Oscilloscope probe

Discussion in 'The Projects Forum' started by QMD, Jun 30, 2010.

  1. QMD

    Thread Starter Member

    Jun 30, 2010
    I recently purchased an old oscilloscope on eBay. Unfortunately I did not purchase a probe. Could I take an old BNC coaxial cable and splice it open and use that as a probe for now, or do actual probes have some sort of ratings that I need to stay within?

  2. Dx3


    Jun 19, 2010
    Scope probes are available for as little as $10, and even those cheapo's are better than I could make for myself. (They work at audio frequencies.) I recommend buying probes that are rated for the bandwidth your scope can do, or better. Fighting your tools is very distracting.
  3. SgtWookie


    Jul 17, 2007
    You really need an O-scope probe. I've bought 1:1/10:1 probes on an auction site that are OK for low-speed stuff; I bought 8 for around $80 + shipping, 100MHz. I have a couple of 60MHz scopes, and the probes seem to work just fine within the bandwidth limitations.

    If you don't use a proper probe, you will load down the signal quite a bit, and you will see LOTS of distortion on the 'scope.

    With the "cheapie" probes I have, there is a 1:1/10:1 switch on the probe end. There's a very high-impedance resistor in the probe end; I don't have a good enough meter to measure it. The switch shorts across the resistor for the 1:1 ratio. On the 'scope end, there is a trimmer cap that's probably somewhere in the vicinity of 20pF to 60pF, and another high-value resistor from the center of the coax to ground.

    If you want to cobble something up, I'd leave the BNC connectors on the cable as-is. Get a bulkhead mount BNC connector that you can plug in to one end of the coax. Solder a 9.1 MEG resistor to the center pin, and use the other "loose" end for your probe. Use a short piece of stranded wire from the flange of the connector to an alligator clip. Remember that the clip will have earth ground on it, as the 'scope is grounded.

    For the other end, you'll need both a male and a female bulkhead BNC connector, and a metal box. Connect a 1 MEG resistor and a 60pF trimmer cap from the center conductor to ground. Connect both center conductors of the BNC connectors.

    Use the 1v p-p 1kHz probe signal output to adjust the appearance of the waveform to be as symmetrical and square as possible.

    It won't be a perfect 10:1 ratio, but it'll be close, and just a bit more expensive than the $10 probes.
  4. studiot

    AAC Fanatic!

    Nov 9, 2007
    Remember that BNC connections and cable come in two impedances 50Ω and 75Ω. Your scope is likely to be the 50Ω variety. RF coax cable is usually 75Ω.
  5. SgtWookie


    Jul 17, 2007
    Just a bit of qualification; TV coax is generally 75 Ohms.

    RG-59, RG-6, RG-11 are all 75 Ohms.

    RG-58, RG-8 are 50 Ohms.
    There are many, many more specifications for coax cables, but those are the most common in the consumer market.