Probes
9 subcategories
This guide covers the technical specifications and operational categories of electrical measurement probes, which act as the critical physical interface between a circuit under test and instruments such as oscilloscopes, logic analyzers, or spectrum analyzers. Choosing the right probe requires balancing bandwidth, signal loading, voltage range, and physical connectivity. The probe's primary task is to transmit signal voltages, currents, or fields to the host test instrument with minimal distortion or disturbance to the device under test (DUT). This page examines the main classes of probes—including active, passive, high-voltage, differential, current, logic, and electromagnetic field types—and clarifies how key parameters like bandwidth, input capacitance, and interface compatibility affect measurement accuracy.
- Bandwidth Range
- DC up to 16 GHz (model dependent)
- Active Probe Input Capacitance
- Under 0.3 pF to 2 pF typical
- High-Voltage Limits
- Up to 40 kV peak passive / 5.6 kV active differential
- Current Probe Limits
- Up to 500 A continuous / 750 A peak
- Common Interfaces
- TekVPI, TekConnect, TEKPROBE, ProBus, AutoProbe
Subcategories
Passive vs. Active Voltage Probes
Passive voltage probes are robust, general-purpose tools suited for everyday measurements where high circuit loading is not a critical factor. For example, the Keysight Technologies (Agilent HP) 1161A is a 500 MHz passive probe offering a 10:1 attenuation ratio, 10 MΩ input resistance when terminated into a 1 MΩ scope input, and a typical input capacitance of 10 pF.
In contrast, active probes use built-in active electronics, typically FET-based, to achieve much higher input impedance at high frequencies by drastically reducing input capacitance. This minimizes high-frequency circuit loading. The Tektronix TAP2500 active probe features a bandwidth of at least 2.5 GHz with an input capacitance of less than 0.8 pF and a 40 kΩ DC input resistance. For higher bandwidth demands, the Tektronix TAP3500 operates up to 3.5 GHz with similar capacitive loading. Other active models, like the Keysight Technologies (Agilent HP) 85024A, operate from 300 kHz to 3 GHz with an input capacitance below 0.7 pF, presenting nominal 1 MΩ DC input resistance and outputting via a 50 Ω Type-N male connector.
Differential and High-Voltage Probing
Differential probes measure the voltage difference between two test points without requiring a connection to earth ground. High-speed active differential probes, such as the Tektronix P7380 (8.0 GHz) and Tektronix P7313 (13.0 GHz), provide low AC loading (capacitance under 0.27 pF or 0.3 pF) for high-bandwidth differential signal analysis. The Tektronix P7516 extends this capability to 16 GHz using TriMode technology for differential, single-ended, and common-mode measurements through one connection.
For power electronics and safety, high-voltage active differential probes allow safe floating measurements. The Tektronix P5205 offers a 100 MHz bandwidth and can handle differential voltages up to ±1300 V, while the Tektronix P5210 provides a 50 MHz bandwidth and measures up to ±5.6 kV. For single-ended high-voltage requirements, passive probes like the Tektronix P6015A scale down heavy-duty voltages up to 40 kV peak pulses with a 1000:1 attenuation ratio and 75 MHz bandwidth (using its standard 3 m cable).
Non-Intrusive Current Measurement Systems
Current probes utilize Hall-effect sensors and transformer windings to measure alternating and direct current without breaking the circuit. Many high-performance current probes require dedicated amplifiers to supply power, scale the signal, and provide auto-zeroing or degaussing.
The Tektronix TCP312 (DC to 100 MHz, 30 A max continuous) and Tektronix TCP303 (DC to 15 MHz, 150 A max continuous) connect to the Tektronix TCPA300 amplifier, which outputs a conditioned signal to the oscilloscope via a TekProbe-BNC connection. For high-current industrial or power applications, the Tektronix TCP404XL current probe measures up to 500 A continuous DC/RMS and 750 A peak, pairing exclusively with the Tektronix TCPA400 amplifier. Other systems, like the LeCroy (Teledyne LeCroy) CP150, provide DC to 10 MHz measurements up to 150 A continuous and are powered directly by the host oscilloscope via the ProBus interface.
Logic Analyzer and Magnetic Field Probes
Specific measurement tasks require specialized probe architectures. Logic analyzer probes are designed to acquire multi-channel digital states simultaneously. The Keysight Technologies (Agilent HP) E5346A is a 34-channel single-ended probe utilizing a 38-pin Mictor connection with a 100 kΩ input resistance. Similarly, the Tektronix P6434 is a 34-channel high-density logic analyzer probe with a 20 kΩ input resistance and 2.0 pF input capacitance.
For electromagnetic compatibility (EMC) troubleshooting, close-field magnetic (H-field) probes isolate local magnetic field emissions. The Keysight Technologies (Agilent HP) 11945A is a close-field probe set containing the passive Keysight Technologies (Agilent HP) 11940A (30 MHz to 1 GHz) and the Keysight Technologies (Agilent HP) 11941A (9 kHz to 30 MHz) H-field probes. These passive sensors offer a dual-loop design for high E-field suppression and output via SMA female connectors to spectrum analyzers or oscilloscopes.
Frequently asked questions
- What is the difference between single-ended active probes and active differential probes?
- Single-ended active probes (such as the Tektronix TAP1500 or TAP2500) measure voltage relative to ground and are designed for high-speed single-ended signals. Active differential probes (such as the Tektronix P7330 or P7380) measure the voltage difference between two unique signal paths, which is critical for floating measurements and rejecting common-mode noise.
- Why do some current probes require external amplifiers like the TCPA300 or TCPA400?
- High-performance AC/DC current probes like the Tektronix TCP312 or TCP404XL require specialized power, control signals, and precise signal conditioning. Amplifiers like the TCPA300 and TCPA400 provide these resources, along with front-panel autozero and demagnetization functions, converting the current signal into a calibrated voltage output for standard 50-ohm oscilloscope inputs.
- How does input capacitance affect high-frequency measurements?
- High input capacitance loads the device under test (DUT) at high frequencies, acting as a low-pass filter that rounds fast rising edges. Passive probes typically have higher capacitance (like the Keysight 1161A at 10 pF), whereas high-speed active probes have very low capacitance (like the Tektronix TAP2500 at < 0.8 pF or Keysight 1169A at 0.27 pF) to preserve signal integrity and avoid disrupting the circuit's operation.