标签: N2890A

I didn't have any X1 probes around, so put a 100-pf capacitor on the node to simulate a really crappy probe. Rise time spiked to 5.5 nsec, more than a five times increase, and the signal was delayed by almost a nsec. I suggest immediately combing your lab for X1 probes and donating them to Goodwill. And be very wary of ad hoc connections—like clip leads and soldered-in wires—whose properties you haven't profiled.   But 100 pf is a really crummy probe. I soldered a 30 pf cap on the node to simulate one that's somewhat like an ad hoc connection or a moderately-cheap probe. In Figure 4 , the orange trace is the gate's output with no load—just the 21X probe. The green is with the additional 30 pf. The distortion is significant. So a 30-pf probe grossly reshapes the node's signal. What effects could that cause? First, everything this signal goes to will see a corrupt input. If it goes to a flip flop's clock input the altered rise time could cause data to be incorrectly latched. Or, if the flop's data input(s) are changing at roughly the same time, the flop's output could become metastable—it'll oscillate for a short time and then settle to a random value. If it goes to a processor's non-maskable interrupt input the leading-edge bounce could cause the CPU to execute two or more interrupts rather than one. (Generally this is not a problem for normal maskable interrupts since the first one disables any others). But wait, there's more. Note that the signal extends from well below ground (about -600 mV) to 3.7V (be sure to factor in the attenuation of the 21X probe), which is much higher than the 2.5V Vcc. Depending on the logic family this signal goes to, those values could exceed the absolute maximum ratings. It's possible the driven device will go into SCR latchup, where it internally tries to connect power to ground, destroying the device. I have seen this happen: the chips explode. Really. It's cool.     So far I haven't shown any signals acquired by the N2890A. The yellow trace in Figure 5 , is the gate's output using that probe. It's pretty ugly! The distortion is entirely in the probe, and not on the board, so does not represent the signal's true shape. In this case the probe is grounded using the normal 3-inch clip lead. Using the formula from last month, that loop has 61 nH of inductance. In orange the same signal is displayed, but in this case I removed the probe's grabber and connected a very short, about 5 mm, ground wire to the metal band that encircles the tip. The signal is still not displayed correctly—it extends below ground and has a total magnitude of about four volts, much more than the 2.5 Vcc. But the better grounding did clean up the shape. The point is that poor grounding can cause the scope to display waveforms that don't reflect the node's real state. Electronics matters Many in the digital world find themselves divorced from electronics. We think in ones and zeroes, simple ideas that brook little subtlety. A one is a one, a zero a zero, and in between is a no-man's land as imponderable as the "space" that separates universes in the multi-verse. But electronics remains hugely important to digital people. Ignore it at your peril. Power supplies have crawled below a volt so the margin between a one and a zero is ever-tighter. On some parts the power supply must be held ±0.06V or the vendor makes no promises about correct operation. On a 74AUC08, typical fast logic, at 0.8 Vcc there's only a quarter volt between a high and a low. Improper probing can easily skew the node's behaviour by that much. And, as we've seen, capacitance and inductance are so vital to digital engineering that we dare not ignore their effects when troubleshooting. Reactance, impedance, and electromagnetics are big subjects that I've only lightly touched on. They're pretty interesting, too! I highly recommend the book High-Speed Digital Design for a deep and dirty look at working with high-speed systems. 1 The ARRL Handbook from the American Radio Relay League is possibly the best introduction to electronics available. 2 It doesn't skimp on the math, but never goes beyond complex numbers. The focus is decidedly on radios, since this is the bible of ham radio, but the basics of electronics are covered here better than any other book I've found. There's a new edition every year; my dad bought me a copy in 1966, and since then I've "upgraded" every decade or so. Endnotes 1. Johnson, Howard and Martin Graham. High-Speed Digital Design ,1993 PTR Prentice-Hall Inc, Englewood Cliffs, NJ. 2. The ARRL Handbook , American Radio Relay League. Published afresh every year. www.arrl.org .