Lo Z Probes
Z0 probes are a specialized type of low-capacitance passive probe used in low-impedance, very-high-frequency circuits. They are similar in design to 10× passive probes but at much lower impedance levels. The probe cables usually have a characteristic impedance of 50 ohms and connect to oscilloscopes with a matched 50 ohm (rather than 1 megohm) input impedance. At the tip, these probes use a 450 ohm (for 10× attenuation) or 950 ohm (for 20× attenuation) series resistor (rather than the 9 megohm resistor of the 10× probe). High-impedance scope probes are designed for the conventional 1 megohm oscilloscope, but at high frequencies mismatch between the scope input impedance and the cable impedance presents a transmission line mismatch. A high frequency oscilloscope presents a matched load (usually 50 ohms) at its input, which minimizes reflections at the scope.
In principle this type of probe can be used at any frequency, but at DC and lower frequencies the low 50 ohm impedance will load the circuit under test unacceptably—parasitic impedances limit very-high-frequency circuits to operating at low impedance, but at lower frequencies high impedances are normal. An attenuator can be used to minimize loading. These probes are also called resistive divider probes, since a 50 ohm transmission line presents a purely resistive load. A 21× divider probe consists of a 1000 ohm series resistor and a short 50 ohm transmission line (Johnson & Martin 1993, p. 98). Tektronix sells a 10× divider probe with 9 GHz bandwidth with a 450 ohm series resistor.
The Z0 name refers to the characteristic impedance of the oscilloscope and cable. The matched impedances provide better high-frequency performance than an unmatched passive probe can achieve, but at the expense of the low 500-ohm load offered by the probe tip to the DUT. Parasitic capacitance at the probe tip is very low so, for very high-frequency signals, the Z0 probe can offer lower loading than any hi-Z probe and even many active probes.