High impedance active probe for spectrum analyzer, or also could be used on a 'scope. Only around 0.7 pf input capacitance, the idea being not to load the circuit under test. This homemade probe took many hours to make and perfect the design. The object of the exercise was to produce a high impedance active probe with as close to 0 db gain as possible from low frequencies to 1 GHz Uses dual gate mosfet which has very low gate capacitance, and MMIC to bring the gain up to near 0 db. See the schematic. Think I did a fairly good job if you look at the spectrum analyzer plots. It's not got a completely flat response and does have some peaks, but it is incredibly hard to improve it and it works well up to 1.4 GHz and a bit beyond. -3 db points are: 1.2 GHz at the high end and -2 db at 28 kHz, below that was impossible to measure with my spectrum analyzer. All measurements were done across a 50 ohm load. Works on any voltage between 8 and 12 roughly. I included a 5 volt regulator, not shown on the schematic and its reverse polarity protected.
Draws 45 mA.
Dimensions of tube 100mm × 15mm. Cable lengths around 1 metre.