Probe Usage Q&A

Q1：What are the main specifications of a probe?

Answer: Mainly bandwidth, loading effect (input impedance, input capacitance), test signal range, and other important specifications.

Q2：What are the key factors in probe selection?

Answer: Probe bandwidth, test signal type, connection method, and loading effect are generally considered.

Q3：How can the probe input impedance be selected accurately?

Answer: Select according to the signal impedance characteristics.

Q3-1：Are probe impedances all50ohm-matched?

Answer: No, they are divided into1Mohm and50ohm matching.

Q3-2：If it is direct-current (DC)coupling, is the same rule applicable?

A: Generally, in an oscilloscope, the oscilloscope's50ohm matching is DC (DC) coupling.

Q4：What probe-to-oscilloscope interface types are available?

A: This depends on the oscilloscope's interface form factor，Generally, the common one is aBNCinterface

Q5：What voltage range can a passive probe measure?

A: Passive probes have relatively high attenuation, so the voltage range they can withstand is also relatively large, generally up to several hundredV. For exampleMXR-B mixed-signal oscilloscope's standard passive probe500 MHz N2873A can measure up to400VVoltage.

Q6：RCRC and RC Does the architecture apply to all probes?

Answer: Generally, high-bandwidth active differential probes will exhibit two different impedance model effects,RCRCandRC。

Q7：What is common-mode signal measurement?

Answer: A common-mode signal is a signal that appears on both wires of a two-wire cable with the same amplitude and phase; it is half the sum of the two line voltages.

Q7-1：Is the suppression capability for different types of common-mode interference considered in the probe design?

Answer: It will be considered; the probe will haveCMRRspecification curve.

Q7-2：At different frequency ranges and common-mode voltage amplitudes, the probe's CMRR How does performance change?

Answer: Probes generally have aCMRRspecification; generally, the higher the frequency,CMRRthe smaller it is.

Q8：This specifications page lists differential input impedance and differential input capacitance. If I use the differential probe in single-ended mode, how will this specification change?

Answer: A differential probe can be used in single-ended mode. Generally, the differential input impedance is2a multiple of the single-ended input impedance; the single-ended input capacitance will be somewhat higher than the differential input capacitance.

Q9：Why are power-rail probes recommended for measuring power-supply noise and ripple? Wouldn't a passive probe suffice?

Answer: Power-rail probes have a low attenuation ratio, reducing system-introduced noise; they also have a large DC offset range, can test ripple under high DC conditions, provide a relatively low loading effect, and at the same time provide high bandwidth for testing fast power-supply transients.

Q10：There are so many types of oscilloscope probes. If test performance requirements are not high, is there a general-purpose type?

Answer: For low-speed single-ended signals (100M) a passive probe can be considered; for high-speed test scenarios, it depends on the actual test scenario.

Q11：Are probes from different brands compatible?

Answer: Oscilloscope interface form factors differ among brands, and the identification and power-supply mechanisms also differ; mixed use is not recommended.

Reprinted from Keysight Technologies