The Fast-Links Project: Recent Results

Recent Fast-Links Results

This page describes some key results from our testing of the first test chip. Many of the results will also be described in our paper at Hot Interconnects '97.

Equalizing transmitter

A series of experiments were performed to demonstrate the effectiveness of equalization in overcoming the equalization of the transmission line. The measurements shown here were all performed with the transmitter driving a short PC-board transmission line, 1 meter of 30 gauge twisted pair, another short PC-board line, and a resistive terminator. This first pair of waveforms result from isolated 1 and 0 bits superimposed on each other to create an ``eye'' diagram. Without the equalizer, there is no eye at all: the level of an isolated 1 bit is below that of the isolated zero bit. With equalization, the overall level signal level is reduced, but the data eye is clearly present.
isolated 0/1, EQ off isolated 0/1, EQ on
Equalization Off Equalization On
This next pair of waveforms show a pseudorandom bit pattern, again with equalization both off and on. Without equalization, there is no data eye at all. Equalization recovers a recognizable data eye, but attenuation in this 1m line is high enough that this probably represents the limit of the technique at its present state of development.
LFSR, EQ off LFSR, EQ on
Equalization Off Equalization On
For these experiments we set all of the weights (except the first) in the filters to the same value; with only observation of eye patterns, there was no improvement in the pseudo-random sequence eye patterns with further adjustment of weights for non-adjacent bits. We speculate that a 2-tap transition filter may be sufficient to obtain most of the benefits of this equalization technique, allowing much simpler transmitter circuitry than we used in the test chip.

Transmitter Problems

In the isolated-bit waveforms of the previous figures, various effects that arise from transmitter imperfections and package parasitics can be seen. The ``ripple'' in an otherwise quiet series of 0s, for example, is due to a slight mis-adjustment of On and Off clock overlap. Some overshoot is evident following an isolated bit (this overshoot contributes much of the amplitude noise in the pseudo-random bit patterns); this is caused by the LC tank circuits formed by the transmitter's output capacitance and bond-wire inductance, as confirmed by SPICE simulations. This effect is probably unavoidable in conventional packaging, but can be overcome almost entirely by terminating the transmitter as well as the receiver. Our future signaling chips will have both ends of the transmission line terminated. A second phenomenon that contributes significant noise is reflection off the capacitance at the receiver. These reflections are reflected again at the (high-impedance) transmitter, causing intersymbol interference at the receiver. Simulations suggest that on-chip transmitter termination will very effectively remove these reflections. Two design ``features'' of the transmitter also contributed to signaling problems. First, the transmitter has a clocked (On-clock) current tail transistor (this circuit structure was the only one we could find at the time that would deliver the necessary switching speed). When the Off-clock turns on, well in advance of the time when the DAC cell is supposed to transmit its data bit, charge is injected into the output from the common source node. In our signaling system, this effects introduces an unintentional signal onto the line at about 15% of the amplitude of the actual signal. Second, and more seriously, the gating function within the filter changes the strength by completely turning off one or two of the three drivers in a DAC. The result is that, when equalization is turned on, the output signal is not truly differential. This has the unfortunate effect of sending a common-mode signal down the line, where various non-linearities transform the common-mode signal into a differential (noise) signal. A new transmitter design will take care of both of these problems.

Tracking Receiver

Because of the previously described problems with package-parasitic-induced noise, we were unable to test the receiver at 4Gb/s. We were, however, able to fully test its ability to acquire the framing sequence and to operate the LFSR checker at 2.5Gb/s. Because of various problems with the data transmitter, described above, bit-error rates were unacceptably high (about 10-4). Simulation suggests that the BER can be brought to acceptable levels with a redesigned transmitter.

About the figures on this page

The waveforms shown on this page were captured with a Tektronix 11801A sampling oscilloscope. For most of these measurements, the 'scope was set in its infinite-persistence color-grading mode; pixels representing more common data points are assigned "hotter" colors with the most common points colored yellow. The oscilloscope screen was stored in a PPM file using a serial connection and software written for the purpose. For display on the web, the screen images have been reduced to half of their original size and converted to JPEG format.
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Last modified: Wed Jul 15 10:52:13 EDT 1998 by Steve Tell.