Measuring clocks and critical signals accurately is often inconvenient. Normal 10× probes usually have about 10 pF of input capacitance, which can considerably affect the signal. FET probes with 1 to 3 pF of input capacitance are expensive and often require an extra power supply or other special setup. Unless ground leads are short (10 to 20 mm), these leads introduce considerable distortion to most probes.

Figure 1a shows a 100× probe that is easy to make and use, has a high bandwidth, and has a small (<1-pF) input capacitance. The probe is insensitive to ground-lead length; the difference in waveforms is almost imperceptible for lead lengths of 10 and 180 mm. This lack of sensitivity solves the inconvenience of finding a new ground point for each accurate measurement of rise time, undershoot, or other high-bandwidth parameter. The needle tip is convenient for measuring surface-mount chips.

When using this probe, you should turn on the oscilloscope’s 50-(ohm) terminator (Figure 1b). The probe’s 5000V dc resistance has little effect on most driven logic signals, but you should carefully use the probe on high-resistance or floating nodes. Also, due to the low input capacitance, even adding just 100 mm of extra wire to the probe tip results in considerable ringing with a fast rise-time signal. (DI #1814)