How to build a probe for an oscilloscope?

Answer 1

Essentially you have to make a device that doesn't load down the circuit being tested and adapts to the input configuration of your oscilloscope.

Start with a suitable length of coaxial cable with a BNC connector attached to one end.

In order to present a high-impedance to the probe tip and simultaneously scaling the sensitivity of the 'scopes input, you will need to construct a voltage divider circuit at the other end of the coax. This voltage divider circuit could be for instance a 5 mega ohm resistor in series with the core of the coax and a probe tip along with a 1 meg resistor in parallel with the coax conductors. That is, across the shield to the center conductor.

A suitable non-conductive handle is used to house the voltage divider, which has the other end of the 5 meg resistor attached to a pointed object of some type that extends from the end of the probe handle. A short length of wire with an alligator clip attached to the end is then soldered to an exposed section of coax shield thus forming the ground reference for the probe.

A suitable housing for the probe could be a plastic writing pen that has been gutted for this purpose.

A web site that shows this operation can be found at

http://www.cromwell-intl.com/radio/probes.html

where I found this clever ascii artwork that explains what is described above:

R1

5 Mohm ----------------------

test <----/\/\/\/\----+-----------(o) )--- scope input

point | +--------------------+ ~ 1 Mohm

| | 50-75 ohm | 10-20 pF

+--/\/\/\/\-- + cable |

1.25 Mohm | |

R2 | |

----- -----

--- ---

- -

\__________________________/ \______________/ \_____________/

| | |

Probe Cable Oscilloscope