Probes are essential equipment in electronic testing and measurements, and many electronic companies make specialized probes suited for specific applications. Conventional probes are passive and commonly used for electrical measurements. The importance of high-quality probes cannot be overstated as the maximum frequency that can be measured by an oscilloscope is limited not just by the oscilloscope but also by the probe. High voltage probes are another class of probes that must perform well at high voltages, especially for Gallium Nitride (GaN) and Silicon Carbide (SiC) applications, which require probes that can withstand high voltages and sense small voltages.
Power distribution network (PDN) measurements are critical in processor power supply designs, and specialized probes from companies such as Picotest make these challenging measurements easy. They need to measure ultra-low impedance of the order of micro-ohms, which requires 2-port shunt-through impedance measurement techniques using vector network analyzers (VNAs). Specialized probes for high-temperature probing are also essential to test industrial-grade electronics at high temperatures to ensure they meet the required specifications.
Electromagnetic interference (EMI) probes are another type of probe used in EMI/electromagnetic compatibility (EMC) testing. They come in two types: E field and H field probes, which sense electric fields and magnetic fields, respectively. E field probes are stub types, and current probes are loop types.
Time domain reflectometry (TDR) is a measurement technique used to study the electrical impedance of a conductor by observing the electromagnetic wave reflections. The sensors used in TDR measurement systems are called TDR probes. TDR is widely used to determine faults in aircraft wiring and measure printed circuit board (PCB) transmission line impedances. It is a large and expensive instrument that includes a high-speed edge pulse and a sampling oscilloscope. TDT is a similar instrument used to measure transfer time domain transmission (TDT) and is used to measure crosstalk, rise time degradation, insertion loss, dielectric loss, skin effect, and characterize lossy transmission line parameters.
Nowadays, a low-cost, pocket-sized signal edge generator is available in the market that can be used with a real-time oscilloscope to make TDR and TDT measurements with excellent accuracy. Together with a sampling oscilloscope, it can also generate an eye diagram. To accurately measure low impedances, VNAs must be correctly calibrated, and fixtures should be de-embedded mathematically to remove their impact on the device under test. Very low impedances are accurately measured using 2-port shunt through measurements, while series measurements are suitable for impedances in the ohms range. Two-port shunt through impedance has an inherent ground loop, which can be resolved using a coaxial transformer or a semi-floating amplifier.
In summary, probes play a crucial role in electronic tests and measurements, and specialized probes are needed for specific applications. Different types of probes are used to measure high voltages, ultra-low impedances, EMI, and to perform TDR and TDT measurements. The importance of high-quality probes cannot be overstated, as they directly impact the quality and accuracy of electronic measurements.
TDR probes
Time Domain Reflectometry (TDR) is a powerful measurement technique used to study the electrical impedance of conductors by observing the reflections of electromagnetic waves. The TDR technique uses a special type of probe called the TDR probe to perform these measurements. TDR probes are designed to generate high-speed edge pulses and to measure the reflected wave’s response to the transmitted signal. TDR probes are typically large and expensive instruments that include a high-speed edge pulse generator and a sampling oscilloscope. They are used to measure a wide range of parameters, including PCB coupons, signal traces, interconnects, cables, and dielectric constants.
TDR probes come in a variety of types and designs, each suited for different applications. The most common type of TDR probe is the coaxial probe, which is used for measuring the impedance of cables and transmission lines. Coaxial TDR probes consist of a center conductor surrounded by a dielectric material and an outer conductor. When the probe is connected to the transmission line or cable under test, a high-speed pulse is generated, and the reflected wave is measured. The time delay between the transmitted and reflected waves is used to calculate the impedance of the cable or transmission line.
Another type of TDR probe is the microstrip TDR probe, which is used to measure the impedance of printed circuit board (PCB) traces. Microstrip TDR probes consist of a small pad that is connected to the PCB trace under test. A high-speed pulse is applied to the pad, and the reflected wave is measured. The time delay between the transmitted and reflected waves is used to calculate the impedance of the PCB trace.
TDR probes can also be used to detect faults in aircraft wiring. By measuring the impedance discontinuity caused by a fault, a TDR probe can locate and identify the fault. TDR probes can also be used to measure crosstalk, rise time degradation, insertion loss, dielectric loss, skin effect, and other transmission line parameters.
In recent years, low-cost, pocket-sized signal edge generators have become available that can be used with a real-time oscilloscope to make TDR measurements with excellent accuracy. Along with a sampling oscilloscope, they can also be used to generate an eye diagram, which is a powerful tool for analyzing signal integrity issues in high-speed digital circuits.
In conclusion, TDR probes are an essential tool for measuring the impedance of cables, transmission lines, PCB traces, and other conductors. They come in a variety of types and designs, each suited for different applications, and can be used to detect faults, measure crosstalk, and characterize transmission line parameters. With the advent of low-cost, pocket-sized signal edge generators, TDR measurements are now more accessible than ever before.
