Electronic – Using a miniature BNC connector on a PCB to measure high-frequency content with an oscilloscope


I am a power electronics engineer in R&D and often measure fast switching waveforms, specifically high bandwidth MOSFET gate signals. When connecting oscilloscope probes in the past I have found that the ground clip often induces unwanted HF content due to the inductance of the loop and the very high dI/dt and dV/dt in the area (switching 800 A at 900 V).

To mitigate this I was always taught to create little ground hoops with a wire and slot my probes in. However, due to smaller and smaller designs, the access is becoming increasingly difficult.

I was considering incorporating some of those miniature 2 mm RF connectors (U.FL) on the next PCB design and running very small diameter coax cable out of the device to monitor accurate waveforms on an oscilloscope. However, I am not experienced with RF and cable transmission specifics and was wondering how the connector impedance might affect/load my real signal or the measured signal on the oscilloscope.

Could anyone shed some light on this idea?

Best Answer

You might consider adding an attenuator probe to your PCB consisting of a single series resistor. The 50-ohm coax cable to your oscilloscope can be any length - the oscilloscope input at the far end of this cable must be 50-ohm-terminated, not left to its default 1MEG


simulate this circuit – Schematic created using CircuitLab

The series resistor Rs will load the MOSfet gate driver only when the coax to the 'scope is connected. This resistor, combined with 50-ohm 'scope termination is a wideband attenuator - its attenuation should likely be made an easy-to-calculate 20:1 or 50:1. You should be able to arrange the short PCB path from Rs -to- coax connector to have 50-ohm impedance to PCB ground plane.