As published in US Tech, April/May 2025
The rapidly growing 5G market brings about new challenges for digital systems. With slopes of just a few picoseconds and decreasing voltage levels, any discontinuity in impedance and impairment of inductance or capacitance on the PCB or back drilling and blind vias on the complex multilayer PCB can have a massive impact on signal quality. This creates a growing need for high-speed RF functional testing of printed circuit boards.
To test a high-frequency PCBA, one can use devices such as oscilloscopes, spectrum analyzers, vector network analyzers, signal generators, and power
meters. High-quality probes, cables, and connectors that are compatible with the impedance and bandwidth of the circuit should also be used.
High-frequency testing best practices should be followed, such as minimizing parasitic effects, avoiding ground loops, and shielding the circuit from external interference.
Also, signal integrity analysis should be performed, such as eye diagram analysis, return loss analysis, and time domain reflectometry (TDR), so verify the high-frequency performance of the PCB design.
RF Probes
There are different probes for testing RF signals. Active probes offer the advantages of greater bandwidth and lower probe loading. For example, active oscilloscope probes have active components inside the probe that serve to keep the capacitance value low.
These provide unmatched levels of performance at high bandwidth frequencies, significantly lower capacity load and more precise insights into fast signals. The measurement is taken between a test point relative to a ground point. Typically, the measured frequency is below 10 GHz.
Differential probes increase the value of ground isolation and reduce common mode effects or noise. Differential probes are essentially two probes in one. Instead of measuring a test point relative to a ground point (like single-ended active probes), differential probes measure differential signals by measuring the amplitude and phase of the signal coming in from the ports. The measured frequency is usually 40 GHz or higher.
These probes are generally available in a variety of combinations, like ground-signal-ground or signal-ground, and ground-signal-signal-ground configuration. There are also active differential probes and when utilizing them, higher frequency measurements up to 10 GHz are possible.
However, manual RF testing is time-consuming, driving the need for automation.
Automated Probing Systems
Robotic positioning systems are highly sophisticated mechanisms that enable precise movement and placement of test probes. These systems are engineered for accurate, repeatable, and efficient operations, making them ideal for tasks demanding precision and consistency while reducing or eliminating the need for manual intervention. Automated RF testing can reduce test time by 90% when compared with manual testing.
An automated robotic system with integrated RF measurement capability can simultaneously engage many probes on the PCB, freeing engineers to quickly perform necessary measurements without manual intervention.
Multi-pin probes can be installed on both the top and bottom of the flying probe system, ensuring comprehensive test coverage. All multi-pin probes can be rotated based on the PCB test point distribution. This will increase the overall test coverage.
The Acculogic FLS980Dxi Flying Scorpion is a robotic system for testing PCBs and PCBAs, renowned in the flying probe market for its excellent flexibility and open architecture.
High-Speed Manufacturing
A client who operates in high-speed PCB manufacturing conducted extensive research to find the best match of cables and interfaces to extend the limit of measured frequency with differential probes from DC to 56 GHz. For this unique application, critical automation is provided by the Acculogic FLS980Dxi Flying Scorpion.
It was important to understand the need for a controlled impedance transition from the probe tip to the planar PCB structure to minimize signal degradation and improve accuracy in calibrating the contact pin and associated cables.
To do this, Acculogic developed a solution to add a force sensor with feedback to control the contact impedance transition and avoid damaging the probe tips. The company also added another axis to the existing VPM400 RF head for differential head rotation module to expand overall test coverage.
Acculogic then introduced a borescope integrated with the RF probe module for monitoring the RF probe landing. This feature allows users to visually inspect a live image of the probe landing, particularly beneficial during debugging and testing. The capability ensures thorough inspections, leading to precise testing procedures, improved debugging results, and streamlined engineering manual probe landing checks.
This is designed to perform vector network analysis tests. Matched cable sets connect the VNA instrument to specially designed probe modules that enable integration of various VNA Probe Types instead of the standard test pogo-pin type probe tips used in the FLS. In addition, the tester can accommodate a mixture of standard test probe modules to perform typical FLS test and measurement functions.
The capability of the RF Probe module includes measuring full channel S-parameters (four-port measurements including insertion and return loss) and saving single-ended, differential mode, mixed mode, or common mode measurements. S-parameters in frequency domain and TDR in time domain. Furthermore, limits on the VNA can be applied in production to insertion loss, return loss and TDR for creating a pass-fail criteria in automated testing.
S-parameters provide a framework for describing RF networks based on the ratio of incident and reflected microwaves. This is useful for circuit design,
because one can use these ratios to calculate properties such as input impedance, frequency response, isolation and graphically overlaid on each other for
easy identification.
The Acculogic flying probe RF tester is an excellent tool for high-speed digital and RF ATE production requirements. Users gain:
- Efficiency. It cuts manual measurement time by 90%, facilitating performance evaluations for high-volume manufacturing demands.
- Defect Detection. It identifies RF component defects, performance issues, and manufacturing flaws pre-shipment, ensuring top-notch quality control.
- Reliable Performance. It is designed to ensure 100% board performance, averting delays and missed deadlines caused by faulty hardware.
- Simplified debugging. It streamlines RF troubleshooting by pinpointing problematic components during production.
- Correlation and repeatability. It exhibits strong alignment with bench instruments and delivers consistent results, enhancing accuracy.
- Time savings. What used to require a week for bench instrument testing can now be accomplished in just one or two hours with the automated RF testing
facilitated by Acculogic.