I have recently acquired a Tek 460A scope and a mixed bag of oscilloscope probes. I have a very basic question about the probes:

One of the probes is a Tek P2200. This is a 1X/10X switchable probe with a simple BNC connector on the back. If I connect this up to a 16V power supply and set the probe to 10X, the scope reads 1.6V. No surprise there.

The other probe is a Tek P6121, which has a chunkier BNC connector on the back that includes an additional pin on the outside ring (that is obviously engaging with some contacts on the scope). The probe is labelled 10X, but when connected to the same 16V power supply the scope registers 16V. Is this because (a) the probe is somehow communicating its attenuation factor to the scope, or (b) is this really a 1X probe?

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    \$\begingroup\$ Do you have a picture? \$\endgroup\$
    – jippie
    Commented Mar 15, 2013 at 21:38

1 Answer 1


The extra pin is connected to ground via a resistor which is used to communicate the scaling factor of the probe. When you connect the probe it should show up somewhere on the scope channel settings that you are using a 10x probe.

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    \$\begingroup\$ On used probes, the little pin sometimes get stuck and doesn't contact the ring very well. I hate using probes that flip themselves from 1x to 10x to 1x to 10x as I use them (I generally just set the scope to 10x manually because of this) \$\endgroup\$ Commented Mar 15, 2013 at 21:26
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    \$\begingroup\$ Gorloth: Ah, spotted it; when I plugin the P6121 the voltage/div changes by the attenuation factor (so instead of "Ch1 100 mV" it says "Ch1 1.00 V"). Thanks! \$\endgroup\$
    – larsks
    Commented Mar 16, 2013 at 0:34

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