB Trace
A signal incurs loss as it travels over a PCB trace. The loss originates from two different mechanisms: the copper resistance and the dielectric loss. Both of these loss components increase as the signal frequency goes up. As shown in the graph below, the conductor loss is greater than the dielectric loss at lower frequencies; however, the dielectric loss becomes the dominant loss mechanism at higher frequencies.
Also, the blue curve in the figure shows the total loss of the trace. As depicted in the following figure, with today’s FR4 PCBs, the total trace loss exhibits a linear dependence on a frequency above about 0.5 GHz.
Graph of conductor loss and dielectric loss. Image (modified) used courtesy of Polarinstruments
Moreover, the loss increases as the trace width is reduced and its length is increased. The figure below shows how losses in a trace change with the trace width, length, and signal frequency.
PCB trace insertion loss compared to signal frequency. Image used courtesy of Texas Instruments
The first graph above shows that high-frequency components of the signal experience a larger attenuation. This low-pass filtering effect limits the bandwidth of the transmitted digital signal and increases the signal rise/fall times as it reaches the far end of the interconnect—a concept I discuss in more detail in my article on the relationship between rise time and bandwidth in digital signals.
The figure below, however, shows that long rise/fall times prevent the received signal from reaching its final values.
Depiction of how insertion loss can degrade signal integrity. Image courtesy of Texas Instruments
Whether or not a given transition will reach its final value depends on the bit pattern. For example, if several consecutive bits have the same value, a longer pulse is transmitted and the received signal can get closer to its final. Besides, the bit pattern affects the initial value of a transition. Therefore, the transitions vary with the transmitted bit pattern. This creates a data-dependent timing jitter often referred to as intersymbol interference (ISI).
The figure above also depicts the eye diagrams of the transmitted and received signals. Note how the distortion manifests itself with a closed eye diagram.
How Does a Redriver Compensate for Distortion?
The block diagram of a basic redriver is shown below.
Block diagram of Intel's differential SerDes redriver. Image used courtesy of Intel
The first block is the continuous-time linear equalizer (CTLE). This is a linear amplifier with a peaking gain. As shown in the following figure, the amplifier exhibits a higher gain (relative to its DC gain) for some high-frequency components. In this way, the CTLE can compensate for the low-pass filtering effect of the long PCB trace.
Example of a CTLE equalizer's amplitude and phase Bode plots. Image used courtesy of Intel
As the example transfer function in the above image suggests, the CTLE might not have a large gain. Therefore, redrivers usually employ a second amplifier to achieve the desired gain. The overall gain of a redriver is usually programmable so the device can be used in different applications. The graph below shows transfer functions that can be achieved by different equalization settings of the TI TUSB1046-DCI.
The DisplayPort EQ settings curves of TI's TUSB1046-DCI. Image courtesy of Texas Instruments
Some datasheets, such as that of the PI3EQX10612, provide the equalization information in a table rather than a plot. The following figure shows the equalization settings of the PI3EQX10612. These settings can be made via either I2C or the corresponding control pins.
Equalization settings of the PI3EQX10612. Image used courtesy of Diodes Incorporated
With the PI3EQX10612, we can choose the flat gain of the equalizer and the output swing as specified by the following table:
Flat gain of the equalizer and output swing of the PI3EQX10612. Image used courtesy of Diodes Incorporated
The compression point setting allows us to specify the linearity of the redriver.
Block Diagram of the PI3EQX10612
The PI3EQX10612 is a two-port linear redriver and includes 1:2/2:1 mux/demux functionality. It supports 100Ω differential common mode logic (CML) signal pathways for receiving and transmitting.
Block diagram of PI3EQX10612. Image (modified) used courtesy of Diodes Incorporated
Redrivers are usually employed in power-sensitive applications and need to incorporate judicious power management systems. The PI3EQX10612 utilizes a low-level input signal detection mechanism similar to that depicted in the block diagram of Intel's differential SerDes redriver.
The output of this level detection circuitry controls the squelch function. When the input remains below a threshold for a certain period of time, the squelch function turns off the output driver to save power and reduce idle bus noise. The squelch function of each channel operates fully independently.
In a large system, such as a laptop or desktop PC, we might need to send USB signals down a 12-inch path. The loss introduced by such a long path can significantly distort data and lead to system failure. In these cases, a redriver, such as the PI3EQX10612, can be used to amplify and condition the signal to make data transfer over long traces possible.
Several parameters of the PI3EQX10612, such as its equalization gain and linearity, are made programmable so that the device can be adjusted for different applications.
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Featured image used courtesy of Diodes Incorporated
If you're a desktop or laptop designer, what other roadblocks do you face with long PCB traces? How do you overcome those obstacles? Share your experiences in the comments below.