Peace at the plugfest
Ransom Stephens- April 24, 2012
Two engineers walk into a plugfest. Each one is convinced that they’re bringing definitive bad news: proof that an important principle of the specification won’t work. And not only won’t work, but can’t work.
The task at hand is data transmission at 25 Gb/s on backplanes.
Of course, being engineers, they’re both a bit smug. After all, they’ve got a secret to tell. Bad news, good news, whatever, the answers are here and they’re definitive.
The first engineer, we’ll call her Marge, breaks out her finest backplane. In her mind, it’s state-of-the-art, cutting/bleeding/name-your-own-cliché edge technology.
Being a good news first sort of person, she connects the backplane to the pattern generator of a BERT (bit error ratio tester), dials in the appropriate settings, connects the output to the error detector and presses play.
And it’s beautiful: 25 Gb/s running error free for a full minute before the error detector rings up a foul – a BER better than 1E-12. Nice.
Polite applause from the five guys paying attention.
She says, “It works, but there’s a problem.” The word problem attracts more people. She connects the pattern generator to a scope and projects the eye diagram onto the big screen.
The other of the two engineers, we’ll call her Jackie, makes the sort of sound you’d expect from an opossum as an F-150 crosses its tail.
Confusing Jackie’s response for misunderstanding, a guy leans over and says, “It’s supposed to look like that. It’s PAM-4, not NRZ.” Even though Jackie is ignoring him, the guy keeps talking: “PAM-4 encodes two bits in each symbol. See how it’s multilevel? That’s why the eye looks like it’s winking.” He winks at her as he says this. “On the other hand, NRZ is like the baseband of a digital signal, the symbols are the bits. I don’t know why they call it Non Return to Zero.”
Jackie looks at him, radiating the disdain she usually reserves for software jocks who ask for soldering tips. She says, “Maybe it’s because the signal doesn’t return to zero in each bit period.”
Marge makes a quick adjustment to the pattern generator, presses play, and a horribly messy eye diagram appears on the screen. “This is NRZ,” she switches to the far more intelligible multilevel diagram, “this is PAM-4. Even with advanced backplane technology like ours, 25 Gb/s NRZ signals can’t propagate more than a few inches.” She pauses to let it sink in. “I’m afraid we’ll have to change the spec to require PAM-4.”
Jackie whips a backplane out of her brief case. It’s red, and the copper traces look like gilding. Without a word, she connects one end to a pattern generator and the other end to a scope. She presses play, waits a minute, and projects the image onto yet another screen (this is the legendary plugfest that has ample equipment for everyone, and it all works). The NRZ eye diagram, while hardly pristine, shows an elliptical opening of nearly 20 ps by 130 mV. She saves the image, fiddles with the pattern generator and announces, “This is PAM-4.” A murky blob appropriate for T-shirts sold at reggae concerts begins to form. “NRZ is just fine, it’s PAM-4 that won’t work.”
Marge takes a step toward Jackie. Jackie takes a step toward Marge. Some idiot makes a meowing sound and everyone glares at him.
A tall thin man sporting a fedora steps between them. “Hmm, I think I understand what is happening.” He sets his hat on an oscilloscope. The lights reflect from his skull. Jackie steps forward. Marge covers her eyes.
The man says, “It’s the bandwidth on one hand and the spectral response on the other.” He points to Marge’s results. “Your backplane has insufficient bandwidth to support an NRZ signal, but that very filtering prevents the intersymbol interference from destroying your PAM-4 signal.”
He swivels around to face Jackie and says, “Your backplane has ample bandwidth for NRZ signals but the spectral response is too severe for the PAM-4 signal. You see,” He says, now raising his hat as if he were Gene Kelly about to find a dark rainy street appropriate for dancing. “There is room for everyone in our specification.”
(With due thanks and appropriate apologies to Pavel Zivny, Domain Expert – Serial Data, Tektronix, who told me a barely related story from which I concocted this offense to literature.)