oscilloscope probes VS BNC cable

john · 2007-09-14T14:10:39+00:00

Hi,Is BNC cable is appropiate cable to make oscilloscope readings oroscilloscope probes? My co-worker is insisting that BNC Cable is moreshielded than the probes? I suspect that its not all about shieldingonly. There is a reason that probes exsists.We are trying to measure from an amplifer ( multiclamp700B). PleaseadviceThanksJohn

Sep 14, 2007 23 Replies

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whit3rd 19 years ago

BNC connectors are commonly affixed to 50 ohm, 75 ohm, sometimes 93 ohm transmission-line coaxial cables. If you have a suitable source, and use the appropriate terminator at the oscilloscope, those coaxial cables are very good input wiring.

An audio amplifier with 6 ohm output impedance will best be measured with

50 ohm cable fitted with 43 ohms series resistance to the amplifier output. With a 50 ohm terminator at the 'scope, expect 2:1 attenuation, of course.

If, on the other hand, your source has high impedance, it will possibly fail to drive the termination resistor, or will be loaded by the cable capacitance. The termination resistor will attenuate the signal, the cable capacitance will cause additional high-frequency attenuation. If your source has low impedance (lower than the terminator) the cable inductance can cause some overshoot (high frequency boost).

And if you omit the terminator resistance, and the source is not series-terminated, one would expect high frequency distortions from standing-waves in that transmission line. For short cables (10 feet) and low frequencies (under 10 MHz) it isn't a big problem. Not usually.

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Phil Allison 19 years ago

"whit3rd"

** Works just fine even without those restrictions.

** Complete bollocks.

** So leave it out.

** All (passive) probes have cable capacitance.

** Total bollocks.

A correctly terminated BNC lead is a resistive load.

....... Phil

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freda 19 years ago

Most scopes only support 1Mohm inputs until you get into high frequency models that also include 50-ohm inputs.

Use a 50-Ohm Coax to drive a 50-Ohm terminated scope input direcly. To make a 10X probe for 50-Ohm system, use a 450 Ohm resistor at the circuit end of a 50-ohm coax, again into the 50-ohm input of a scope....you'll need to factor this attenuator into the scopes readout as it won't know about the 10X unless you can force it somehow. This forms a high quality probe for cheap, but only for 50-ohm systems. Keep the braid over the resistor as best a possible to minimize impeadance discontinuities.

Watch how much power you dump into the attenuator. Most won't take more then 5W. Some scopes don't have any over temp protection on the input so you can fry them pretty quickly if you are not careful.

If you aren't in a 50-ohm system OR if your scope only has 1 Mohm inputs, use a probe designed to match it's input capacitance.

You NEED to compensate the probe to match the input - this is a LF comp and is usually done by using the calibrator and tweaking until the square-wave input has flat tops and bottoms.

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