Some time ago I built a signal generator, which is a reasonably handy bit of equipment. But I was periodically frustrated with not having a suitable probe to easily inject a signal into a circuit. The standard BNC – BNC cable has its uses, but isn’t too great for that kind of thing. And a dangling bit of wire causes nothing but troubles.
What I really wanted was an oscilloscope probe. Scope probes have a nice sharp tip on them, they have a ground lead with a clip, they even have a hooky tip for grabbing onto things. Scope probes are great. But some experiments proved that a scope probe makes a terrible signal generator cable, because scope probes contain resistor-divider circuits, trimmer capacitors, etc, which get in the way of the signal generator output.
So I decided to hack a scope probe, to build a signal generator cable / probe. From DealExtreme I ordered a cheap oscilloscope probe – their SKU 32422 for a big $6. About three weeks later it landed in my mailbox. I quickly found it can easily be taken apart. Like so:
The strain relief (on the left of the photo) can simply be pulled off. That holds the little square box covering the PCB in place – with the strain relief gone the cover now slides off as well. A minute of quality time with the soldering iron removes the resistors and trimmer capacitor on the PCB (done before I took these photos), then I soldered a wire across the middle of the PCB to connect the BNC connector back to the centre of the cable. Here’s a close-up:
Then the PCB cover and strain relief were slipped back into place, and the X1/X10 switch on the probe handle was placed in the X1 position, then wrapped in tape to make sure it couldn’t be accidently switched to X10. When the switch is in the X10 position a big resistor is switched in series with the tip – don’t want that! Finally I stuck a label on the probe so I won’t confuse it with the regular scope probes. Done!
This is awesome. For a few dollars and maybe 10 minutes work we get a nice signal generator probe complete with sharp tip, hooky tip, ground lead, etc. Can’t beat that.
Hi,
I just did that to a tektronix scope probe but the cable itself has ~280 ohms, nothing attached to it !!!
What do you think about that ?
That’s what I originally believed, but in fact it’s not the case. And a quick check with the multimeter proved it. Try it yourself.
I don’t know why you needed to modify the probe… in the X1 mode, it should be a straight-through connection, except for the resistance of the probe cable itself.
Great idea! I think I’ll make one to replace my current SG connector which has alligator clips.
Frank,
great write-up! I can’t wait to make my own now.