Which Probe Type for This Situation?#

The default probe isn’t always the right probe. Each probe type makes trade-offs between bandwidth, loading, voltage range, and what it can physically connect to. Picking the right one avoids getting a technically correct but practically useless reading.

Quick Selection Guide#

Passive 10x Probe#

Bandwidth: 100–500 MHz | Input: 10 MΩ ‖ ~12–15 pF | Max voltage: ~300V CAT I

The default for almost everything: digital logic, analog signals, power rail checks. Uses a compensated RC divider — a 9 MΩ resistor in series with the tip, combined with the scope’s 1 MΩ input, gives 10:1 attenuation. A trimmer capacitor compensates for cable capacitance so the divider ratio is flat across frequency.

The 10:1 attenuation means the scope’s vertical noise is 10x more significant relative to the signal. For millivolt-level signals, 1x or direct BNC is better.

Passive 1x Probe#

Bandwidth: 6–15 MHz | Input: 1 MΩ ‖ ~100+ pF | Max voltage: ~300V CAT I

For audio-frequency signals where sensitivity matters, or low-level DC/slow AC where millivolts need to be seen without 10x attenuation noise. The high capacitance (~100 pF) loads the circuit significantly at high frequencies — this probe is effectively a low-pass filter on high-impedance nodes.

Active Probes#

Bandwidth: 500 MHz – 10+ GHz | Input: ~100 kΩ ‖ ~1 pF | Max voltage: typically ±8V or less

For signals with frequency content above a few hundred MHz, or high-impedance nodes where even 15 pF is too much loading. Contains a FET amplifier at the probe tip that presents high impedance to the circuit and drives the cable capacitance with low-impedance output.

Low voltage range makes them easily damaged by overvoltage. Expensive, require power from the scope, often proprietary interface, and ESD-sensitive at the tip.

Current Probes#

AC (transformer-based): Clamps around a conductor; changing magnetic field induces proportional voltage. AC only, needs a single conductor (supply and return together cancel).

DC/AC (Hall-effect): Measures both DC and AC. More expensive, needs periodic degaussing (DC offset drifts), lower bandwidth than best AC-only probes.

Both types work best above 100 mA–1A. Position conductor in center of clamp for accuracy. External magnetic fields can affect readings.

Differential Probes#

Bandwidth: 25 MHz – 1 GHz+ | Common-mode: 600V – 1500V

For measuring voltage across two nodes that are both not at earth ground — high-side current sense resistors, motor phase voltages, bridge circuits, any measurement where connecting a scope ground clip would create a short.

CMRR degrades with frequency. Separate ratings exist for differential voltage (between inputs) and common-mode voltage (both inputs to earth).

Near-Field Probes#

H-field (magnetic loop) or E-field (capacitive stub) probes for localizing EMI sources on a board. Measurements are relative, not calibrated — comparing “more here” vs “less there.” H-field probes pick up current loops; E-field probes pick up voltage nodes. Orientation matters.

DMM Accessories#

Kelvin clips (4-wire): For resistances below ~1 Ω where lead resistance affects the reading. Four terminals: two carry test current, two sense voltage. Essential for milliohm measurements.

Tips#

• Start with passive 10x for most measurements — switch to specialized probes when loading, bandwidth, or voltage range demands it
• Compensate 10x probes whenever using a new probe or moving between scopes
• Use 10x rather than 1x for anything where loading or bandwidth matters
• For current measurement, single conductor only — supply and return together cancel
• Check both differential and common-mode voltage ratings on differential probes

Caveats#

• 10x probe compensation must be checked — under-compensated shows drooping edges, over-compensated shows overshoot
• Active probes are easily damaged by overvoltage — verify voltage before probing power rails
• Current probe sensitivity decreases for small currents — most work best above 100 mA–1A
• Cheap differential probes may have CMRR of only 30–40 dB even at low frequency
• Standard DMM leads contribute ~0.1–0.5 Ω of lead resistance — negligible above 10 Ω, significant below 1 Ω
• Probe tip contact quality matters more at low resistance — dirty or oxidized contacts add resistance

In Practice#

• Signal that looks clean on 10x but distorted on 1x indicates 1x bandwidth is insufficient or capacitive loading is affecting the circuit
• Oscillator that works normally but stops when probed needs an active probe with lower capacitance
• Measurement across a floating node that causes smoke or trips a breaker indicates the ground clip created a short — differential probe needed
• Current reading that seems too low with clamp probe may be caused by conductor not centered in clamp, or nearby magnetic field interference
• Resistance reading that changes significantly between 2-wire and 4-wire mode indicates lead resistance is a substantial fraction of the measurement