Bandwidth study of recessed probe launch variations for broadband measurement of embedded PCB structures

In this paper the recessed probe launch technique for measurement of embedded (internal) printed circuit board (PCB) structures such as striplines and vias (plated through holes) will be reviewed and its bandwidth will be studied in experiments. The recessed probe launch (RPL) uses high frequency microprobes in combination with a milling technique that removes upper board layers and exposes internal probing points. By providing direct access to the PCB layers of interest a broadband launch into embedded structures is thus provided. Here, measurements of different RPL designs show that reflections of less than -25 dB from DC up to 40 GHz can be achieved. First investigations show that this level of reflection is mostly due to a mismatched stripline impedance in the 52 Ohm range and not due to the recessed probe launch itself. Full-wave simulations are underway to corroborate the results obtained by measurements.