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I understand the difference between passive and active probes, and between x1, x10, x100, ...

I also understand that at high frequencies the capacitance of the oscilloscope has to be considered, so a 1 MOhm probe in x1 or x10 cannot be used at, for example, 300 MHz, because a capacitance of 9 pF means a 59 ohm impedance, and a 50 Ohm input oscilloscope would be much better.

But I also saw that sometimes a coaxial cable is connected between the DUT (device under test) and the oscilloscope. When is better to use a coaxial? Why would I use a coaxial instead of a probe?

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  • \$\begingroup\$ Intense dI/dT of magnetic fields, as when probing inside switching power supplies, need the probing-return-path to be coaxial with the signal. Otherwise the RETURN wire(even at the probe tip) forms a loop and large error voltages are imposed atop the signal you wish to examine. \$\endgroup\$ – analogsystemsrf Nov 20 '18 at 18:11
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This is not about what is "better" or not. It is a question whether the source of signal has low enough impedance or not.

In many cases the oscilloscope probing is meant to be "non-invasive", and not alter the signals. So if the signal have relatively high impedance, even 50-65 Ohms (as many modern signals on PCBs do), the scope probe must have an input impedance much higher than that. In some cases even 500 Ohms probe tip can be considered "high impedance probe", and there are "passive" probes that are 10:1, and look like a simple coaxial cable, see for example the P6150 Tektronix "probe". This probe is essentially a high-quality 50-Ohm coaxial cable with a tip that has a 450 Ohm series resistor, which forms 10:1 divider into 50-Ohm cable:

enter image description here

The resulting input impedance is just 500 Ohms, but for a 50-Ohms signal source it changes the signal just by 10%, which is frequently good enough. The major advantage of this kind of probes is that they are limited only by quality of cable (and bare input bandwidth of the scope), and P6150 works up to 9 GHz.

If you have signals with 2-3 kOhms or more at source, you must use more sophisticated active probes (if the range is 500 MHz ++), or passive probes, with mega-ohms impedance range if your signal frequencies are DC or low MHz.

However in some cases of devices the signals are straight 50-Ohm outputs, and they are supposed to be loaded with 50-Ohm loads for normal operations. In this case a direct cable connection to 50-Ohms coaxial scope inputs can and will be used. An example would be testing of signal quality (amplitudes, jitter, eye diagrams) on USB 3.0 channels (from USB.org):

enter image description here

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Basically, when it is possible to use coaxial, use it. Coaxial connectors are more reliable than a probe. With a probe, you need to hook it to a test pin or even sometimes hold it in place. You have a grater risk of slipping off of your test point or getting a dodgy connection. With a probe you also get ground from 'somewhere' but not necessarily at the right place. On a coax, you get ground from the connector, where it should be. (this can matter when propagation times become relevant)

Also, test points are often exposed to more dust, dirt, oxidation, etc as protected coax connectors. This can add small resistance / impedance to the link and result in measurement inaccuracies.

Finally, although the cable is the same in a probe and a coax (mostly), on a coax the connector is still shielded, whereas on a probe, the end point is not and can therefore pick up more RF noise. I you have a coax adapter for your probe and stick the probe in the coax connector, then only this point applies.

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  • \$\begingroup\$ You say "when it is possible to use coaxial, use it", but when is that possible? For example, if I'm using a pulse generator, then a DUT and then an oscilloscope. In the input of the DUT I can put a T to connect 2 cables/probes, so one would be for the output of the pulse generator and the other one for the input of the oscilloscope. When a probe or a coaxial would be better? \$\endgroup\$ – Dylan Nov 20 '18 at 17:44
  • \$\begingroup\$ @Dylan I'm not sure I understand your comment... but I would for sure choose a T and two coax over one coax and a probe. \$\endgroup\$ – Vince Nov 20 '18 at 18:00
  • \$\begingroup\$ Perfect, but why? \$\endgroup\$ – Dylan Nov 20 '18 at 18:01
  • \$\begingroup\$ Because there is a good quality reliable connector available, which provides low resistance/impedance and rf shielding. I would use that over a probe, of which the tip is basically an antenna and relies on the conductive properties of whatever test pin you stick it on \$\endgroup\$ – Vince Nov 20 '18 at 18:09
  • \$\begingroup\$ Ohhh perfect! So if you're using a circuit (for example, the DUT connected to a parallel resistance), you would use a probe because there is not a good quality reliable connector available or why? \$\endgroup\$ – Dylan Nov 20 '18 at 18:17

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