HP-41800A clone ideas?

[WB8PFZ]

My HP spectrum analyzer needs an active probe similar to the HP41800. Lots of money for something not used very often. Does any one have any ideas to a comparable circuit. I am not doing lab stuff just probing around radio circuits. I was thinking a few MMICs. Thanks Mike

[SuzyC]

Just use a n-chan enhancement mode JFET common-drain follower to make a very high impedance low cap(<5pf) active probe tip that you can make using a tiny piece of PCB copper laminate to attach to the end of a scope probe.  Only limit:  low-voltage circuits, otherwise a coupling capacitor(5pf)/22-meg input resistor G-S would be needed.

You can supply +12 V to the JFET source and set the JFET drain resistor to 50 to 500 ohms to -12V,  maybe OC or  22-Meg gate to source pull-down. The gate to source resistor is not needed once the gate is connected to the circuit point of interest.