How does a passive 1x Scope probe work?

Question

I understand about input impedance and capacitance... My question relates to the cable: What is the characteristic impedance of the cable connecting the probe to the scope? It must be terminated in a matched impedance to avoid reflections but the input impedance is typically 1 M ohm in parallel with a few pF - that can't be the characteristic impedance. So what gives?

I have noticed that the cable has a centre conductor that is very small in diameter - is that to create the correct Z0?

I've been asking this question of professional EEs for many years and no one knows...

Answer 1

Probe cable is lossy coax.

Achieving a matched condition with an oscilloscope probe is virtually impossible because the source impedance of the circuit under test is unknown and generally different from the scope's 1MΩ or 50Ω input impedance.

On top of that the input impedance of the oscilloscope has a reactive component as do most circuits under test, which makes it impossible to achieve impedance matching over the full BW of the scope (eg 100MHz). To dampen the catastrophic frequency response as a result of the reflection on the probe cable, the latter is made of lossy coax. If you measure the core resistance between the tip and the BNC connector with a multimeter you can observe this.

You may want to read "Tektronix ABC's of Probes" primer and "The Secret World of Probes" from Doug Ford. Both are excellent primers on a correct understanding of how probes work.

Answer 2

I hardly ever use a x1 probe. But grabbing one off the wire rack, I measured ~330 ohm of series resistance and ~100 pf parallel capacitance. (The 100 pF number is a bit squishy.) The 100 pf is a killer at high frequencies. And this probe has a 3dB point near 7 MHz. (about where 100 pf is 300 ohms) The lossy coax is important for operation as a x10 probe... I've read about the details, but to be honest I have forgotten them. Jim Williams had a nice discussion of probes... I think in this app note. And this is a classic... but not too much about x1 probes.

Answer 3

It's probably 50 ohms. It's basically just coax cable.

One reason for choosing 50 ohms over any other impedance is that some testgear has switchable 50 ohm terminations on its inputs, to improve accuracy for HF measurements, and in that case a 1x probe could be terminated correctly.