oscilloscope probes VS BNC cable

[Helmut Sennewald]

Hi,

Is BNC cable is appropiate cable to make oscilloscope readings or
oscilloscope probes?

Hello John,

It depends on your requirement. It would be ok for signals up to 100kHz.
The disadvantage would be the higher capacitance of the normal coaxial
cable. Probe cables have only half the capacitance of 50Ohm coaxial
cables.

My co-worker is insisting that BNC Cable is more
shielded than the probes?

I don't agree.

I suspect that its not all about shielding
only. There is a reason that probes exsists.

The most important thing is the distributed series resistance of the
inner conductor. A value of 100 to 200 Ohm per meter gives the
best pulse performance. If you would make a 10:1 probe using
a normal coaxial cable, you would have nearly 100% overshoot
for rise times below ten nanoseconds.

Best regards,
Helmut

 

[whit3rd]

Is BNC cable is appropiate cable to make oscilloscope readings or
oscilloscope probes?

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.

 

[Phil Allison]

"whit3rd"

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.

** Works just fine even without those restrictions.

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.

** Complete bollocks.

If, on the other hand, your source has high impedance, it will
possibly fail to drive the termination resistor,

** So leave it out.

or will be loaded by the cable capacitance.

** All (passive) probes have cable capacitance.

If your source has low impedance
(lower than the terminator) the cable inductance can cause
some overshoot (high frequency boost).

** Total bollocks.

A correctly terminated BNC lead is a resistive load.



........ Phil

 

[freda]

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.