By Dr. Eric Bogatin
www.BeTheSignal.com
June 2007
It is often said, “Everything you ever wanted to know about a differential channel is contained in its 4-port differential S-parameters.” While this is perfectly true, it is not enough. In long backplane channels, which are strongly dominated by losses, crosstalk may pose the limitation to receiving the highest bit rate. This effect is not detectable with 4-port S-parameters in the standard configuration.
To incorporate the impact of crosstalk between channels in high-speed serial links, at least 8 ports are needed—12 ports are even better.
With 12 ports, three adjacent differential pairs can be measured simultaneously and fully characterized. The properties of each differential channel and all possible coupling paths between the channels can also be characterized.
Recently, Agilent Technologies announced a new family of multiport Vector Network Analyzers (VNAs), shown in Figure 1.
Figure 1. Multiport VNA From Agilent Technologies
With 12-port S-parameters there are 144 different matrix elements, each with a magnitude and phase at every frequency point, which usually spans 10 MHz to 20 GHz at 10-MHz intervals. This is 2000 frequency points for each of 288 sets of data, or more than 500,000 data points. Even though only 54 percent of the data is unique, this is still a huge amount of data just to characterize three adjacent differential channels in a high-speed serial interconnect.
The new information in 12-port measurements is the coupling between adjacent channels. If the labeling scheme proposed by Agilent Technologies is used, where the differential ports on the left side are the odd numbers and the differential ports on the right side are the even numbers, the through connection is SDD21. The near-end crosstalk between adjacent channels is SDD31, the near end between one channel and the pair two channels away is SDD51.
Ordinarily, the far-end noise in stripline channels, SDD41 and SDD61, is typically at least 10 dB lower than the near-end crosstalk between the same channels. For near-end crosstalk to play a role, the receiver (RX) of the victim line would have to be adjacent to the transmitter (TX) of the aggressor channel, resulting in the worst-case noise at the receiver from the aggressor line (see Figure 2).
Figure 2. Worst-Case Noise at Receiver From Aggressor Line
The acceptable amount of near-end noise depends on the received signal strength. As a rough rule of thumb, crosstalk-limited bandwidth is when the crosstalk is within 10 dB of the received signal.
In a very lossy backplane, leveraging pre-emphasis and equalization, an acceptable received signal could be down by -30 dB. With roughly 10 dB of margin on top of this, the acceptable differential near-end noise is in the order of -40 dB.
Figure 3 shows an example of the measured crosstalk in a typical backplane compared to the transmitted signal. Unless special care is taken, near-end crosstalk can exceed -40 dB, even at frequencies as low as 2 GHz. This may be an additional motivation for using low-loss backplane materials, which push the 30-dB interconnect bandwidth to higher frequencies.
Figure 3. Measured Crosstalk in a Typical Backplane
In this example, the bandwidth for a 10-dB crosstalk margin is about 4 GHz, which corresponds to signals in the order of 8 Gbps.
The first step in minimizing crosstalk is identifying its root cause. You can identify the source of the excessive crosstalk using the time domain view of the SDD31 response. The source of the crosstalk usually comes from tightly routed differential pairs in the backplane and from the connector and its via field.
The coupling between signal paths in a via field is more about the return current distributions, in which case six or more channels may interact. This suggests that 12 ports are not the limit. How many ports do we need to fully characterize high bit-rate backplane channels? In the words of Frank Schonig, expert interconnect design engineer, "A lot is good, more is better, and too much is just right."
This and other signal integrity topics are covered in Eric’s public classes and online lectures, available from his website, www.BeTheSignal.com. Send your signal integrity technical questions to DoctorIsIn@BeTheSignal.com.
Bio: Eric is president of Bogatin Enterprises, whose mission is to set the standard for signal integrity training. He is the author of Signal Integrity - Simplified, published by Prentice Hall. Check out his public signal integrity classes posted on www.BeTheSignal.com. He can be reached at eric@BeTheSignal.com.
