Overview of Coaxial Probes (Cable Mount Probes)
While prototyping and testing RF equipment, there are many occasions where it can be helpful to test at non-ported locations in a circuit for optimizing a design or during troubleshooting. Unfortunately, this is a much greater challenge to perform at higher frequencies, as inserting a typical voltage probe would dramatically skew the results due to the lack of impedance matching between the RF transmission line and the voltage probe.
Hence, RF test probes must have matched impedances with the test point. What is commonly done is to have RF coaxial cables (pigtails) soldered at various pre-planned test points within a design. This helps to ensure that there is adequate impedance match, and the test points can be chosen in areas that would reduce the impact to the overall design performance. Other methods include soldering specialized RF probes onto a custom pad, or leads design, for much less invasive probing. High performance test equipment vendors offer these specialized probes with options that can reach frequencies into the millimeter-waves. Unfortunately, these probe ends are generally expensive, and cannot always physically access the component-populated circuits. Therefore, they are limited in large volume testing applications or troubleshooting, and are more geared toward prototyping and R&D.
And yet other options, when available, include wafer probe stations and associated pogo-pin probes, or otherwise flexible, probe heads. This last option is likely the least impactful and provides the highest fidelity configuration, as calibration cards can be used to provide extremely well calibrated results at the test site accounting for nearly the exact probe position.
A recently available option that combines some of the accuracy and repeatability features of probe stations, as well as the accessibility of specialized probes is the use of coaxial RF cable mount probes. These probes feature an edge similar to a probe station probe fixture– either ground-signal-ground (GSG) or ground-signal (GS)–with pogo-pin probes on one end and a typical coaxial connector on the other end. These new probes come with options that can reach 40 GHz and provide return loss better than 10 dB. Probe pin pitches range from 800 microns to 1500 microns, and the termination end commonly used for these probes is either a female 3.5 mm or 2.92mm coaxial connector. As with probe stations, a minimally impactful pad port can be designed into specific test areas, or the probes can be positioned on-near component terminals or microstrip transmission lines.
Cable mount probes can either be used by hand, or with a multi-axis probe positioner. Unlike typical probe stations, these probes are large enough to be reliably used by an operator by hand. The probe positioner provides a much-improved level of position accuracy and repeatability, and can itself be positioned conveniently for testing. Unlike probe stations, a coaxial cable mount probes and positioners can be used at an engineer or technician typical test bench as an accessory to network analyzers, signal generators, spectrum analyzers, oscilloscopes, and other test equipment for RF/microwave, Millimeter-wave, and high-speed digital applications. The small footprint, non-damaging, and otherwise non-invasive design of cable mount RF probes allows for testing in high density applications, such as antenna arrays, metamaterials, fractal antennas, microstrip transmission lines, compact assemblies, and PCB with small surface mount package components.