PLA Scope Probe Accessory Improves Signal Fidelity

This page describes an earlier version, and is retained for historical interest.

See this new page for details of the theory, the performance and manufacturing.

All too often signal integrity aberrations are visible in digital signals displayed on an oscilloscope. Fortunately signal fidelity can be significantly improved by a simple homebrew 3D-printed accessory that can be retrofitted to any standard scope probe.

The 3D-printed probe ground accessory shown below was inspired by the HP10020A.

HP10074 With Adaptor

HP10074 With Homebrew Ground Spear

The body was designed in OpenSCAD, printed in PLA on a RepRap printer in ~20 minutes

  • the body, designed in OpenSCAD, printed in PLA on a RepRap printer in ~20 minutes
  • the spear, made from 0.8mm piano wire
  • the ground connection, cut from 0.2mm phosphor bronze using a jeweller’s fretsaw
  • the hinge, a 1.7mm/1.5mm spring pin in a 1.6mm hole

The phosphor bronze ground connection fits into the slot and protrudes into the central cavity. It is then deformed so that when the probe is inserted it rubs against the probe’s ground sleeve. Finally the piano wire spear is inserted into the slot against the phosphor bronze, and held in place by the spring pin. The phosphor bronze can just be seen through the translucent PLA.

Adaptor Component Parts

Accessory Component Parts

The problems fabricating the accessory were:

  • holes and slots that were narrower than specified. Cure: file/sand/drill to correct dimensions, or slightly change the OpenSCAD model
  • “swarf” threads. Cure: pick or knock or file them them off
  • score marks in HP10074C plastic body caused by interference fit between the probe and rough hard PLA. Cure: use circular file to gently ream hole until it fits more loosely
  • falling/knocked over during printing. Cure: add sprues to make tall thin body more stable

It would be interesting to see if a commercially printed nylon body had the same defects. Update: it didn’t, as described here.

Acknowledgements

Thanks to Russell Dicken and Ian Stratford for help with the RepRap, and Bristol Hackspace for the use of their RepRap.

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