A FET Oscilloscope Probe For Higher Frequencies

It’s a problem that has dogged electronic engineers since the first electrons were coaxed along a wire: that measuring instruments can themselves disrupt the operation of a circuit. Older multimeters for example had impedances low enough to pull resistor values, thus our multimeters today have high-impedance FET inputs. [Christoph] faced it with his oscilloscope probe, its input capacitance was high enough to put unacceptable load on a crystal oscillator and stop it oscillating. He thus built a FET input probe for higher RF frequencies, and its construction is an accessible view of wideband RF instrumentation design.

The circuit is a very simple one using a dual-gate FET, but the interest comes in the PCB and screening can design to ensure good RF performance. Off-the-shelf cans have four sides, so to accommodate the circuit one wall of the can had to be removed. The end result is a tiny PCB with miniature co-ax connectors for power and signal, which when characterised was found to have a 1.3 GHz bandwidth and a very low input capacitance.

If the language of RF design is foreign to you, may we recommend [Michael Ossmann]’s talk at a Superconference a few years ago.