Now, let's talk specifically about digital circuits. Given the explanation on the previous page, you'd think you never need to worry about a 1MHz digital signal, right? Well, there's another rule of thumb specifically for digital circuits: You should treat a wire or interconnect as a transmission line if the propagation delay is more than 1/6 of the rise time of the digital signal. A square wave is made of many frequencies. Recall your college days (way back when) when your instructor talked about the Fourier Series? This is a perfect example. A square wave is the prime frequency plus the odd harmonics.
The propagation delay of a signal through a medium is also dependent on the signal's frequency. Different components of our square wave reach the end of the line at different times. If I have a square wave with a slow edge (say, 5µs), then a three-inch trace is no big deal. The distance traveled is so small that all the frequencies get there at almost the same time. But if the edge is, say, 1ns, maybe we'll have ringing on the signal.
"Yes, we already know all this, and your explanation isn't all that good," you say. "Why are you bugging us with this ancient news?"
When I was working on those F-4s, we occasionally had a broken wire. Look closely at the picture below. This bird has cables that go from the near the tip of the nose all the way to the tip of the tail. That would be more than 40 feet if it were a straight line, but there's nothing straight about it. Believe me.
A real F-4 aircraft.
There are two ways to find the location of a fault in one of those wires. The first is long, hard, and laborious. It involves finding an access panel where the cable runs. You open the panel, break the wire (this is usually a splice point), and use an ohm meter to shoot the cable both ways. Now you know if the problem is in the front half or the back half. You keep going like this until you isolate the bad section and replace it. Then you go about repairing all the breaks you made and sealing the jet back up.
Now, let's talk about the second way to find the fault -- one of the greatest troubleshooting tools of all time. It's a time domain reflectometer (TDR). It makes use of the principle of reflections to help find faults in long cables. It is really an oscilloscope combined with a pulse generator.
A TDR is really an oscilloscope with a signal generator.
The pulse is sent down the cable, and you use the oscilloscope to measure the signals on the wire. A break in the wire looks like a pulse up. A short looks like a pulse down, and a connector usually looks like a glitch. Places where the cable is going bad (e.g., due to corrosion from water infiltration) look, well, crummy.
TDRs are essentially oscilloscopes, except the time-per-division knob is continuously variable and is calibrated in tenths of feet. We could just start dialing the knob until we saw the fault, look at the knob, and know it's about 37.5 feet down the cable (likely in the engine compartment). That's a lot easier than the other way of doing it.
Some people think TDRs can be used only with coax cables, but I'll tell you that just isn't true. Of course, that's how they work best, but they work well on twisted pairs, as well. In fact, they work rather well if you have any kind of multi-conductor cable where you can probe both lines together. In fact, the TDR is so sensitive that you could just lay a long wire on the ground outside the plane and still get a pretty decent reading on the location of the fault -- within a few feet. (I've done this once, and the manual for the TDR referenced it.)
TDRs come in two varieties -- electronic (like I've been discussing) and optical (for optical fibers). If you ever need to deplay or troublwshoot some long cables, look into one of these devices. Unfortunately, they're kind of expensive (imagine that) to have one of your own, especially given how rarely you need them. The cheapest electrical TDR I've seen recently costs more than $400. However, if you'd like to do a fun project at home, take a look at this. This guy is basically building his own and telling us all about it.
As for my opinion about the best electrical troubleshooting tool of all time, take a look at the video below.
What's your favorite troubleshooting tool?
Editor's note: This article originally appeared on The Connecting Edge.