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Probing pointers, Take 2

March 25, 2012

Jack Ganssle-March 25, 2012

Jack tests several probes to see how different probes change the results.

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Last month I gave a mostly theoretical overview of the effect probes--like scope and logic analyzer probes--have on the nodes being tested. The most important effects stem from the capacitance of the probe tip. To reiterate, the reactance, or resistance to AC, at the tip is:



This reactance loads the node and can alter a device's operation--or worse.

To explore this, I built a circuit on a printed circuit board with ground and power planes, keeping all wires very short. A 50-MHz oscillator drives two AND gates. The 74AUC08 is spec'd with a propagation delay between 0.2 and 1.6 nsec at the 2.5 volts I used for the experiment. The second gate is a slower 74LVC08 whose propagation delay is 0.7 to 4.4 nsec. Still speedy, but slower than the first gate. I was not able to find rise-time specifications but assumed the faster AUC would switch with more alacrity and thought it would be interesting to compare effects with differing rise times. Alas, it was not to be; the LVC wasn't much slower than the AUC. So I'll generally report on the slower gate's results.

These parts are in miniscule SOT-23 packages, which keeps inductances very low but means one solders under a microscope, sans coffee.

I wanted to see the effect that probes have on nodes, but that posed a meta-problem: if probing causes distortion, how can one see the undistorted signal? Thankfully there's a simple solution. I made a pair of meter-long probes from RG-58/U coax cable. A BNC connector on one end goes to the scope. A short bit of braid is exposed and soldered to the ground plane very close to the node being probed, and a ¼-watt 1K resistor goes from the inner conductor to the node. I used an Agilent MSO-X-3054A scope with selectable input impedance, set to 50 ohms. This is critical for the shop-made probe; the normal 1 MΩ simply will not work. If your scope doesn't have a 50-Ω mode, use a series attenuator such as the 120082 from Test Products International (this part doesn't seem to be on their web page, but Digikey resells them). Agilent's N5442A is a more expensive but better-quality alternative.

RG/58U is 50-Ω cable; add the resistor and the total is 1,050 ohms. The scope's 50-Ω input forms a 21:1 divider, but the resistor's very low capacitance (remember, a ¼-watt resistor runs only 0.5 pf) means the probe's tip looks extremely resistive, with little reactance. The scope thinks a 1X probe is installed, so to accommodate the oddball 21:1 ratio one multiplies the displayed readings by 21.



Figure 1
Click on image to enlarge.
The first experiment showed Fourier at work. The blue trace in Figure 1 shows the output of the fastest gate using a 21X probe. Note that it's far from perfect since the circuit had its own reactive properties. The rise time (measured with a faster sweep rate than shown) is about 690 psec (picoseconds). "About" is the operative word, as the scope has a 500-MHz bandwidth (though samples at 4 GS/sec). I found that having the instrument average readings over 128 samples gave very consistent results.

The pink trace is the Fourier Transform of the gate's output. Unlike the blue trace, this one is not in the time domain (e.g., time across the horizontal axis) but is in the frequency domain. From left to right spans 2 GHz, with 500 MHz at the center. The vertical axis is dBm, so is a log scale. Each peak corresponds to a term in the Fourier series. Point "A" is exactly 50 MHz, the frequency of the oscillator. Most of the energy is concentrated there. Peak "B" is 48 dBm down from "A." That's on the order of 100,000 times lower than "A."

"B" is at 900 MHz. Remembering that little energy remains in frequencies above with F=900 MHz the rise time is 555 psec, close enough to the 690 measured. The same experiment using the slower 74LVC08 gate yielded 48 dBm down at 450 MHz, or a rise time of 1.1 nsec. That's close to the 0.95 nsec reported by the scope.

Next, I connected a decent-quality $200 Agilent N2890A 500-MHz probe (11-pf tip capacitance) on the 74LVC08's output. The 21X probe saw an additional third of a nanosecond in rise time due to the N2890A's capacitance. In other words, connect a probe and the circuit's behavior changes.



Figure 2
Click on image to enlarge.
In Figure 2 the orange trace is the gate's output measured, as usual, with the 21X probe, although now there's 10 inches of wire dangling from it. That trace is stored as a reference, and the green one is the same point, with the same probe, but the N2890A is connected to the end of that 10 inches of wire. Note that the waveform has changed--even though that other probe is almost a foot away--and the signal is slightly delayed. This is probably not going to cause much trouble.



Figure 3
Click on image to enlarge.
Gates typically have a very low output impedance, so it's unsurprising there's so little effect. Often, though, we're sensing signals that go to more than one place. For instance, the "read" control line probably goes from the CPU to quite a few spots on the board. To explore this situation, I put the 21X probe five inches down that wire, captured the waveform into the reference (orange in Figure 3), and then connected the same N2890A at the end of the 10 inches of wire. The signal (green) at the 5-inch point shifted right and was distorted.

Consider the clock signal: On a typical board, it runs all over the place. The impedance at the driver is very low, but the long PCB track will have a varying reactance. Probe it and the distortion can be enough to cause the system to fail.

The ringing is caused by an impedance mismatch. The N2890A has changed the node's impedance, so it no longer matches that of the driver. Part of the signal is reflected back to the driver, and this reflection is the bounciness on the top and bottom of the pulses.

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.



Figure 4
Click on image to enlarge.
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 3.7 volts (be sure to factor in the attenuation of the 21X probe), which is much higher than the 2.5-volt 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.



Figure 5
Click on image to enlarge.
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 multiverse.

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 ±0.06 volts 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 behavior 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.

Jack Ganssle (jack@ganssle.com) is a lecturer and consultant specializing in embedded systems development. He has been a columnist with Embedded Systems Design and Embedded.com for over 20 years. For more information on Jack, click here.

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.


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This material was first printed in April 2012 Embedded Systems Design magazine.
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