Differential Probe for Oscilloscopes

Probes for oscilloscopes are easier to build than most people beleive! The differential probe described here roughly features the following parameters:

Attenuation:    -20dB (1/10)

Input impedance:    390K

Bandwidth:    80MHz (provided a good layout is used)

Maximum differential input voltage (begins clipping): 20V

Special care must be taken for propper layout.  It must be absolutely symmetrically in the RF part (IC3, Q2, Q3, R10, R11, R14, R15, R17, R18, R6, R7). Ground should be routed exactly on the  symmetry line. A printed board is not mandatory (I used a simple hand wired evaluation board and only used a very wide wire for ground to minimize degradation due to skin effect). Blocking capacitors C10 and C11 must be placed as close to IC3 as possible.

The component list

Schematic

Application