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The frequency may be quite high. That's where the second part of a probe's spec, the tip capacitance, comes in. This ranges from under a pF to pF or more. And that has a huge effect on the probe's impedance. Tektronix is one of the few vendors that gives graphs of probe impedance. At MHz the impedance appears to be around ohms. The probe's capacitance is 4 pF; running the reactance numbers we get ohms at that frequency, pretty darn close. Cheap probes may be considerably worse. That's 93 ohms at MHz.

In a 5 volt circuit the probe will add a 54 ma load. Surfing the net one finds lots of cheapies at pF or worse, which nets a terrifying 20 ohms at MHz. To put that into perspective, in a 2 volt circuit it would take ma to drive the probe alone, and few gates can do that. Even at 10 MHz we're talking 10 ma. Other parts, notably processors, may be capable of driving less current.

Let's do a little more analysis. When connected to a MHz signal it has ohms of impedance. Looking at the 74AUC08 gate specification one finds that with a 2. In other words, connect one of these puppies to your board and the circuit may very well stop working. I think some of those homeless people I see in Baltimore are engineers who cracked when they found their troubleshooting tools made troubleshooting impossible.

Agilent has a really nifty 33 GHz scope. Thankfully the company sells much better probes. Think impedance, which is related to tip capacitance. A side note for those working with analog. Generally analog signals aren't as fast as digital so the probe-loading issues may be less severe.


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On the other hand, many analog nodes have a much higher impedance than digital. Better probes exist. Tek's 4 GHz P has a tip capacitance of only 0. Which explains the sign held aloft by one gaunt street person last week: "Please help. Will work for a pair of Ps. Logic analyzer probes also exhibit capacitive characteristics.

Agilent, for instance, sells sets that range from 0. The probe sets for their MSOs run 8 to 12 pF. Cheapies advertised on the Internet have higher capacitances, and an astonishing number don't have a rating at all. Never connect a logic analyzer to a circuit unless you've thought through the probe impedance issues. That seminal work has tormented generations of electrical engineering students and no doubt others. Discontinuous functions - like square waves - are very resistant to mathematical analysis with calculus unless one does horrible things like use unit step functions.

But Fourier showed that one can represent many of these periodic functions as the sum of sine waves of different amplitudes and frequencies. The Fourier Series for a square wave is:. The series goes on forever, so all square waves have frequency components going to infinity.

Special Report: PAM4, PCIe, jitter limits move the needle in high-speed digital

However, the amplitude of these decrease rapidly due to the division by an ever-larger odd number. The point, though, is that the "frequency" of a square wave is composed of many frequencies higher than that of the baseband. Pulses, like the ones that race around every digital board, the ones we probe with our scopes and logic analyzers, are square-wave-ish. Pulses are also, happily, imperfect. Fourier's analysis assumed that the signal transitions between zero and one instantaneously.

In the real world every pulse has a finite rise and fall time. If Tr is the rise time, then frequency components above F in the following formula will be so far down they're not important:. This does mean that, assuming a 1 nsec rise time, even if your clock is ticking along at a leisurely rate about the same as a Florida old-timer's speedometer, the signals have significant frequencies up to MHz.

Long troubleshooting sessions often see a board covered with connections to test equipment, data loggers, etc. Long lengths of wire-wrap wire get soldered between a node and an instrument. These connections all change the AC properties of the nodes by adding inductance and capacitance. Here are some useful formulas with which one can estimate the effects. These are derived from reference 1, and there's much more useful data in that book. Most of us use multilayer PCBs that have one or more ground and power planes. Solder a wire to a node and drape it across the board as it runs to a scope or other instrument, and you'll add capacitance.

If d is the diameter of the wire in inches, h is the height above the PCB, and l is the length of the wire, then the capacitance in pF is:. AWG 30 wire-wrap wire is 0. Typical hook-up wires are AWG 20 0. The inductance of a round loop of wire for example a scope probe's ground lead in nH is, if d is the diameter of the wire and x is the diameter of the loop:. While Missouri may be the "show me" state, most engineers also want to see real-world demonstrations to prove a point.

So I built a circuit on a PCB with ground and power planes, keeping all wires very short. The 74AUC08 is spec'd with a propagation delay between 0. The second gate is a slower 74LVC08 whose propagation delay is 0. 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 was be interesting to compare effects with differing rise times. So I'll generally report on the slower gate's results.

These parts are in miniscule SOT 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 derived from reference 1. A BNC connector on one end goes to the scope. Two views follow:. Agilent's NA is a more expensive but better- quality alternative.

The scope thinks a 1X probe is installed, so to accommodate the oddball ratio one multiplies the displayed readings by Look at the phenomenal behavior of this probe the red curve :. The first experiment showed Fourier at work. The blue trace in following figure 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 psec picoseconds.

I found that having the instrument average readings over 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.

From left to right spans 2 GHz, with 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 close to the 0. The 21X probe saw an additional third of a nanosecond in rise time due to the NA's capacitance.

In other words, connect a probe and the circuit's behavior changes. In the next figure the orange trace is the gate's output measured, as usual, with the 21X probe, although now there's 10" 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 NA is connected to the end of that 10" 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. 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.

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Book Review, Pulse Generator, and Probing

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 the following figure , and then connected the same NA at the end of the 10" of wire. The signal green at the 5" point shifted right and was distorted as follows:. 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.

Building Your Own Oscilloscope Probes

The NA has changed the node's impedance, so it no long 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 pf capacitor on the node to simulate a really crappy probe. Rise time spiked to 5. 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 pf is a really crummy probe. I am using a single resistor in this version. Trim the braid. Remember that this must fit into the pen tip, so you need to be careful in the above step of connecting the resistor R2 and this step of trimming the braid. Again, I am using just a single resistor as R2. Twist the center conductor around the R1-R2 junction, and solder this new connection.

Wind the far lead of the R1 resistor around the nail shaft near its head, and solder that junction. I used the pen tip as a guide to cut the lengths of the center conductor and resistor leads to appropriate lengths. The secton picture below shows the pen tip slid onto this assembly, but I simply held it next to the end of the cable to gauge appropriate lengths. Mix the two-part epoxy and stuff it into the pen tip. The tricky part was getting the viscous epoxy into the tip.

My solution was to "wind" as much epoxy as possible around the large toothpick I used to mix it, and let it drip into the pen tip. After several iterations of that, use the toothpick to work the epoxy into the tip. In this picture I have pushed the probe nail into and through the tip, and wiped the epoxy off the probe.

Slide the pen body into place and mark the cable jacket at the upper end of the pen body. Then slide the pen body back up the cable.

Cut away a short section of jacket just above where you marked the position of the uppend end of the pen body. This will allow a connection for a ground clip wire.

How Probes Work

Mix another small puddle of epoxy and work it into the upper end of the pen body, between the pen body and the cable. Click here to see my page about repairing that Tektronix A oscilloscope. You could worry about the precision, or just call it "close enough" Alternatively, here is a 1X probe with a lower but still reasonably high impedance:. Here is the complete bill of materials: The pen A 2-meter piece of coaxial test cable with a BNC connector on one end Epoxy adhesive One alligator clip Copper-plated nail — 0.

Discard the remainder. That is, unless you have a use for those pieces