How To Probe USB Waveforms on iPad Oscilloscope

Part of the fun in electronics is taking stuff apart to see how things work. Sometimes this might be as involved as disassembling a smartphone or other high tech device, or as simple as stripping the insulation off of conductors in a cable assembly to get access for probing.

In this picture, a USB cable has been dissected with razor for the benefit of scientific exploration and engineering study. This article talks about preparing a cable like this one for use in probing, and also includes some helpful suggestions about avoiding mistakes that could ruin the cable.

The first step in modifying any cable assembly is understanding how it is constructed. Like surgeons study human anatomy before making an incision, electronics enthusiasts must know something about the cable so they make cuts and remove material without causing so much damage that the cable is useless. Here is a simple diagram that shows how most USB cables are constructed.

Correct Setup Key In Measuring USB Waveforms

In the figure below, a section of the cable’s external insulation and foil shield have been cut away to expose the signal and power conductors. The next step, peeling off the conductor insulation, requires a delicate touch to avoid damaging the conductors and ruining the cable. The trick is to apply enough pressure to score and cut the insulation, but not so much pressure that individual strands of the conductor are either nicked or cut. It’s best to expose only the minimum amount of the conductor to get access with the probe. When connecting the probes to the exposed conductors, be sure to put adequately space between adjacent probes to prevent the exposed conductors from shorting together and to reduce coupling. This second risk, radiated coupling, is a problem now when it wasn’t before for two reasons: 1) the conductive foil shield have been removed from this section of the cable, and 2) the clips/probes can act as coupling antennas between high frequency signals.